scs@adam.mit.edu (Steve Summit) (07/18/90)
(This diff listing has been edited for readability and is not suitable for the patch program.) 93,98c68,71 < Although a null pointer does not "point anywhere," it is different < from an uninitialized pointer, about which we cannot say where it < points. A null pointer points explicitly nowhere (i.e. not at any < object); an uninitialized pointer might point anywhere (at some < random object, or at a garbage or unallocated address). See also < question 29. --- > A null pointer is different from an uninitialized pointer. A null > pointer is known not to point to any object; an uninitialized > pointer might point anywhere (that is, at some random object, or at > a garbage or unallocated address). See also question 30. 114,115c87,88 < in an assignment or comparison when one side is a variable of < pointer type, the compiler can tell that a constant 0 on the other --- > in an initialization, assignment, or comparison when one side is a > variable or expression of pointer type, the compiler can tell that a 165,169c142,145 < A: Not since the early days. It is now merely guaranteed that a < pointer to an object may be cast to a "suitably capacious" integral < type, and back, without change, but how large the type is is not < specified (it may be larger than a long int) and the rule does _not_ < apply to pointers to functions. (Any object pointer may be cast to --- > A: Not since the early days. Attempting to push pointers into > integers, or build pointers out of integers, has always been > machine-dependent and unportable, and doing so is strongly > discouraged. (Any object pointer may be cast to the "universal" 224,227c200,204 < different kinds of pointers for different types of data. The < suggested #definition would make uncast NULL arguments to functions < expecting pointers to characters to work correctly, but pointer < arguments to other types would still be problematical. --- > different internal representations for pointers to different types > of data. The suggested #definition would make uncast NULL arguments > to functions expecting pointers to characters to work correctly, but > pointer arguments to other types would still be problematical, and > legal constructions such as 228a206,209 > FILE *fp = NULL; > > could fail. > 240,241c221,222 < A: This definition, like (void *)0, helps incorrect programs work on < some common architectures. --- > A: This definition (the third approved one, besides 0 and (void *)0) > helps incorrect programs work on some architectures. 249,250c230,231 < operators), a false value is produced when the expression is equal < to zero, and a true value otherwise. That is, whenever one writes --- > operators), a false value is produced when the expression compares > equal to zero, and a true value otherwise. That is, whenever one 287,288c271,272 < and that an uncast "0" is perfectly acceptable in assignment and < comparison contexts. Any usage of "NULL" (as opposed to "0") should --- > and that an uncast "0" is perfectly acceptable in initialization, > assignment, and comparison contexts. Any usage of "NULL" (as 300,302c285,288 < A: No. NULL is defined as a preprocessor macro _not_ because its value < might change. That source-code 0's generate null pointers is < guaranteed by the language. NULL is used only as a stylistic --- > A: No. Although preprocessor macros are often used in place of numbers > because the numbers might change, this is _not_ the reason that NULL > is used in place of 0. The language guarantees that source-code 0's > (in pointer contexts) generate null pointers. NULL is used only as 315,316c301,302 < When the phrase "null pointer" is casually used, one of three things < may be meant: --- > When the term "null" or "NULL" is casually used, one of several > things may be meant: 318a304,305 > 1. The conceptual null pointer, the abstract language concept > defined in question 1. It is implemented with... 327a316,319 > Finally, as a red herring, we have > > 5. The ASCII null character (NUL), which does have all bits zero, > but has no relation to the null pointer except in name. 336,342c329,338 < about the underlying machine implementation. The fact that null < pointers are represented both in source code, and internally to most < machines, as zero invites unwarranted assumptions. The fact that a < preprocessor macro (NULL) is often used suggests that this is done < because the value might change later, or on some weird machine. < Furthermore, the distinction between the three uses of the term < "null" (listed above) is often overlooked. --- > about the underlying machine implementation. The construct > "if(p == 0)" is easily misread as calling for conversion of p to an > integral type, rather than 0 to a pointer type, before the > conversion. The fact that null pointers are represented both in > source code, and internally to most machines, as zero invites > unwarranted assumptions. The fact that a preprocessor macro (NULL) > is often used suggests that this is done because the value might > change later, or on some weird machine. Furthermore, the > distinction between the four uses of the term "null" (listed above) > is often overlooked. 383,389c379,390 < array, usually unsized) holds _only_ for formal arguments to < functions. This identity falls out of the fact that arrays "turn < into" pointers in expressions. That is, when an array name is < mentioned in an expression, it is converted immediately into a < pointer to the array's first element. Therefore, an array is never < passed to a function; rather a pointer to its first element is < passed instead. --- > array declaration, usually unsized) holds _only_ for formal > parameters to functions. This identity is related to the fact that > arrays "turn into" pointers in expressions. That is, when an array > name is mentioned in an expression, it is converted immediately into > a pointer to the array's first element. Therefore, an array is > never passed to a function; rather a pointer to its first element is > passed instead. Allowing pointer parameters to be declared as > arrays is a simply a way of making it look as though the array was > actually being passed. Some programmers prefer, as a matter of > style, to use this syntax to indicate that the pointer parameter is > expected to point to the start of an array rather than to a single > value. 409,410c413,414 < A: Perhaps no aspect of C is more confusing than pointers, and no < statement has compounded this confusion more than the one above. --- > A: Perhaps no aspect of C is more confusing than pointers, and the > confusion is compounded by statements like the one above. 455a458,460 > The order of other embedded side effects is similarly undefined. > For example, the expression i + (i = 2) may or may not have the > value 4. 459a465,473 > 18. But what about the &&, ||, and ?: operators? > I see code like "if((c = getchar()) == EOF || c == '\n')" ... > > A: There is a special exception for those operators; each of them does > imply a sequence point (i.e. left-to-right evaluation is > guaranteed). > > References: ANSI X3.159-1989 Sec. 3.3.2.2, 3.3.13, 3.3.14, 3.3.15 . > 489c506,507 < The cost is approximately $50.00, plus $6.00 shipping. --- > The cost is approximately $50.00, plus $6.00 shipping. Quantity > discounts are available. 505,507c523,525 < with the old-style definition "int blort(x) float x;". Old C < silently promoted doubles to floats when passing them as arguments, < and made a corresponding silent change to formal argument --- > with the old-style definition "int blort(x) float x;". Old C (and > ANSI C, in the absence of prototypes) silently promotes floats to > doubles when passing them as arguments, and makes a corresponding 547c567,569 < of a character constant) and no newlines inside quotes. --- > of a character constant) and no newlines inside quotes. Therefore, > natural-language comments should always be written between the > "official" comment delimiters /* and */. 627c651 < Using varargs, rather than stdarg, requires a few changes which are --- > Using the older varargs package, rather than stdarg, requires a few 638c662 < A: Use v*printf. --- > A: Use vprintf, vfprintf, or vsprintf. 677c696,697 < between the calls to va_start and vfprintf. --- > between the calls to va_start and vfprintf. (Note that there is no > semicolon after va_dcl.) 686c706 < systems have a nonstandard nargs() function available, but even this --- > systems have a nonstandard nargs() function available, but its use 707,709c727,729 < example above. If the arguments must be passed as arguments (not < indirectly through a va_list pointer) to another function which is < itself varargs (for which you do not have the option of creating an --- > example above. If the arguments must be passed directly as actual > arguments (not indirectly through a va_list pointer) to another > function which is itself variadic (for which you do not have the 716c736 < 29. Why doesn't this program work? --- > 30. Why doesn't this program work? 721c741 < printf("Type something: "); --- > printf("Type something:\n); 759c779,781 < treatment of the trailing \n.) --- > treatment of the trailing \n.) It would also be possible to use > malloc to allocate the answer buffer, and/or to parameterize its > size (#define ANSWERSIZE 100). 760a783,797 > 31. What is alloca and why is its use discouraged? > > A: alloca allocates memory which is automatically freed when the > function from which alloca was called returns. That is, memory > allocated with alloca is local to a particular function's "stack > frame" or context. > > alloca cannot be written portably, and is difficult to implement on > machines without a stack. Its use is problematical (and the obvious > implementation on a stack-based machine fails) when its return value > is passed directly to another function, as in > fgets(alloca(100), stdin, 100). > > For these reasons, alloca cannot be used in programs which must be > widely portable, no matter how useful it might be. 764,764c802,802 < and from functions, but K&R I says no. --- > and from functions, but K&R I says not. 807c845 < this case, by the C start-up code). Storing a structure into memory --- > this case, by the C start-up code). Attempting to store a structure 836c874,875 < #define offset(type, mem) ((size_t)(char *)&(((type *)0)->mem)) --- > #define offsetof(type, mem) ((size_t) \ > ((char *)&(((type) *) 0)->mem - (char *)&(((type) *) 0))) 838,839c877,878 < This implementation's use of the null pointer, however, is said not < to be completely portable. --- > This implementation is not 100% portable; some compilers may refuse > to accept it. 867c908 < but the compiler gave me errors. Can't a struct in C contain a --- > but the compiler gave me error messages. Can't a struct in C 936,937c977,978 < within, the definitions for struct a and struct b. The problem < arises when an attempt is made to define and use a typedef within --- > within, the definitions for struct a and struct b. Problems arise > only when an attempt is made to define and use a typedef within the 972,974c1012,1013 < A: Several public-domain versions are available. A file called < "cdecl.shar" is available for anonymous ftp from mimsy.umd.edu < (128.8.128.8), or check your local archive. --- > A: Several public-domain versions are available. One is in volume 14 > of comp.sources.unix . 1007c1045 < These don't really buy much (see below). --- > These don't buy anything (see below). 1024c1061,1063 < _not_ necessarily 1. --- > _not_ necessarily 1. A good rule of thumb is to use TRUE and FALSE > (or the like) only for assignment to a Boolean variable or as the > return value from a Boolean function, never in a comparison. 1043,1049c1083,1089 < For now, the advantages of enums (other than that the numeric values < are automatically assigned) are that a compiler may generate < nonfatal warnings when enums and ints are indiscriminately mixed < (such mixing can still be considered bad style even though it is not < strictly illegal) or when enum cases are left out of switch < statements; and that a debugger may be able to display the symbolic < values when enum variables are examined. --- > The advantages of enums are that the numeric values are > automatically assigned, that a debugger may be able to display the > symbolic values when enum variables are examined, and that a > compiler may generate nonfatal warnings when enums and ints are > indiscriminately mixed (such mixing can still be considered bad > style even though it is not strictly illegal) or when enum cases are > left out of switch statements. 1062,1063c1102,1105 < standardized by the C language. Under UNIX, use ioctl to play with < the terminal driver modes. --- > standardized by the C language. If you are using curses, use its > cbreak() function. Under UNIX, use ioctl to play with the terminal > driver modes (CBREAK or RAW under "classic" versions; ICANON, > c_cc[VMIN] and c_cc[VTIME] under System V or Posix systems). 1074,1075c1117,1119 < your operating system, you may be able to use "nonblocking I/O", or < a system call named "select", or the FIONREAD ioctl. --- > your system, you may be able to use "nonblocking I/O", or a system > call named "select", or the FIONREAD ioctl, or O_NDELAY, or a > kbhit() routine. 1090c1134,1137 < changed, a special-case scheme can be set up. --- > changed, a special-case scheme can be set up. (Under Unix, a child > process cannot directly affect its parent at all. Other operating > systems have different process environments which could > intrinsically support such communication.) 1117a1164,1191 > 51. When I read from the keyboard with scanf(), it seems to hang until I > type one extra line of input. > > A: scanf() was designed for free-format input, which is seldom what you > want when reading from the keyboard. In particular, "\n" in a > format string does not mean "expect a newline", it means "discard > all whitespace". But the only way to discard all whitespace is to > continue reading the stream until a non-whitespace character is seen > (which is then left in the buffer for the next input), so the effect > is that it keeps going until it sees a nonblank line. > > 52. So what should I use instead? > > A: You could use a "%c" format, which will read one character that you > can then manually compare against a newline; or "%*c" and no > variable if you're willing to trust the user to hit a newline; or > "%*[^\n]%*c" to discard everything up to and including the newline. > Or you could use fgets() to read a whole line, and then use sscanf() > or other string functions to parse the line buffer. > > 53. Can someone tell me how to write itoa (the inverse of atoi)? > > A: Just use sprintf. 1137c1212 < your compiler, operating system, and processer can be supplied. You --- > your compiler, operating system, and processor can be supplied. You 1148,1149c1223,1224 < p2c written by Dave Gillespie, and posted to comp.sources.unix in < March, 1990. --- > p2c written by Dave Gillespie, and posted to > comp.sources.unix in March, 1990 (Volume 21). 1151,1152c1226,1228 < ptoc another comp.sources.unix contribution, this one written in < Pascal. --- > ptoc another comp.sources.unix contribution, this one > written in Pascal (comp.sources.unix, Volume 10, also > patches in Volume 13?). 1157a1235,1246 > FOR_C Available from: > > Cobalt Blue > 2940 Union Ave., Suite C > San Jose, CA 95124 > (408) 723-0474 > > Promula.Fortran Available from > > Promula Development Corp. > 3620 N. High St., Suite 301 > Columbus, OH 43214 > (614) 263-5454 > 1162a1250,1251 > 57. How can I call Fortran (BASIC, Pascal, ADA, LISP) functions from C? > (And vice versa?) > > A: The answer is entirely dependent on the machine and the specific > calling sequences of the various compilers in use, and may not be > possible at all. Read your compiler documentation very carefully; > sometimes there is a "mixed-language programming guide," although > the techniques for passing arguments correctly are often arcane. 1166,1168c1259 < A: C comments do not nest. For this reason, it is usually better to < "comment out" large sections of code, which might contain comments, < with #ifdef. --- > A: Nested comments would cause more harm than good, mostly because of > the possibility of accidentally leaving comments unclosed by > including the characters "/*" within them. For this reason, it is > usually better to "comment out" large sections of code, which might > contain comments, with #ifdef or #if 0. 1181,1182c1278,1279 < floating-point versions of printf and scanf do not handle %e, %f, < and %g. Occasionally the heuristics for "is the program using --- > floating-point versions of printf and scanf save space by not > including code to handle %e, %f, and %g. Occasionally the 1196c1294,1295 < A: Plum Hall, among others, sells one. --- > A: Plum Hall, (1 Spruce Ave., Cardiff, NJ 08232, USA), among others, > sells one. 1198a1297,1303 > 61. Where can I get a YACC grammar for C? > > A: Several grammars are floating around; keep your eyes open. There is > one on uunet.uu.net (192.48.96.2) in net.sources/ansi.c.grammar.Z . > FSF's GNU C compiler contains a grammar, as does the appendix to > K&R II. Several have recently been posted to the net. 1217c1304 < 57. Where can I get the "Indian Hills Style Guide" and other coding --- > 62. Where can I get the "Indian Hill Style Guide" and other coding 1220a1307,1315 > A: Various standards are available for anonymous ftp from: > > site file/directory > > cs.washington.edu ~ftp/pub/cstyle.tar.Z > (128.95.1.4) > > cs.toronto.edu doc/programming > > giza.cis.ohio-state.edu pub/style-guide > > prep.ai.mit.edu pub/gnu/standards.text 1234a1320,1325 > 63. Where can I get extra copies of this list? > > A: For now, just pull it off the net; it is normally posted on about > the first of the month, with an Expiration: line which should keep > it around all month. Eventually, it may be available for anonymous > ftp, or via a mailserver. 1238,1240c1329,1332 < Dunn, Tony Hansen, Blair Houghton, Kirk Johnson, Andrew Koenig, John < Lauro, Christopher Lott, Rich Salz, and Joshua Simons, who have < contributed, directly or indirectly, to this article. --- > Dunn, Tony Hansen, Guy Harris, Karl Heuer, Blair Houghton, Kirk Johnson, > Andrew Koenig, John Lauro, Christopher Lott, Rich Salz, Joshua Simons, > and Erik Talvola, who have contributed, directly or indirectly, to this > article.
scs@adam.mit.edu (Steve Summit) (08/01/90)
This article contains the changes between the previous posting of the frequently-asked questions list (July 18) and the new one (coming up next). (These diffs have been edited for readability and are not suitable for the patch program.) 178,179c166,167 < doing so sends the wrong stylistic message. (The ANSI #definition < of NULL is allowed to be (void *)0, which will not work in non- --- > doing so sends the wrong stylistic message. (ANSI allows the > #definition of NULL to be (void *)0, which will not work in non- 301a286,291 > 10. But I once used a compiler that wouldn't work unless NULL was used. > > A: This compiler was broken. In general, making decisions about a > language based on the behavior of one particular compiler is likely > to be counterproductive. 416c406,410 < formal argument declarations, nowhere else. --- > formal parameter declarations, nowhere else. If this conversion > confuses you, don't use it; many people have concluded that the > confusion it causes outweighs the small advantage of having the > declaration "look like" the call and/or the uses within the > function. 446a441,528 > 17. My compiler complained when I passed a two-dimensional array to a > routine expecting a pointer to a pointer. > > A: The rule by which arrays decay into pointers is not applied > recursively. An array of arrays (i.e. a two-dimensional array in C) > decays into a pointer to an array, not a pointer to a pointer. > Pointers to arrays are confusing, and it is best to avoid them. > (The confusion is heightened by incorrect compilers, including some > versions of pcc and pcc-derived lint's, which incorrectly accept > assignments of multi-dimensional arrays to multi-level pointers.) > If you are passing a two-dimensional array to a function: > > int array[YSIZE][XSIZE]; > f(array); > > the function's declaration should match: > f(int a[][XSIZE]) {...} > or > f(int (*a)[XSIZE]) {...} > > In the first declaration, the compiler performs the usual implicit > rewriting of "array of array" to "pointer to array;" in the second > form the pointer declaration is explicit. The called function does > not care how big the array is, but it must know its shape, so the > "column" dimension XSIZE must be included. In both cases the number > of "rows" is irrelevant, and omitted. > > If a function is already declared as accepting a pointer to a > pointer, an intermediate pointer would need to be used when > attempting to call it with a two-dimensional array: > > int *ip = &a[0][0]; > g(&ip); > ... > g(int **ipp) {...} > > Note that this usage is liable to be misleading (if not incorrect), > since the array has been "flattened" (its shape has been lost). > > 18. How do I declare a pointer to an array? > > A: Usually, you don't want one. Think about using a pointer to one of > the array's elements instead. Arrays of type T decay into pointers > to type T, which is convenient; subscripting or incrementing the > resultant pointer accesses the individual members of the array. > True pointers to arrays, when subscripted or incremented, step over > entire arrays, and are generally only useful when operating on > multidimensional arrays. (See the question above.) > > 19. How can I dynamically allocate a multidimensional array? > > A: It is usually best to allocate an array of pointers, and then > initialize each pointer to a dynamically-allocated "row." The > resulting "ragged" array often saves space, although it may not be > contiguous in memory as a real array would be. > > int **array = (int **)malloc(nrows * ncolumns * sizeof(int *)); > for(i = 0; i < nrows; i++) > array[i] = (int *)malloc(ncolumns * sizeof(int)); > > (In "real" code, of course, malloc's return value should be > checked.) > > You can keep the array's contents contiguous, while losing the > ability to have rows of varying and different lengths, with a bit of > explicit pointer arithmetic: > > int **array = (int **)malloc(nrows * ncolumns * sizeof(int *)); > array[0] = (int *)malloc(nrows * ncolumns * sizeof(int)); > for(i = 1; i < nrows; i++) > array[i] = array[0] + i * ncolumns; > > In either case, the elements of the dynamic array can be accessed > with normal-looking array subscripts: array[i][j]. > > If the double indirection implied by the above scheme is for some > reason unacceptable, you can simulate a two-dimensional array with a > single, dynamically-allocated one-dimensional array: > > int *array = (int *)malloc(nrows * ncolumns * sizeof(int)); > > However, you must now perform subscript calculations manually, > accessing array[i, j] with array[i * ncolumns + j]. (A macro can > hide the explicit calculation, but invoking it then requires > parentheses and commas which don't look exactly like > multidimensional array subscripts.) 471c553,554 < value 4. --- > value 4. ANSI allows compilers to reject code which contains such > ambiguous or undefined side effects. 476c559 < 18. But what about the &&, ||, and ?: operators? --- > 21. But what about the &&, ||, ?:, and comma operators? 549a633,635 > (In this case, it would be clearest to change the old-style > definition to use double as well). 660c745,746 < Note the cast on the last argument. --- > Note the cast on the last argument. (Also note that the caller must > free the returned, malloc'ed storage.) 713a799,804 > 31. How can I write a function analogous to scanf? > > A: Unfortunately, vscanf and the like are not standard. You're on your > own. 794a885,907 > 35. You can't use dynamically-allocated memory after you free it, can > you? > > A: No. Some early man pages for malloc stated that the contents of > freed memory was "left undisturbed;" this ill-advised guarantee is > not universal and is not required by ANSI. > > Few programmers would use the contents of freed memory deliberately, > but it is easy to do so accidentally. Consider the following > (correct) code for freeing a singly-linked list: > > struct list *listp, *nextp; > for(listp = base; listp != NULL; listp = nextp) { > nextp = listp->next; > free((char *)listp); > } > > and notice what would happen if the more-obvious loop iteration > expression listp = listp->next were used, without the temporary > nextp pointer. 885,889c1000,1004 < #define offsetof(type, mem) ((size_t) \ < ((char *)&(((type) *) 0)->mem - (char *)&(((type) *) 0))) < < This implementation is not 100% portable; some compilers may refuse < to accept it. --- > #define offsetof(type, mem) ((size_t) \ > ((char *)&((type *) 0)->mem - (char *)((type *) 0))) > > This implementation is not 100% portable; some compilers may > legitimately refuse to accept it. 1125a1239,1244 > Operating system specific questions are not appropriate for > comp.lang.c . Several common questions are answered in frequently- > asked questions postings in the comp.unix.questions and > comp.sys.ibm.pc newsgroups. 1265a1382,1391 > 62. Where can I get copies of all these public-domain programs? > > A: If you have access to Usenet, see the regular postings in the > comp.sources.unix and comp.sources.misc newsgroups, which describe, > in some detail, the archiving policies and how to retrieve copies. > Otherwise, you can try anonymous ftp and/or uucp from a central, > public-spirited site, such as uunet.uu.net, but this article cannot > track or list all of the available sites and how to access them.
scs@adam.mit.edu (Steve Summit) (09/01/90)
This article contains the changes between the previous posting of the frequently-asked questions list (August 1) and the new one (coming up next). The changes this month were obviously minimal, correcting only one glaring error. (I have been on vacation for two weeks and, in fact, still am; these articles are being graciously posted for me by the at program.) If you have sent me corrections or suggestions which are not reflected here, don't worry; they will most likely be rolled in to the October posting. 509c509 < int **array = (int **)malloc(nrows * ncolumns * sizeof(int *)); --- > int **array = (int **)malloc(nrows * sizeof(int *)); 520c520 < int **array = (int **)malloc(nrows * ncolumns * sizeof(int *)); --- > int **array = (int **)malloc(nrows * sizeof(int *));
scs@adam.mit.edu (Steve Summit) (11/01/90)
This article contains the changes between the previous posting of the frequently-asked questions list (October 1) and the new one. (Do _not_ worry if you have not seen the new one yet; it's coming up next.) (These diffs have been edited for readability and are not suitable for the patch program.) < This article, which will be reposted periodically, attempts to answer --- > This article, which is posted monthly, attempts to answer these common < some of the myths which this article attempts to debunk. Two invaluable < references, which are an excellent addition to any serious programmer's < library, are: < < The C Programming Language, by Brian W. Kernighan and Dennis M. < Ritchie. < < C: A Reference Manual, by Samuel P. Harbison and Guy L. Steele, Jr. < < Both exist in several editions. Andrew Koenig's book _C Traps and < Pitfalls_ also covers many of the difficulties frequently discussed < here. < --- > some of the myths which this article attempts to debunk. Several > noteworthy books on C are listed in this article's bibliography. < please try to answer it by referring to these or other books, or to < knowledgeable colleagues, before posing your question to the net at --- > please try to answer it by checking a few of the referenced books, or by > asking knowledgeable colleagues, before posing your question to the net < your comments to scs@adam.mit.edu and/or scs%adam.mit.edu@mit.edu; this < article's From: line may be unuseable. --- > your comments to scs@adam.mit.edu, scs%adam.mit.edu@mit.edu, and/or > mit-eddie!adam!scs; this article's From: line may be unusable. > The questions answered here are divided into several categories: > > 1. Null Pointers > 2. Arrays and Pointers > 3. Order of Evaluation > 4. ANSI C > 5. C Preprocessor > 6. Variable-Length Argument Lists > 7. Memory Allocation > 8. Structures > 9. Declarations > 10. Boolean Expressions and Variables > 11. Operating System Dependencies > 12. Stdio > 13. Miscellaneous < object. That is, the address-of operator & will never "return" a --- > object. That is, the address-of operator & will never yield a null < A null pointer is different from an uninitialized pointer. A null < pointer is known not to point to any object; an uninitialized --- > A null pointer is conceptually different from an uninitialized > pointer. A null pointer is known not to point to any object; an < null pointer is required, so it can make the distinction if --- > type of null pointer is required, so it can make the distinction if < A: Not since the early days. Attempting to push pointers into < integers, or build pointers out of integers, has always been --- > A: Not since the early days. Attempting to turn pointers into > integers, or to build pointers out of integers, has always been > 7. I use the preprocessor macro > > #define Nullptr(type) (type *)0 > > to help me build null pointers of the correct type. > > A: This trick, though popular with beginning programmers, does not buy > much. It is not needed in assignments and comparisons; see question > 2. It does not even save keystrokes. Its use suggests to the > reader that the author is shaky on the subject of null pointers, and > requires the reader to check the #definition of the macro, its > invocations, and _all_ other pointer usages much more carefully. < in a non-pointer context generates an integer zero. If the null < pointer keyword were "nil" the compiler could emit an error message < for an ambiguous usage, but since it is "0" the compiler may end up < emitting incorrect code. --- > in a non-pointer context generates an integer zero instead of an > error message, and if that uncast 0 was supposed to be a null > pointer, the code may not work. < 2. If the usage of "0" or "NULL" is in a function call, cast it to < the pointer type expected by the function being called. --- > 2. If the usage of "0" or "NULL" is an argument in a function > call, cast it to the pointer type expected by the function > being called. < pointers, which you shouldn't need to know. --- > pointers, which you shouldn't need to know. Understand questions 1, > 2, and 4, and consider 9 and 13, and you'll do fine. < arrays "turn into" pointers in expressions. That is, when an array --- > arrays "decay" into pointers in expressions. That is, when an array < expected to point to the start of an array rather than to a single < value. --- > expected to point to the start of an array rather than to some > single value. < A: Perhaps no aspect of C is more confusing than pointers, and the < confusion is compounded by statements like the one above. Saying < that arrays and pointers are "equivalent" does not by any means --- > A: Much of the confusion surrounding pointers in C can be traced to > this statement. Saying that arrays and pointers are "equivalent" < (The confusion is heightened by incorrect compilers, including some < versions of pcc and pcc-derived lint's, which incorrectly accept < assignments of multi-dimensional arrays to multi-level pointers.) --- > (The confusion is heightened by the existence of incorrect > compilers, including some versions of pcc and pcc-derived lint's, > which improperly accept assignments of multi-dimensional arrays to > multi-level pointers.) < If a function is already declared as accepting a pointer to a < pointer, an intermediate pointer would need to be used when < attempting to call it with a two-dimensional array: < < int *ip = &a[0][0]; < g(&ip); < ... < g(int **ipp) {...} < < Note that this usage is liable to be misleading (if not incorrect), < since the array has been "flattened" (its shape has been lost). --- > If a function is already declared as accepting a pointer to a > pointer, it is probably incorrect to pass a two-dimensional array > directly to it. < A: Usually, you don't want one. Think about using a pointer to one of --- > A: Usually, you don't want to. Consider using a pointer to one of the > If you really need to declare a pointer to an entire array, use > something like "int (*ap)[N];" where N is the size of the array. If > the size of the array is unknown, N can be omitted, but the > resulting type, "pointer to array of unknown size," is almost > completely useless. < resulting "ragged" array often saves space, although it may not be < contiguous in memory as a real array would be. --- > resulting "ragged" array often saves space, although it is not > necessarily contiguous in memory as a real array would be. < (In "real" code, of course, malloc's return value should be < checked.) --- > (In "real" code, of course, each return value from malloc would have > to be checked.) < You can keep the array's contents contiguous, while losing the < ability to have rows of varying and different lengths, with a bit of --- > You can keep the array's contents contiguous, while making later > reallocation of individual rows difficult, with a bit of explicit < value 4. ANSI allows compilers to reject code which contains such < ambiguous or undefined side effects. --- > value 4. > > The ANSI C standard declares that code which contains such ambiguous > or undefined side effects is not merely undefined, but illegal. > Don't even try to find out how your compiler implements such things > (contrary to the ill-advised exercises in many C textbooks); as K&R > wisely point out, "if you don't know _how_ they are done on various > machines, the innocence may help to protect you." < A: There is a special exception for those operators; each of them does < imply a sequence point (i.e. left-to-right evaluation is < guaranteed). --- > A: There is a special exception for those operators, (as well as ?: ); > each of them does imply a sequence point (i.e. left-to-right > evaluation is guaranteed). Any book on C should make this clear. < arduous process, this C standard was finally ratified as an American --- > arduous process, the committee's work was finally ratified as an < library support routines, an unprecedented effort. --- > library support routines. < The cost is approximately $50.00, plus $6.00 shipping. Quantity < discounts are available. --- > The cost from ANSI is $50.00, plus $6.00 shipping. Quantity > discounts are available. (Note that ANSI derives revenues to > support its operations from the sale of printed standards, so > electronic copies are _not_ available.) < used, but it will not work for floating-point values or pointers. --- > used, but it will not work for floating-point values or pointers > (and the "obvious" supercompressed implementation for integral types > a^=b^=a^=b is, strictly speaking, illegal due to multiple side- > effects; and it will not work if the two values are the same > variable, and...). > 28. I have some old code that tries to construct identifiers with a > macro like > > #define Paste(a, b) a/**/b > > but it doesn't work any more. > > A: That comments disappeared entirely and could therefore be used for > token pasting was an undocumented feature of some early preprocessor > implementations, notably Reiser's. ANSI affirms (as did K&R) that > comments are replaced with white space. However, since the need for > pasting tokens was demonstrated and real, ANSI introduced a well- > defined token-pasting operator, ##, which can be used as follows: > > #define Paste(a, b) a##b > > Reference: ANSI Sec. 3.8.3.3 p. 91, Rationale pp. 66-7. < that an apostrophe within a contracted word looks like the beginning < of a character constant) and no newlines inside quotes. Therefore, --- > that an apostrophe within a contracted word in a comment looks like > the beginning of a character constant), and no newlines inside > 30. What's the best way to write a multi-statement cpp macro? > > A: The usual goal is to write a macro that can be invoked as if it were > a single function-call statement. This means that the "caller" will > be supplying the final semicolon, so the macro body should not. The > macro body cannot be a simple brace-delineated compound statement, > because syntax errors would result if it were invoked (apparently as > a single statement, but with a resultant an extra semicolon) as the > if branch of an if/else statement with an explicit else clause. > > The best solution is to use > > #define Func() do { \ > /* declarations */ \ > stmt1; \ > stmt2; \ > /* ... */ \ > } while(0) /* (no trailing ; ) */ > > When the "caller" appends a semicolon, this expansion becomes a > single statement regardless of context. (An optimizing compiler > will remove any "dead" tests or branches on the constant condition > 0, although lint may complain.) > > If all of the statements in the intended macro are simple > expressions, a simpler technique is to separate them with commas and > surround them with parentheses. > > Reference: CT&P Sec. 6.3 pp. 82-3. < extern char *malloc(); /* redundant */ < int len = 0; --- > size_t len = 0; < Using the older varargs package, rather than stdarg, requires a few < changes which are not discussed here, in the interests of brevity. --- > Under a pre-ANSI compiler, rewrite the function definition without a > prototype ("char *vstrcat(first) char *first; {"), #include > <stdio.h> rather than <stddef.h>, replace "#include <stdlib.h>" with > "extern char *malloc();", and use int instead of size_t. You may > also have to delete the (void) casts, and use the older varargs > package instead of stdarg. > (If you know enough about your machine's architecture, it is > possible to pick arguments off of the stack "by hand," but there is > little reason to do so, since portable mechanisms exist.) > 38. I can't get strcat to work. I tried > > #include <string.h> > main() > { > char *s1 = "Hello, "; > char *s2 = "world!"; > char *s3 = strcat(s1, s2); > printf("%s\n", s3); > } > > but I got strange results. > > A: Again, the problem is that space for the concatenated result is not > properly allocated. C does not provide a true string type. C > programmers use char *'s for strings, but must always keep > allocation in mind. The compiler will only allocate memory for > objects explicitly mentioned in the source code (in the case of > "strings," this includes character arrays and string literals). The > programmer must arrange (explicitly) for sufficient space for the > results of run-time operations such as string concatenation, > typically by declaring arrays, or calling malloc. > > The simple strcat example could be fixed with something like > > char s1[20] = "Hello, "; > char *s2 = "world!"; > > Note, however, that strcat appends the string pointed to by its > second argument to that pointed to by the first, and merely returns > its first argument, so the s3 variable is superfluous. > > Reference: CT&P Sec. 3.2 p. 32. < struct is pushed on the stack, which may involve significant < overhead for large structures. It may be preferable in such cases < to pass a pointer to the structure instead. --- > struct is typically pushed on the stack, using as many words as are > required. (Pointers to functions are often chosen precisely to > avoid this overhead.) < Structures are returned from functions either in a special, static < place (which may make struct-valued functions nonreentrant) or in a < location pointed to by an extra, "hidden" argument to the function. --- > Structures are typically returned from functions in a location > pointed to by an extra, "hidden" argument to the function. Older > compilers often used a special, static location for structure > returns, although this made struct-valued function nonreentrant, > which ANSI disallows. < 45. How do I declare a pointer to a function returning a pointer to a < double? --- > 49. How do I declare an array of pointers to functions returning > pointers to functions returning pointers to characters? < A: There are at least three answers to this question: --- > A: This question can be answered in at least three ways (all assume the > hypothetical array is to have 5 elements): < 1. double *(*p)(); --- > 1. char *(*(*a[5])())(); < typedef double *pd; /* pointer to double */ < typedef pd fpd(); /* func returning ptr to double */ < typedef fpd *pfpd; /* ptr to func ret ptr to double */ < pfpd p; --- > typedef char *cp; /* pointer to char */ > typedef cp fpc(); /* function returning pointer to char */ > typedef fpc *pfpc; /* pointer to above */ > typedef pfpc fpfpc(); /* function returning... */ > typedef fpfpc *pfpfpc; /* pointer to... */ > pfpfpc a[5]; /* array of... */ > < cdecl> declare p as pointer to function returning pointer to double < double *(*p)(); --- > cdecl> declare a as array 5 of pointer to function returning > pointer to function returning pointer to char > char *(*(*a[5])())() > Any good book on C should explain tricks for reading these > complicated C declarations "inside out" to understand them > ("declaration mimics use"). > 51. I finally figured out the syntax for declaring pointers to > functions, but now how do I initialize one? > > A: Use something like > > extern int func(); > int (*fp)() = func; > > When the name of a function appears in an expression but is not > being called (i.e. is not followed by a "("), its address is > implicitly taken, just as is done for arrays. > > An explicit extern declaration for the function is normally needed, > since implicit external function declaration does not happen in this > case (again, because the function name is not followed by a "("). < as long as you are consistent within one program or project. --- > or use raw 1 and 0, as long as you are consistent within one program > or project. > or define "helper" macros such as > > #define Istrue(e) ((e) != 0) < _not_ necessarily 1. A good rule of thumb is to use TRUE and FALSE < (or the like) only for assignment to a Boolean variable or as the < return value from a Boolean function, never in a comparison. --- > _not_ necessarily 1. (Besides, if you believe that > "if((a == b) == TRUE)" is an improvement over "if(a == b)", why stop > there? Why not use "if(((a == b) == TRUE) == TRUE)"?) A good rule > of thumb is to use TRUE and FALSE (or the like) only for assignment > to a Boolean variable or as the return value from a Boolean > function, never in a comparison. < comp.lang.c . Several common questions are answered in frequently- < asked questions postings in the comp.unix.questions and < comp.sys.ibm.pc newsgroups. --- > comp.lang.c . Many common questions are answered in frequently- > asked questions postings in such groups as comp.unix.questions and > comp.os.msdos.programmer . Note that the answers are often not > unique even across different versions of Unix. Bear in mind when > answering system-specific questions that the answer that applies to > your system may not apply to everyone else's. < A: In general, it cannot. If the calling process is prepared to listen < explicitly for some indication that its environment should be < changed, a special-case scheme can be set up. (Under Unix, a child < process cannot directly affect its parent at all. Other operating < systems have different process environments which could < intrinsically support such communication.) --- > A: In general, it cannot. Different operating systems implement > name/value functionality similar to the Unix environment in many > different ways. Whether the "environment" can be usefully altered > by a running program, and if so, how, is entirely system-dependent. > > Under Unix, a process can modify its own environment (Some systems > provide setenv() or putenv() functions to do this), and the modified > environment is passed on to any child processes, but it is not > propagated back to the parent process. (The environment of the > parent process can only be altered if the parent is explicitly set > up to listen for some kind of change requests. The conventional > execution of the BSD "tset" program in .profile and .login files > effects such a scheme.) > 59. How can a file be shortened in-place without completely clearing or > rewriting it? > > A: BSD systems provide ftruncate(), and some MS-DOS compilers supply > chsize(), but there is no portable solution. < slightly depending on whether stdout is a terminal or not. To make < this determination, these implementations perform an operation which < fails (with ENOTTY) if stdout is not a terminal. --- > slightly if stdout is a terminal. To make the determination, these > implementations perform an operation which fails (with ENOTTY) if > stdout is not a terminal. < Or you could use fgets() to read a whole line, and then use sscanf() < or other string functions to parse the line buffer. --- > Usually the best solution is to use fgets() to read a whole line, > and then use sscanf() or other string functions to parse the line > buffer. < A: Just use sprintf. --- > A: Just use sprintf. (You'll have to allocate space for the result > somewhere anyway; see questions 37 and 38.) < Otherwise, you can try anonymous ftp and/or uucp from a central, < public-spirited site, such as uunet.uu.net, but this article cannot < track or list all of the available sites and how to access them. --- > The usual approach is to use anonymous ftp and/or uucp from a > central, public-spirited site, such as uunet.uu.net. However, this > article cannot track or list all of the available sites and how to > access them. < might want to print them. --- > might want to print them. It is hard to imagine why anyone would > want or need to place a comment inside a quoted string. It is easy > to imagine a program needing to print "/*". < A: Several grammars are floating around; keep your eyes open. There is --- > A: The definitive grammar is of course the one in the ANSI standard. > Several copies are floating around; keep your eyes open. There is < 69. Where can I get extra copies of this list? --- > 75. Where can I get extra copies of this list? What about back issues? 1469,1472c1668,1674 < A: For now, just pull it off the net; it is normally posted on about < the first of the month, with an Expiration: line which should keep < it around all month. Eventually, it may be available for anonymous < ftp, or via a mailserver. --- > A: For now, just pull it off the net; it is normally posted on the > first of each month, with an Expiration: line which should keep it > around all month. Eventually, it may be available for anonymous > ftp, or via a mailserver. (Note that the size of the list is > monotonically increasing; older copies are obsolete and don't > contain anything, except the occasional typo, that the current list > doesn't.) > Bibliography > > ANSI American National Standard for Information Systems -- > Programming Language -- C, ANSI X3.159-1989. > > H&S Samuel P. Harbison and Guy L. Steele, C: A Reference Manual, > Second Edition, Prentice-Hall, 1987, ISBN 0-13-109802-0. > > PCS Mark R. Horton, Portable C Software, Prentice Hall, 1990, ISBN > 0-13-868050-7. > > K&R I Brian W. Kernighan and Dennis M. Ritchie, The C Programming > Language, Prentice Hall, 1978, ISBN 0-13-110163-3. > > K&R II Brian W. Kernighan and Dennis M. Ritchie, The C Programming > Language, Second Edition, Prentice Hall, 1988, ISBN 0-13- > 110362-8, 0-13-110370-9. > > CT&P Andrew Koenig, C Traps and Pitfalls, Addison-Wesley, 1989, ISBN > 0-201-17928-8. > > There is a more extensive bibliography in the revised Indian Hill style > guide; see question 74. < Thanks to Mark Brader, Joe Buehler, Christopher Calabrese, Stephen M. < Dunn, Tony Hansen, Guy Harris, Karl Heuer, Blair Houghton, Kirk Johnson, < Andrew Koenig, John Lauro, Christopher Lott, Rich Salz, Joshua Simons, < and Erik Talvola, who have contributed, directly or indirectly, to this < article. --- > Thanks to Mark Brader, Joe Buehler, rayc, Christopher Calabrese, Ray > Dunn, Stephen M. Dunn, Bjorn Engsig, Doug Gwyn, Tony Hansen, Joe > Harrington, Guy Harris, Karl Heuer, Blair Houghton, Kirk Johnson, Andrew > Koenig, John Lauro, Christopher Lott, Evan Manning, Mark Moraes Francois > Pinard, randall@virginia, Rich Salz, Joshua Simons, Henry Spencer, Erik > Talvola, Chris Torek, and Freek Wiedijk, who have contributed, directly > or indirectly, to this article.
scs@adam.mit.edu (Steve Summit) (12/01/90)
This article contains changes between the previous posting of the frequently-asked questions list (November 1) and the new one. (Do _not_ worry if you have not seen the new one yet; it's coming up next.) (These diffs have been edited for readability and are not suitable for the patch program.) < [Last modified 10/31/90 by scs.] --- > [Last modified 11/30/90 by scs.] ========== < (On some machines the internal value is 0; on others it is not.) --- > (On some machines the internal value is all-bits-0; on others it is > not.) ========== < which may be different for different pointer types. The actual < values should be of concern only to compiler writers. --- > which may or may not be all-bits-0 and which may be different > for different pointer types. The actual values should be of > concern only to compiler writers. ========== < confuses you, don't use it; many people have concluded that the --- > bothers you, don't use it; many people have concluded that the ========== < A: Much of the confusion surrounding pointers in C can be traced to < this statement. --- > A: Much of the confusion surrounding pointers in C can be traced to a > misunderstanding of this statement. ========== < multidimensional arrays. (See the question above.) --- > multidimensional arrays, if at all. (See question 18 above.) When > people say "pointer to array" casually, they usually mean "pointer > to array's first element," which is the more useful type. ========== < resulting "ragged" array often saves space, although it is not --- > resulting "ragged" array can save space, although it is not ========== < accessing array[i, j] with array[i * ncolumns + j]. --- > accessing the i,jth element with array[i * ncolumns + j]. ========== < The ANSI C standard declares that code which contains such ambiguous < or undefined side effects is not merely undefined, but illegal. --- > The behavior of code which contains such ambiguous or undefined side > effects has always been undefined. ========== < committee, X3J11, to standardize the C language. After a long and < arduous process, the committee's work was finally ratified as an --- > committee, X3J11, to standardize the C language. After a long, > arduous process, including several widespread public reviews, the > committee's work was finally ratified as an American National ========== > 27. Why does the ANSI Standard not guarantee more than six monocase > characters for external identifier significance? > > A: The main problem is older linkers which are neither under the > control of the ANSI standard nor the C compiler developers on the > systems which have them. The limitation is only that identifiers be > _significant_ in the first six characters, not that they be > restricted to six characters in length. This limitation is > annoying, but certainly not unbearable, and is marked in the > Standard as "obsolescent," i.e. a future revision will likely relax > it. > > This concession to current, restrictive linkers really had to be > made, no matter how vehemently some people oppose it. (The > Rationale notes that its retention was "most painful.") If you > disagree, or have thought of a trick by which a compiler burdened > with a restrictive linker could present the C programmer with the > appearance of more significance in external identifiers, read the > excellently-worded X3.159 Rationale, which discusses several such > schemes and describes why they can't be mandated. > > References: ANSI Sec. 3.1.2 p. 21, Sec. 3.9.1 p. 96, Rationale Sec. > 3.1.2 pp. 19-21. ========== < a single statement, but with a resultant an extra semicolon) as the --- > a single statement, but with a resultant extra semicolon) as the if ========== < The best solution is to use --- > The traditional solution is to use ========== < expressions, a simpler technique is to separate them with commas and < surround them with parentheses. --- > expressions, with no declarations, another technique is to separate > them with commas and surround them with parentheses. ========== < little reason to do so, since portable mechanisms exist.) --- > little reason to do so, since portable mechanisms exist. If you > know how to access arguments "by hand," but have access to neither > <stdarg.h> nor <varargs.h>, you could as easily implement one of > them, leaving your code portable.) ========== > Q: But the man page for strcat said that it took two char *'s as > arguments. How was I supposed to know to allocate things? > > A: In general, when using pointers you _always_ have to worry about > memory allocation, at least to make sure that the compiler is doing > it for you. > > The Synopsis section at the top of a Unix-style man page is often > misleading. The code fragments presented there are closer to the > function definition used by the call's implementor than the > invocation used by the caller. In particular, many routines accept > pointers (e.g. to strings or structs), yet the caller usually passes > the address of some object (an array, or an entire struct). Another > common example is stat(). ========== < required. (Pointers to functions are often chosen precisely to --- > required. (Pointers to structures are often chosen precisely to ========== < returns, although this made struct-valued function nonreentrant, --- > returns, although this made struct-valued functions nonreentrant, ========== > 46. I came across some code that declared a structure like this: > > struct name > { > int namelen; > char name[1]; > }; > > and then did some tricky allocation to make the name array act like > it had several elements. Is this legal and/or portable? > > A: This trick is popular, although Dennis Ritchie has called it > "unwarranted chumminess with the compiler." It is surprisingly > difficult to determine whether the ANSI C standard allows or > disallows it, but it is hard to imagine a compiler or architecture > for which it wouldn't work. ========== < being called (i.e. is not followed by a "("), its address is < implicitly taken, just as is done for arrays. --- > being called (i.e. is not followed by a "("), it "decays" into a > pointer (i.e. its address is implicitly taken), analagously to the > implicit decay of an array into a pointer to its first element. ========== > 54. I've seen different methods used for calling through functions to > pointers. Wht's the story? > > A: Originally, a pointer to a function had to be "turned into" a "real" > function, with the * operator (and an extra pair of parentheses, to > keep the precedence straight), before calling: > > int r, f(), (*fp)() = f; > r = (*fp)(); > > Another argument says that functions are always called through > pointers, but that "real" functions decay implicitly into pointers > and so cause no trouble. This argument, which was adopted by the > ANSI standard, means that > > r = fp(); > > is legal and works correctly (it has always been unambiguous; > there's nothing you ever could have done with a function pointer > except call through it). The explicit * is harmless, and still > allowed (and recommended, if portability to older compilers is > important). > > References: ANSI Sec. 3.3.2.2 . ========== < will succeed (if a, in fact, equals b and TRUE is 1), but this code < is obviously silly. --- > will work as expected (as long as TRUE is 1), but it is obviously > silly. ========== > somewhere anyway; see questions 38 and 39. Don't worry that sprintf > may be overkill, potentially wasting run time or code space; it > works well in practice.) ========== > 69. How can I write data files which can be read on other machines with > different word size, byte order, or floating point formats? > > A: The best solution is to use a text file (usually ASCII), written > with fprintf and read with fscanf or the like. Be very skeptical of > arguments that text files are too big, or that reading and writing > them is too slow. Not only is their efficiency frequently adequate > in practice, but the advantages of being able to manipulate them > with standard tools can be overwhelming. > > If you must use a binary format, you can improve portability, and > perhaps take advantage of prewritten I/O libraries, by making use of > standardized formats such as Sun's XDR or OSI's ASN.1 . ========== < the techniques for passing arguments correctly are often arcane. --- > the techniques for passing arguments and ensuring correct run-time > startup are often arcane. ========== > 75. My floating-point calculations are acting strangely and giving me > different answers on different machines. > > A: Most digital computers use floating-point formats which provide a > close but by no means exact simulation of real number arithmetic. > Among other things, the associative and distributive laws do not > hold exactly (that is, order of calculation may be important, and > repeated addition is not necessarily equivalent to multiplication). > > Don't assume that floating-point results will be exact, and > especially don't assume that floating-point values can be compared > for equality. (Don't stick random "fuzz factors" in, either.) > > These problems are no worse for C than they are for any other > language. Languages usually define floating-point semantics as > "however the processor does them;" otherwise a compiler for a > machine without the "right" model would have to do prohibitively > expensive emulations. > > This article cannot begin to list the pitfalls associated with, and > workarounds appropriate for, floating-point work. A good > programming text should cover the basics. (Beware that subtle > problems can occupy numerical analysts for years.) > > References: K&P Sec. 6 pp. 115-8. ========== > 79. What's the best style for code layout in C? > > A: K&R, while providing the example most often copied, also supply a > good excuse for avoiding it: > > The position of braces is less important; we have > chosen one of several popular styles. Pick a style > that suits you, then use it consistently. > > It is more important that the layout chosen be consistent (with > itself, and with nearby or common code) than that it be "perfect." > If your coding environment (i.e. co-workers or company policy) does > not suggest a style, and you don't feel like inventing your own, > just copy K&R. (The tradeoffs between various indenting and brace > placement options can be exhaustively and minutely examined, but > don't warrant repetition here.) > > Reference: K&R I Sec. 1.2 p. 10. ========== > 81. How do you pronounce "char"? What's that funny name for the "#" > character? > > A: You can make "char" rhyme with "far" or "bear;" the choice is > arbitrary. Bell Labs once proposed the (now obsolete) term > "octothorpe" for the "#" character. > > Trivia questions like these aren't any more pertinent for > comp.lang.c than they are for any of the other groups they > frequently come up in. The "jargon file" (also published as _The > Hacker's Dictionary_), contains lots of tidbits like these, as does > the official Usenet ASCII pronunciation list, maintained by Maarten > Litmaath. (The pronunciation list also appears in the jargon file > under ASCII, as well as in the comp.unix frequently-asked questions > list.) ========== > Thanks to Mark Brader, Joe Buehler, Raymond Chen, Christopher Calabrese, > Norm Diamond, Ray Dunn, Stephen M. Dunn, Bjorn Engsig, Doug Gwyn, Tony > Hansen, Joe Harrington, Guy Harris, Karl Heuer, Blair Houghton, Kirk > Johnson, Andrew Koenig, John Lauro, Christopher Lott, Evan Manning, Mark > Moraes, Francois Pinard, randall@virginia, Rich Salz, Paul Sand, > Patricia Shanahan, Joshua Simons, Henry Spencer, Erik Talvola, Clarke > Thatcher, Chris Torek, and Freek Wiedijk, who have contributed, Steve Summit scs@adam.mit.edu scs%adam.mit.edu@mit.edu mit-eddie!adam!scs
scs@adam.mit.edu (Steve Summit) (01/01/91)
This article contains changes between the previous posting of the frequently-asked questions list (November 1) and the new one. (Do _not_ worry if you have not seen the new one yet; it's coming up next.) (These diffs have been edited for readability and are not suitable for the patch program.) ========== < [Last modified 11/30/90 by scs.] --- > [Last modified December 16, 1990 by scs.] ========== < multidimensional arrays, if at all. (See question 18 above.) When < people say "pointer to array" casually, they usually mean "pointer < to array's first element," which is the more useful type. --- > multidimensional arrays, if at all. (See question 18 above.) When > people speak casually of a pointer to an array, they usually mean a > pointer to its first element; the type of this latter pointer is > generally more useful. ========== < The behavior of code which contains such ambiguous or undefined side < effects has always been undefined. Don't even try to find out how --- > The behavior of code which contains ambiguous or undefined side > effects has always been undefined. (Note, too, that a compiler's > choice, especially under ANSI rules, for "undefined behavior" may be > to refuse to compile the code.) Don't even try to find out how your ========== > The published Standard includes a "Rationale," which explains many > of its decisions, and discusses a number of subtle points, including > several of those covered here. (The Rationale is "not part of ANSI > Standard X3.159-1989, but is included for information only.") > > The Standard has also been adopted as ISO/IEC 9899:1990, although > the Rationale is currently not included. ========== < A: The main problem is older linkers which are neither under the --- > A: The problem is older linkers which are neither under the control of ========== < appearance of more significance in external identifiers, read the < excellently-worded X3.159 Rationale, which discusses several such < schemes and describes why they can't be mandated. --- > appearance of more significance in external identifiers, read the > excellently-worded X3.159 Rationale, (see question 24) which > discusses several such schemes and explains why they couldn't be > mandated. ========== < call it with a double set of parentheses, which appear to the < compiler to indicate a single argument: --- > call it with a double set of parentheses, which appear to the > preprocessor to indicate a single argument: ========== < #include <stdio.h> < main() < { < char answer[100]; < printf("Type something:\n"); < fgets(answer, 100, stdin); < printf("You typed \"%s\"\n", answer); < } --- > #include <stdio.h> > #include <string.h> > main() > { > char answer[100], *p; > printf("Type something:\n"); > fgets(answer, 100, stdin); > if((p = strchr(answer, '\n')) != NULL) > *p = '\0'; > printf("You typed \"%s\"\n", answer); > } ========== < overly-long line. (Unfortunately, gets and fgets differ in their < treatment of the trailing \n.) It would also be possible to use --- > overly-long line. (Unfortunately, fgets does not automatically > delete the trailing \n, as gets would.) It would also be possible ========== < Q: But the man page for strcat said that it took two char *'s as < arguments. How was I supposed to know to allocate things? --- > 40. But the man page for strcat says that it takes two char *'s as > arguments. How am I supposed to know to allocate things? ========== < A: In general, when using pointers you _always_ have to worry about < memory allocation, at least to make sure that the compiler is doing --- > A: In general, when using pointers you _always_ have to consider memory > allocation, at least to make sure that the compiler is doing it for ========== < The Synopsis section at the top of a Unix-style man page is often < misleading. The code fragments presented there are closer to the --- > The Synopsis section at the top of a Unix-style man page can be > misleading. The code fragments presented there are closer to the ========== < disallows it, but it is hard to imagine a compiler or architecture < for which it wouldn't work. --- > disallows it, but it is hard to imagine a compiler or architecture > for which it wouldn't work. (Debugging, array-bounds-checking > compilers might issue warnings.) ========== < 54. I've seen different methods used for calling through functions to < pointers. Wht's the story? --- > 55. I've seen different methods used for calling through pointers to > functions. What's the story? ========== < Another argument says that functions are always called through < pointers, but that "real" functions decay implicitly into pointers < and so cause no trouble. This argument, which was adopted by the < ANSI standard, means that --- > Another analysis holds that functions are always called through > pointers, but that "real" functions decay implicitly into pointers > (in expressions, as they do in initializations) and so cause no > trouble. This reasoning, which was adopted in the ANSI standard, ========== < is legal and works correctly (it has always been unambiguous; < there's nothing you ever could have done with a function pointer < except call through it). The explicit * is harmless, and still < allowed (and recommended, if portability to older compilers is --- > is legal and works correctly, whether fp is a function or a pointer > to one. (The usage has always been unambiguous; there is nothing > you ever could have done with a function pointer followed by an > argument list except call through it). An explicit * is harmless, > and still allowed (and recommended, if portability to older ========== < may also be able to use "nonblocking I/O", or a system call named < "select", or the FIONREAD ioctl, or O_NDELAY, or a kbhit() routine. --- > may also be able to use "nonblocking I/O", or a system call named > "select", or the FIONREAD ioctl, or the O_NDELAY option to open() or > fcntl(), or a kbhit() routine. ========== < A: BSD systems provide ftruncate(), and some MS-DOS compilers supply < chsize(), but there is no portable solution. --- > A: BSD systems provide ftruncate(), and some PC compilers supply > chsize(), but there is no portable solution. ========== < with fprintf and read with fscanf or the like. Be very skeptical of < arguments that text files are too big, or that reading and writing < them is too slow. Not only is their efficiency frequently adequate --- > with fprintf and read with fscanf or the like. Be very skeptical of > arguments which imply that text files are too big, or that reading > and writing them is too slow. Not only is their efficiency > frequently acceptable in practice, but the advantages of being able ========== < perhaps take advantage of prewritten I/O libraries, by making use of < standardized formats such as Sun's XDR or OSI's ASN.1 . --- > perhaps take advantage of prewritten I/O libraries, by making use of > standardized formats such as Sun's XDR, OSI's ASN.1, or CCITT's > X.409 . ========== < central, public-spirited site, such as uunet.uu.net. However, this < article cannot track or list all of the available sites and how to < access them. --- > central, public-spirited site, such as uunet.uu.net. However, this > article cannot track or list all of the available archive sites and > how to access them. The comp.archives newsgroup contains numerous > announcements of anonymous ftp availability of various items. ========== < Among other things, the associative and distributive laws do not < hold exactly (that is, order of calculation may be important, and < repeated addition is not necessarily equivalent to multiplication). --- > Among other things, the associative and distributive laws do not > hold completely (i.e. order of operation may be important, repeated > addition is not necessarily equivalent to multiplication, and > underflow or cumulative precision loss is often a problem). ========== < especially don't assume that floating-point values can be compared < for equality. (Don't stick random "fuzz factors" in, either.) --- > especially don't assume that floating-point values can be compared > for equality. (Don't stick in random "fuzz factors," either.) ========== < These problems are no worse for C than they are for any other < language. Languages usually define floating-point semantics as < "however the processor does them;" otherwise a compiler for a --- > These problems are no worse for C than they are for any other > computer language. Floating-point semantics are usually defined as > "however the processor does them;" otherwise a compiler for a ========== < If your coding environment (i.e. co-workers or company policy) does < not suggest a style, and you don't feel like inventing your own, < just copy K&R. (The tradeoffs between various indenting and brace < placement options can be exhaustively and minutely examined, but < don't warrant repetition here.) --- > If your coding environment (i.e. local custom or company policy) > does not suggest a style, and you don't feel like inventing your > own, just copy K&R. (The tradeoffs between various indenting and > brace placement options can be exhaustively and minutely examined, > but don't warrant repetition here. See also the Indian Hill Style > Guide.) ========== < A: Various standards are available for anonymous ftp from: --- > A: Various documents are available for anonymous ftp from: ========== < A: You can make "char" rhyme with "far" or "bear;" the choice is < arbitrary. Bell Labs once proposed the (now obsolete) term --- > A: You can pronounce the C keyword "char" like the English words > "char," "care," or "car;" the choice is arbitrary. Bell Labs once ========== > Thanks to Sudheer Apte, Mark Brader, Joe Buehler, Raymond Chen, > Christopher Calabrese, Norm Diamond, Ray Dunn, Stephen M. Dunn, Bjorn > Engsig, Doug Gwyn, Tony Hansen, Joe Harrington, Guy Harris, Karl Heuer, > Blair Houghton, Kirk Johnson, Andrew Koenig, John Lauro, Christopher > Lott, Evan Manning, Mark Moraes, Francois Pinard, randall@virginia, Rich > Salz, Paul Sand, Patricia Shanahan, Joshua Simons, Henry Spencer, Erik > Talvola, Clarke Thatcher, Chris Torek, Ed Vielmetti, and Freek Wiedijk, > who have contributed, directly or indirectly, to this article. Steve Summit scs@adam.mit.edu scs%adam.mit.edu@mit.edu mit-eddie!adam!scs
scs@adam.mit.edu (Steve Summit) (02/06/91)
This article contains changes between the previous posting of the frequently-asked questions list (posted January 1) and the new one. (Do _not_ worry if you have not seen the new one yet; it's coming up next.) (These diffs have been edited for readability and are not suitable for the patch program.) By the way, sorry for the delay in this month's FAQ postings: I was busy traveling and getting engaged. ========== < [Last modified December 16, 1990 by scs.] --- > [Last modified February 5, 1990 by scs.] ========== > 18. I came across some "joke" code containing the "expression" > 5["abcdef"] . How can this be legal C? > > A: Yes, Virginia, array subscripting is commutative in C. This curious > fact follows from the pointer definition of array subscripting, > namely that a[e] is exactly equivalent to *((a)+(e)), for _any_ > expression e and primary expression a, as long as one of them is a > pointer expression. This unsuspected commutativity is often > mentioned in C texts as if it were something to be proud of, but it > finds no useful application outside of the Obfuscated C Contest (see > also question 83). ========== < The Standard has also been adopted as ISO/IEC 9899:1990, although < the Rationale is currently not included. --- > The Standard has also been adopted as an international standard, > ISO/IEC 9899:1990, although the Rationale is currently not included. ========== > Section 7. Lint > > 39. I just typed in this program, and it's acting strangely. Can you > see anything wrong with it? > > A: Try running lint first. Most C compilers are really only half- > compilers, electing not to diagnose numerous source code > difficulties which would not actively preclude code generation. > That the "other half," better error detection, was deferred to lint, > was a fairly deliberate decision on the part of the earliest Unix C > compiler authors, but is inexcusable (in the absence of a supplied, > consistent lint) in a modern compiler. > > 40. How can I shut off the "warning: possible pointer alignment problem" > message lint gives me for each call to malloc? > > A: The problem is that traditional versions of lint do not know, and > cannot be told, that malloc "returns a pointer to space suitably > aligned for storage of any type of object." It is possible to > provide a pseudoimplementation of malloc, using a #define inside of > #ifdef lint, which effectively shuts this warning off, but a > simpleminded #definition will also suppress meaningful messages > about truly incorrect invocations. It may be easier simply to > ignore the message, perhaps in an automated way with grep -v. > > 41. Where can I get an ANSI-compatible lint? > > A: A product called FlexeLint is available (in "shrouded source form," > for compilation on 'most any system) from > > Gimpel Software > 3207 Hogarth Lane > Collegeville, PA 19426 USA > (+1) 215 584 4261 > > Another product is MKS lint, from Mortice Kern Systems. At the > moment, I don't have their address, but you can send email to > inquiry@mks.com . > > 42. Don't ANSI function prototypes render lint obsolete? > > A: No. First of all, prototypes work well only if the programmer works > assiduously to maintain them, and the effort to do so (plus the > extra recompilations required by numerous, more-frequently-modified > header files) can rival the toil of keeping function calls correct > manually. Secondly, an independent program like lint will probably > always be more scrupulous at enforcing compatible, portable coding > practices than will a particular, implementation-specific, feature- > and extension-laden compiler. (Some vendors seem to introduce > incompatible extensions deliberately, perhaps to lock in market > share.) ========== alloca cannot be written portably, and is difficult to implement on machines without a stack. Its use is problematical (and the obvious implementation on a stack-based machine fails) when its return value is passed directly to another function, as in < fgets(alloca(100), stdin, 100). --- > fgets(alloca(100), 100, stdin). ========== A: This trick is popular, although Dennis Ritchie has called it < "unwarranted chumminess with the compiler." It is surprisingly < difficult to determine whether the ANSI C standard allows or < disallows it, but it is hard to imagine a compiler or architecture < for which it wouldn't work. (Debugging, array-bounds-checking < compilers might issue warnings.) --- > "unwarranted chumminess with the compiler." The ANSI C standard > allows it only implicitly. It seems to be portable to all known > implementations. (Debugging, array-bounds-checking compilers might > issue warnings.) ========== of curses have a nodelay() function. Depending on your system, you may also be able to use "nonblocking I/O", or a system call named < "select", or the FIONREAD ioctl, or the O_NDELAY option to open() or < fcntl(), or a kbhit() routine. --- > "select", or the FIONREAD ioctl, or kbhit(), or rdchk(), or the > O_NDELAY option to open() or fcntl(). ========== < A: BSD systems provide ftruncate(), and some PC compilers supply < chsize(), but there is no portable solution. --- > A: BSD systems provide ftruncate(), and several others supply chsize(), > but there is no truly portable solution. ========== > 73. How can I recover the file name given an open file descriptor? > > A: This problem is, in general, insoluble. Under Unix, for instance, a > scan of the entire disk, (perhaps requiring special permissions) > would be required, and would fail if the file descriptor were a pipe > (and could give a misleading answer for a file with multiple links). > It is best to remember the names of open files yourself (perhaps > with a wrapper function around fopen). > > > Section 14. Style > > 74. Here's a neat trick: > > if(!strcmp(s1, s2)) > > Is this good style? > > A: No. This is a classic example of C minimalism carried to an > obnoxious degree. The test succeeds if the two strings are equal, > but its form strongly suggests that it tests for inequality. > > A much better solution is to use a macro: > > #define Streq(s1, s2) (strcmp(s1, s2) == 0) > ========== 79. How can I write data files which can be read on other machines with different word size, byte order, or floating point formats? A: The best solution is to use a text file (usually ASCII), written < with fprintf and read with fscanf or the like. Be very skeptical of < arguments which imply that text files are too big, or that reading < and writing them is too slow. Not only is their efficiency < frequently acceptable in practice, but the advantages of being able < to manipulate them with standard tools can be overwhelming. --- > with fprintf and read with fscanf or the like. (Similar advice also > applies to network protocols.) Be very skeptical of arguments which > imply that text files are too big, or that reading and writing them > is too slow. Not only is their efficiency frequently acceptable in > practice, but the advantages of being able to manipulate them with > standard tools can be overwhelming. > > If the binary format is being imposed on you by an existing program, > first see if you can get that program changed to use a more portable > format. ========== Bellcore, and Carnegie Mellon. To find about f2c, send the mail message "send index from f2c" to < netlib@research.att.com or research!netlib. --- > netlib@research.att.com or research!netlib. (It is > also available via anonymous ftp on > research.att.com, in directory dist/f2c.) ========== > 83. Where can I get the winners of the old Obfuscated C Contests? When > will the next contest be held? > > A: Send mail to {pacbell,uunet,utzoo}!hoptoad!obfuscate . The contest > is usually announced in March, with entries due in May. Contest > announcements are posted in several obvious places. The winning > entries are archived on uunet (see question 82). ========== Don't assume that floating-point results will be exact, and especially don't assume that floating-point values can be compared < for equality. (Don't stick in random "fuzz factors," either.) --- > for equality. (Don't throw haphazard "fuzz factors" in, either.) ========== Thanks to Sudheer Apte, Mark Brader, Joe Buehler, Raymond Chen, > Christopher Calabrese, James Davies, Norm Diamond, Ray Dunn, Stephen M. > Dunn, Bjorn Engsig, Doug Gwyn, Tony Hansen, Joe Harrington, Guy Harris, > Karl Heuer, Blair Houghton, Kirk Johnson, Andrew Koenig, John Lauro, > Christopher Lott, Tim McDaniel, Evan Manning, Mark Moraes, Francois > Pinard, randall@virginia, Rich Salz, Chip Salzenberg, Paul Sand, Doug > Schmidt, Patricia Shanahan, Joshua Simons, Henry Spencer, Erik Talvola, > Clarke Thatcher, Chris Torek, Ed Vielmetti, and Freek Wiedijk, who have > contributed, directly or indirectly, to this article.
scs@adam.mit.edu (Steve Summit) (03/01/91)
This article contains changes between the previous posting of the frequently-asked questions list (posted on February 6) and the new one. (Do _not_ worry if you have not seen the new one yet; it's coming up next.) (These diffs have been edited for readability and are not suitable for the patch program.) ========== < [Last modified February 5, 1991 by scs.] --- > [Last modified February 28, 1991 by scs.] ========== > Besides listing frequently-asked questions, this article also summarizes > frequently-posted answers. Even if you know all the answers, it's worth > skimming through this list once in a while, so that when you see one of > its questions unwittingly posted, you won't have to waste time > answering. ========== discouraged. (Any object pointer may be cast to the "universal" pointer type void *, or char * under a pre-ANSI compiler, when < heterogeneous pointers must be passed around.) --- > heterogeneous pointers must be passed around.) It is no longer > guaranteed that a pointer can be cast to a "suitably capacious" > integer and back, unchanged. ========== < A: As a stylistic convention, many people prefer not to have unadorned < 0's scattered throughout their programs. For this reason, the --- > A: As a matter of style, many people prefer not to have unadorned 0's > scattered throughout their programs. For this reason, the ========== < NULL should _only_ be used for pointers. It should not be used when < another kind of 0 is required, even though it might work, because < doing so sends the wrong stylistic message. (ANSI allows the < #definition of NULL to be (void *)0, which will not work in non- < pointer contexts.) In particular, do not use NULL when the ASCII < null character (NUL) is desired. Provide your own definition < < #define NUL '\0' < < if you must. --- > NULL should _only_ be used for pointers; see question 9. ========== < A: This trick, though popular with beginning programmers, does not buy < much. --- > A: This trick, though popular in some circles, does not buy much. ========== > "Abbreviations" such as if(p), though perfectly legal, are > considered by some to be bad style. ========== < distinguishing pointer 0's from integer 0's. Again, NULL should not < be used for other than pointers. --- > distinguishing pointer 0's from integer 0's. > > NULL should _not_ be used when another kind of 0 is required, even > though it might work, because doing so sends the wrong stylistic > message. (ANSI allows the #definition of NULL to be (void *)0, > which will not work in non-pointer contexts.) In particular, do > not use NULL when the ASCII null character (NUL) is desired. > Provide your own definition > > #define NUL '\0' > > if you must. ========== 11. I once used a compiler that wouldn't work unless NULL was used. < A: That compiler was broken. In general, making decisions about a --- > A: Unless the code being compiled was nonportable (see question 6), > that compiler was probably broken. In general, making decisions ========== < This article always uses the phrase "null pointer" for sense 1, the < character "0" for sense 3, and the capitalized word "NULL" for < sense 4. --- > This article always uses the phrase "null pointer" (in lower case) > for sense 1, the character "0" for sense 3, and the capitalized > word "NULL" for sense 4. ========== > 15. Given all the confusion surrounding null pointers, wouldn't it be > easier simply to require them to be represented internally by > zeroes? > > A: If for no other reason, doing so would be ill-advised because it > would unnecessarily constrain implementations which would otherwise > naturally represent null pointers by special, nonzero bit patterns, > particularly when those values would trigger automatic hardware > traps for invalid accesses. > > Besides, what would this requirement really accomplish? Proper > understanding of null pointers does not require knowledge of the > internal representation, whether zero or nonzero. Assuming that > null pointers are internally zero does not make any code easier to > write (except for a certain ill-advised usage of calloc; see > question 57). Known-zero internal pointers would not obviate casts > in function calls, because the _size_ of the pointer might still be > different from that of an int. (If "nil" were used to request null > pointers rather than "0," as mentioned in question 13, the urge to > assume an internal zero representation would not even arise.) > > 16. Seriously, have any actual machines really used nonzero null > pointers? > > A: "Certain Prime computers use a value different from all- > bits-0 to encode the null pointer. Also, some large > Honeywell-Bull machines use the bit pattern 06000 to encode > the null pointer. On such machines, the assignment of 0 to > a pointer yields the special bit pattern that designates the > null pointer." > > -- Portable C, by H. Rabinowitz and Chaim Schaap, > Prentice-Hall, 1990, page 147. > > The "certain Prime computers" were the segmented 50 series, which > used segment 07777, offset 0 for the null pointer, at least for > PL/I. Later models used segment 0, offset 0 for null pointers in > C, necessitating new instructions such as TCNP (Test C Null > Pointer), evidently as a sop to all the extant poorly-written C > code which made incorrect assumptions. > > The Symbolics Lisp Machine, a tagged architecture, does not even > have conventional numeric pointers; it uses the pair <NIL, 0> > (basically a nonexistent <object, offset> handle) as a C null > pointer. ========== 16. But I heard that char a[] was identical to char *a. < A: This identity (that a pointer declaration is interchangeable with an < array declaration, usually unsized) holds _only_ for formal < parameters to functions. This identity is related to the fact that < arrays "decay" into pointers in expressions. That is, when an array < name is mentioned in an expression, it is converted immediately into < a pointer to the array's first element. Therefore, an array is < never passed to a function; rather a pointer to its first element is < passed instead. Allowing pointer parameters to be declared as < arrays is a simply a way of making it look as though the array was < actually being passed. Some programmers prefer, as a matter of < style, to use this syntax to indicate that the pointer parameter is < expected to point to the start of an array rather than to some < single value. < < Since functions can never receive arrays as parameters, any < parameter declarations which "look like" arrays, e.g. < < f(a) < char a[]; < < are treated as if they were pointers, since that is what the < function will receive if an array is passed: < < f(a) < char *a; < < To repeat, however, this conversion holds only within function < formal parameter declarations, nowhere else. If this conversion < bothers you, don't use it; many people have concluded that the < confusion it causes outweighs the small advantage of having the < declaration "look like" the call and/or the uses within the < function. --- > A: Not at all. (What you heard has to do with formal parameters to > functions; see question 21.) Arrays are not pointers. The > declaration "char a[6];" requests that space for six characters be > set aside, to be known by the name "a." That is, there is a > location named "a" at which six characters can sit. The > declaration "char *p;" on the other hand, requests a place which > holds a pointer. The pointer is to be known by the name "p," and > can point to any char (or contiguous array of chars) anywhere. > > As usual, a picture is worth a thousand words. The statements > > char a[] = "hello"; > char *p = "world"; > char *p2 = a; > > would result in data structures which could be represented like > this: > > +---+---+---+---+---+---+ > a: | h | e | l | l | o |\0 | > +---+---+---+---+---+---+ > ^ > | > +--|--+ > p2: | * | > +-----+ > > +-----+ +---+---+---+---+---+---+ > p: | *======> | w | o | r | l | d |\0 | > +-----+ +---+---+---+---+---+---+ > > 19. You mean that a reference like x[3] generates different code > depending on whether x is an array or a pointer? > > A: Precisely. Referring back to the sample declarations in the > previous question, when the compiler sees the expression a[3], it > emits code to start at the location "a," move three past it, and > fetch the character there. When it sees the expression p[3], it > emits code to start at the location "p," fetch the pointer value > there, add three to the pointer, and finally fetch the character > pointed to. In the example above, both a[3] and p[3] (and p2[3], > for that matter) happen to be the character 'l', but that the > compiler gets there differently. (See also question 100.) ========== 17. So what is meant by the "equivalence of pointers and arrays" in C? < A: Much of the confusion surrounding pointers in C can be traced to a < misunderstanding of this statement. Saying that arrays and pointers < are "equivalent" does not by any means imply that they are < interchangeable. (The fact that, as formal parameters to functions, < array-style and pointer-style declarations are in fact < interchangeable does nothing to reduce the confusion.) < < "Equivalence" refers to the fact (mentioned above) that arrays decay < into pointers within expressions, and that pointers and arrays can < both be dereferenced using array-like subscript notation. That is, < if we have < < char a[10]; < char *p = a; < int i; < < we can refer to a[i] and p[i]. (That pointers can be subscripted < like arrays is hardly surprising, since arrays have decayed into < pointers by the time they are subscripted.) ========== > A: Much of the confusion surrounding pointers in C can be traced to a > misunderstanding of this statement. Saying that arrays and > pointers are "equivalent" does not by any means imply that they are > interchangeable. > > "Equivalence" refers to the following key definition: > > An identifier of type array-of-T which appears in an > expression decays (with three exceptions) into a pointer to > its first element; the type of the resultant pointer is > pointer-to-T. > > (The exceptions are when the array is the operand of the sizeof() > operator or of the & operator, or is a literal string initializer > for a character array.) > > As a consequence of this definition, there is not really any > difference in the behavior of the "array subscripting" operator [] > as it applies to arrays and pointers. In an expression of the form > a[i], the array name "a" decays into a pointer, following the rule > above, and is then subscripted exactly as would be a pointer > variable in the expression p[i]. In either case, the expression > x[i] (where x is an array or a pointer) is, by definition, exactly > equivalent to *((x)+(i)). > > 21. Then why are array and pointer declarations interchangeable as > function formal parameters? > > A: Since arrays decay immediately into pointers, an array is never > actually passed to a function. Allowing pointer parameters to be > declared as arrays is a simply a way of making it look as though > the array was being passed. Some programmers prefer, as a matter > of style, to use this syntax to indicate that the pointer parameter > is expected to point to the start of an array rather than to some > single value. > > Since functions can never receive arrays as parameters, any > parameter declarations which "look like" arrays, e.g. > > f(a) > char a[]; > > are treated by the compiler as if they were pointers, since that is > what the function will receive if an array is passed: > > f(a) > char *a; > > To repeat, however, this conversion holds only within function > formal parameter declarations, nowhere else. If this conversion > bothers you, don't use it; many people have concluded that the > confusion it causes outweighs the small advantage of having the > declaration "look like" the call and/or the uses within the > function. ========== > 22. Someone explained to me that arrays were really just constant > pointers. > > A: That person did you a disservice. An array name is "constant" in > that it cannot be assigned to, but an array is _not_ a pointer, as > the discussion and pictures in question 18 should make clear. ========== A: Yes, Virginia, array subscripting is commutative in C. This curious fact follows from the pointer definition of array subscripting, namely that a[e] is exactly equivalent to *((a)+(e)), for _any_ < expression e and primary expression a, as long as one of them is a < pointer expression. This unsuspected commutativity is often --- > for _any_ expression e and primary expression a, as long as one of > them is a pointer expression and one is integral. This unsuspected ========== < Pointers to arrays are confusing, and it is best to avoid them. --- > Pointers to arrays can be confusing, and must be treated carefully. ========== In the first declaration, the compiler performs the usual implicit rewriting of "array of array" to "pointer to array;" in the second < form the pointer declaration is explicit. The called function does < not care how big the array is, but it must know its shape, so the < "column" dimension XSIZE must be included. In both cases the number < of "rows" is irrelevant, and omitted. --- > the second form the pointer declaration is explicit. Since the > called function does not allocate space for the array, it does not > need to know the overall size, so the number of "rows," YSIZE, can > be omitted. The "shape" of the array is still important, so the > "column" dimension XSIZE (and, for 3- or more dimensional arrays, > the intervening ones) must be included. ========== The order of other embedded side effects is similarly undefined. < For example, the expression i + (i = 2) may or may not have the < value 4. --- > For example, the expression i + (i = 2) does not necessarily yield > 4. ========== > The Rationale, by itself, has been printed by Silicon Press, ISBN > 0-929306-07-4. ========== 26. Does anyone have a tool for converting old-style C programs to ANSI C, or for automatically generating prototypes? < A: There are several such programs, many in the public domain. Check < your nearest comp.sources archive. --- > A: Two programs, protoize and unprotoize, are being written to convert > back and forth between prototyped and "old style" function > definitions and declarations. (These programs are _not_ expected > to handle full-blown conversion between "Classic" C and ANSI C.) > When available, these programs will exist as patches to the FSF GNU > C compiler, gcc. > > Several prototype generators exist, many as modifications to lint. ========== > 32. What's the difference between "char const *p" and "char * const p"? > > A: "char const *p" is a pointer to a constant character (you can't > change the character); "char * const p" is a constant pointer to a > (variable) character (i.e. you can't change the pointer). (Read > these "inside out" to understand them. See question 69.) ========== Old C (and ANSI C, in the absence of prototypes) silently promotes floats to doubles when passing them as arguments, < and makes a corresponding silent change to formal parameter < declarations, so the old-style definition actually says that func < takes a double. --- > as arguments, and arranges that doubles being passed are coerced > back to floats if the formal parameters are declared that way. ========== (In this case, it would be clearest to change the old-style < definition to use double as well). --- > definition to use double as well, as long as the address of that > parameter is not taken.) ========== > 36. What was noalias and what ever happened to it? > > A: noalias was another type qualifier, in the same syntactic class as > const and volatile, which was intended to assert that the object > pointed to was not also pointed to ("aliased") by other pointers. > The primary application, which is an important one, would have been > for the formal parameters of subroutines designed to perform > computations on large arrays. A compiler can not usually take > advantage of vectorization or other parallelization hardware (on > supercomputers which have it) unless it can ensure that the source > and destination arrays do not overlap. > > The noalias keyword was not backed up by any "prior art," and it > was introduced late in the review and approval process. It was > phenomenally difficult to define precisely and explain coherently, > and sparked widespread, acrimonious debate, including a scathing > pan by Dennis Ritchie. It had far-ranging implications, > particularly on several standard library interfaces, for which easy > fixes were not readily apparent. > > Because of the criticism and the difficulty of defining noalias > well, the Committee wisely declined to adopt it, in spite of its > superficial attractions. (When writing a standard, features cannot > be introduced halfway; their full integration, and all > implications, must be understood.) The need for a mechanism to > support parallel implementation of non-overlapping operations > remains unfilled (although the C Numerical Extensions Working Group > is examining the problem). > > References: ANSI Sec. 3.9.6 . > > 37. What are #pragmas and what are they good for? > > A: The #pragma directive (based on a similar feature in Ada, of all > things) provides a single, well-defined "escape hatch" which can be > used for all sorts of implementation-specific controls and > extensions: source listing control, structure packing, warning > suppression (like the old lint /* NOTREACHED */ comments), etc. > > References: ANSI Sec. 3.8.6 . ========== If all of the statements in the intended macro are simple < expressions, with no declarations, another technique is to separate < them with commas and surround them with parentheses. --- > expressions, with no declarations, conditionals, or loops, another > technique is to write a single, parenthesized expression using one > or more comma operators. (This technique also allows a value to be > "returned.") ========== 34. How can I write a function that takes a variable number of arguments? < A: Use varargs or stdarg. --- > A: Use the <stdarg.h> header (or, if you must, the older <varargs.h>). ========== If you know how to access arguments "by hand," but have access to neither < <stdarg.h> nor <varargs.h>, you could as easily implement one of < them yourself, leaving your code portable.) --- > <stdarg.h> nor <varargs.h>, you could as easily implement > <stdarg.h> yourself, leaving your code portable.) ========== < To use varargs, instead of stdarg, change the function header to: --- > To use the older <varargs.h> package, instead of <stdarg.h>, change > the function header to: ========== That the "other half," better error detection, was deferred to lint, was a fairly deliberate decision on the part of the earliest Unix C compiler authors, but is inexcusable (in the absence of a supplied, < consistent lint) in a modern compiler. --- > supplied, consistent lint, or equivalent error checking) in a > modern compiler. ========== > The System V release 4 lint is ANSI-compatible, and is available > separately (bundled with other C tools) from Unix Support Labs (a > subsidiary of AT&T), or from System V resellers. ========== 42. Don't ANSI function prototypes render lint obsolete? < A: No. First of all, prototypes work well only if the programmer works --- > A: Not really. First of all, prototypes work well only if the ========== A: Again, the problem is that space for the concatenated result is not < properly allocated. C does not provide a true string type. C < programmers use char *'s for strings, but must always keep < allocation in mind. The compiler will only allocate memory for < objects explicitly mentioned in the source code (in the case of < "strings," this includes character arrays and string literals). --- > properly allocated. C does not provide an automatically-managed > string type. C compilers only allocate memory for objects > explicitly mentioned in the source code (in the case of "strings," ========== < The simple strcat example could be fixed with something like --- > strcat performs no allocation; the second string is appended to the > first one, in place. Therefore, one fix would be to declare the > first string as an array with sufficient space: ========== < Note, however, that strcat appends the string pointed to by its < second argument to that pointed to by the first, and merely returns < its first argument, so the s3 variable is superfluous. --- > Since strcat returns its first argument, the s3 variable is > superfluous. ========== > 55. How does free() know how many bytes to free? > > A: The malloc/free package remembers the size of each block it > allocates and returns, so it is not necessary to remind it of the > size when freeing. > > 56. Is it legal to pass a null pointer as the first argument to > realloc()? Why would you want to? > > A: ANSI C sanctions this usage (and the related realloc(..., 0), which > frees), but several earlier implementations do not support it, so > it is not widely portable. Passing an initially-null pointer to > realloc can make it easy to write a self-starting incremental > allocation algorithm. > > References: ANSI Sec. 4.10.3.4 . > > 57. What is the difference between calloc and malloc? Is it safe to > use calloc's zero-fill guarantee for pointer and floating-point > values? Does free work on memory allocated with calloc, or do you > need a cfree? > > A: calloc(m, n) is essentially equivalent to > > p = malloc(m * n); > memset(p, 0, m * n); > > The zero fill is all-bits-zero, and does not therefore guarantee > useful zero values for pointers (see questions 1-16) or floating- > point values. free can (and should) be used to free the memory > allocated by calloc. > > References: ANSI Secs. 4.10.3 to 4.10.3.2 . ========== Structures are typically returned from functions in a location < pointed to by an extra, "hidden" argument to the function. Older --- > pointed to by an extra, compiler-supplied "hidden" argument to the ========== A: A missing semicolon causes the compiler to believe that main returns a struct list. (The connection is hard to see because of the < intervening comment.) When struct-valued functions are implemented < by adding a hidden return pointer, the generated code tries to store < a struct with respect to a pointer which was not actually passed (in < this case, by the C start-up code). Attempting to store a structure < into memory pointed to by the argc or argv value on the stack (where < the compiler expected to find the hidden return pointer) causes the < core dump. --- > the intervening comment.) Since struct-valued functions are > usually implemented by adding a hidden return pointer, the > generated code for main() actually expects three arguments, > although only two were passed (in this case, by the C start-up > code). See also question 103. ========== 53. How can I determine the byte offset of a field within a structure? A: ANSI C defines the offsetof macro, which should be used if < available. If you don't have it, a suggested implementation is --- > available; see <stddef.h>. If you don't have it, a suggested ========== > 66. How do you decide which integer type to use? > > A: If you might need large values (above 32767 or below -32767), use > long. If space is very important (there are large arrays or many > structures), use short. Otherwise, use int. If well-defined > overflow characteristics are important and/or sign is not, use > unsigned. > > Similar arguments operate when deciding between float and double. > Exceptions apply if the address of a variable is taken and must > have a particular type. > > In general, don't try to use char or unsigned char as a "tiny" int > type; doing so is often more trouble than it's worth. ========== < Any good book on C should explain techniques for reading these < complicated C declarations "inside out" to understand them --- > Any good book on C should explain how to read these complicated C > declarations "inside out" to understand them ("declaration mimics ========== When the name of a function appears in an expression but is not being called (i.e. is not followed by a "("), it "decays" into a < pointer (i.e. its address is implicitly taken), analagously to the < implicit decay of an array into a pointer to its first element. --- > pointer (i.e. it has its address implicitly taken), much as an > array name does. ========== < The advantages of enums are that the numeric values are < automatically assigned, that a debugger may be able to display the < symbolic values when enum variables are examined, and that a < compiler may generate nonfatal warnings when enums and ints are < indiscriminately mixed (such mixing can still be considered bad < style even though it is not strictly illegal). --- > The primary advantages of enums are that the numeric values are > automatically assigned, and that a debugger may be able to display > the symbolic values when enum variables are examined. (A compiler > may also generate nonfatal warnings when enums and ints are > indiscriminately mixed, since doing so can still be considered bad > style even though it is not strictly illegal). A disadvantage is > that the programmer has little control over the size. ========== 66. How can my program discover the complete pathname to the executable file from which it was invoked? < A: Depending on the operating system, argv[0] may contain all or part < of the pathname. (It may also contain nothing.) You may be able to < duplicate the command language interpreter's search path logic to < locate the executable if the name in argv[0] is incomplete. < However, there is no guaranteed or portable solution. --- > A: argv[0] may contain all or part of the pathname, or it may contain > nothing. You may be able to duplicate the command language > interpreter's search path logic to locate the executable if the > name in argv[0] is present but incomplete. However, there is no > guaranteed or portable solution. ========== 67. How can a process change an environment variable in its caller? Under Unix, a process can modify its own environment (some systems < provide setenv() or putenv() functions to do this), and the modified < environment is passed on to any child processes, but it is _not_ --- > provide setenv() and/or putenv() functions to do this), and the > modified environment is usually passed on to any child processes, ========== < A: It is best to use an explicit fflush(stdout) at any point within < your program at which output should definitely be visible. Several --- > A: It is best to use an explicit fflush(stdout) whenever output should > definitely be visible. Several mechanisms attempt to perform the ========== < The position of braces is less important; we have < chosen one of several popular styles. Pick a style < that suits you, then use it consistently. --- > The position of braces is less important, although > people hold passionate beliefs. We have chosen one > of several popular styles. Pick a style that suits > you, then use it consistently. > > Reference: K&R Sec. 1.2 p. 10. ========== > 89. What can I safely assume about the initial values of variables > which are not explicitly initialized? If global variables start > out as "zero," is that good enough for null pointers and floating- > point zeroes? > > A: Variables (and arrays) with "static" duration (that is, those > declared outside of functions, and those declared with the storage > class static), are guaranteed initialized to zero, as if the > programmer had typed "= 0". Therefore, such variables are > initialized to the null pointer (of the correct type) if they are > pointers, and to 0.0 if they are floating-point. This requirement > means that compilers and linkers on machines which use nonzero > internal representations for null pointers and/or floating-point > zeroes cannot necessarily make use of uninitialized, 0-filled > memory, but must emit explicit initializers for these values > (rather as if the programmer had). > > Variables with "automatic" duration (i.e. local variables without > the static storage class) start out containing garbage, unless they > are explicitly initialized. Nothing useful can be predicted about > the garbage. > > Dynamically-allocated memory obtained with malloc and realloc is > also likely to contain garbage, and must be initialized by the > calling program, as appropriate. Memory obtained with calloc > contains all-bits-0, but this is not necessarily useful for pointer > or floating-point values (see question 57). ========== printable string. How can I perform the inverse operations of converting a struct tm or a string into a time_t? Converting a string to a time_t is harder, because of the wide variety of date and time formats which should be parsed. Public- domain routines have been written for performing this function, as < well, but they are less likely to become standardized. --- > well (see, for example, the file partime.c, widely distributed with > the RCS package), but they are less likely to become standardized. ========== The usual approach is to use anonymous ftp and/or uucp from a < central, public-spirited site, such as uunet.uu.net. However, this --- > central, public-spirited site, such as uunet.uu.net (192.48.96.2). ========== > various items. The "archie" mailserver can tell you which > anonymous ftp sites have which packages; send the mail message > "help" to archie@quiche.cs.mcgill.ca for information. ========== > 99. How can I make this code more efficient? > > A: Efficiency, though a favorite comp.lang.c topic, is not important > nearly as often as people tend to think it is. Most of the code in > most programs is not time-critical. When code is not time- > critical, it is far more important that it be written clearly and > portably than that it be written maximally efficiently. (Remember > that computers are very, very fast, and that even "inefficient" > code can run without apparent delay.) > > It is notoriously difficult to predict what the "hot spots" in a > program will be. When efficiency is a concern, it is important to > use profiling software to determine which parts of the program > deserve attention. Often, actual computation time is swamped by > peripheral tasks such as I/O and memory allocation, which can be > sped up by using buffering and cacheing techniques. > > For the small fraction of code that is time-critical, it is vital > to pick a good algorithm; it is much less important to > "microoptimize" the coding details. Source-level optimizations > rarely make significant improvements, and often render code opaque. > Many of the "efficient coding tricks" which are frequently > suggested (e.g. substituting shift operators for multiplication by > powers of two) are performed automatically by even simpleminded > compilers. Heavyhanded "optimization" attempts can make code so > bulky that performance is degraded. If the performance of your > code is so important that you are willing to invest programming > time in source-level optimizations, you would be better served by > buying the best optimizing compiler you can afford (compilers can > perform optimizations that are impossible at the source level). > > It is not the intent here to suggest that efficiency can be > completely ignored. Most of the time, however, by simply paying > attention to good algorithm choices, implementing them clearly and > obviously, and avoiding obviously inefficient blunders (i.e. shun > O(n**3) implementations of O(n**2) algorithms), perfectly > acceptable results can be achieved. > > 100. Are pointers really faster than arrays? Do function calls really > slow things down? Is i++ faster than i = i + 1? > > A: Precise answers to these and many similar questions depend of course on > the processor and compiler in use. If you simply must know, you'll > have to time test programs carefully. (Often the differences are > so slight that hundreds of thousands of iterations are required > even to see them. Check the compiler's assembly language output, > if available, to see if two purported alternatives aren't compiled > identically.) > > It is "usually" faster to march through large arrays with pointers > rather than array subscripts, but for some processors the reverse > is true. > > Function calls, though obviously incrementally slower than in-line > code, contribute so much to modularity and code clarity that there > is rarely good reason to avoid them. (Actually, by reducing bulk, > functions can improve performance.) > > Before rearranging expressions such as i = i + 1, remember that you > are dealing with a C compiler, not a keystroke-programmable > calculator. A good compiler will generate identical code for i++, > i += 1, and i = i + 1. The reasons for using i++ or i += 1 over > i = i + 1 have to do with style, not efficiency. ========== 86. My floating-point calculations are acting strangely and giving me different answers on different machines. programming text should cover the basics. (Beware, though, that < subtle problems can occupy numerical analysts for years.) --- > subtle problems can occupy numerical analysts for years.) Do make > sure that you have #included <math.h>, and correctly declared other > functions returning double. ========== > 103. This program crashes before it even runs! (When single-stepping > with a debugger, it dies before the first statement in main.) > > A: You probably have one or more very large (kilobyte or more) local > arrays. Many systems have fixed-size stacks, and those which > perform dynamic stack allocation automatically (e.g. Unix) are often > confused when the stack tries to grow by a huge chunk all at once. > > It is often better to declare large arrays with static duration > (unless of course you need a fresh set with each recursive call). ========== Trivia questions like these aren't any more pertinent for comp.lang.c than they are for any of the other groups they < frequently come up in. The "jargon file" (also published as _The < Hacker's Dictionary_) contains lots of tidbits like these, as does --- > frequently come up in. You can find lots of information in the > net.announce.newusers frequently-asked questions postings, the > "jargon file" (also published as _The Hacker's Dictionary_), and ========== 91. Where can I get extra copies of this list? What about back issues? < A: For now, just pull it off the net; it is normally posted on the < first of each month, with an Expiration: line which should keep it --- > A: For now, just pull it off the net; it is normally posted to > comp.lang.c on the first of each month, with an Expiration: line ========== > Thanks to Sudheer Apte, Joe Buehler, Raymond Chen, Christopher > Calabrese, James Davies, Norm Diamond, Ray Dunn, Stephen M. Dunn, Bjorn > Engsig, Ron Guilmette, Doug Gwyn, Tony Hansen, Joe Harrington, Guy > Harris, Blair Houghton, Kirk Johnson, Andrew Koenig, John Lauro, > Christopher Lott, Tim McDaniel, Evan Manning, Mark Moraes, Francois > Pinard, randall@virginia, Pat Rankin, Rich Salz, Chip Salzenberg, Paul > Sand, Doug Schmidt, Patricia Shanahan, Peter da Silva, Joshua Simons, > Henry Spencer, Erik Talvola, Clarke Thatcher, Chris Torek, Ed Vielmetti, > Larry Virden, Freek Wiedijk, and Dave Wolverton, who have contributed, > directly or indirectly, to this article. Special thanks to Karl Heuer, > and particularly to Mark Brader, who (to borrow a line from Steve > Johnson) have goaded me beyond my inclination, and frequently beyond my > endurance, in relentless pursuit of a better FAQ list. ========== < This article is Copyright 1988, 1990 by Steve Summit. --- > This article is Copyright 1988, 1990, 1991 by Steve Summit. ==========
scs@adam.mit.edu (Steve Summit) (04/02/91)
Coming up next is the real comp.lang.c FAQ list. (Accept no substitutes!) There are no major changes this month; a brief diff listing follows. (As always, these diffs have been edited for readability, and are not suitable for the patch program.) ========== < [Last modified February 28, 1991 by scs.] --- > [Last modified April 2, 1991 by scs.] ========== < An identifier of type array-of-T which appears in an expression decays (with three exceptions) into a pointer to its first element; the type of the resultant pointer is pointer-to-T. --- > An lvalue of type array-of-T which appears in an expression ========== as it applies to arrays and pointers. In an expression of the form < a[i], the array name "a" decays into a pointer, following the rule above, and is then subscripted exactly as would be a pointer variable in the expression p[i]. In either case, the expression --- > a[i], the array reference "a" decays into a pointer, following the ========== < Since strcat returns its first argument, the s3 variable is < superfluous. --- > Since strcat returns the value of its first argument, the s3 > variable is superfluous. ========== A: In general, when using pointers you _always_ have to consider memory allocation, at least to make sure that the compiler is doing < it for you. --- > it for you. If a library routine's documentation does not > explicitly mention allocation, it is usually the caller's problem. ========== it is not widely portable. Passing an initially-null pointer to < realloc can make it easy to write a self-starting incremental allocation algorithm. --- > realloc can make it easier to write a self-starting incremental ========== structures), use short. Otherwise, use int. If well-defined < overflow characteristics are important and/or sign is not, use < unsigned. --- > overflow characteristics are important and/or negative values are > not, use unsigned. (But beware mixtures of signed and unsigned.) ========== < In general, don't try to use char or unsigned char as a "tiny" int < type; doing so is often more trouble than it's worth. --- > Although char or unsigned char can be used as a "tiny" int type; > doing so is often more trouble than it's worth. ========== may also generate nonfatal warnings when enums and ints are indiscriminately mixed, since doing so can still be considered bad style even though it is not strictly illegal). A disadvantage is < that the programmer has little control over the size. --- > that the programmer has little control over the size (or over those > nonfatal warnings). ========== < calculator. A good compiler will generate identical code for i++, < i += 1, and i = i + 1. The reasons for using i++ or i += 1 over i = i + 1 have to do with style, not efficiency. --- > calculator. A good compiler will generate identical code for ++i, > i += 1, and i = i + 1. The reasons for using ++i or i += 1 over ========== Acknowledgements > Thanks to Sudheer Apte, Dan Bernstein, Joe Buehler, Raymond Chen, > Christopher Calabrese, James Davies, Norm Diamond, Ray Dunn, Stephen > M. Dunn, Bjorn Engsig, Ron Guilmette, Doug Gwyn, Tony Hansen, Joe > Harrington, Guy Harris, Blair Houghton, Kirk Johnson, Andrew Koenig, > John Lauro, Christopher Lott, Tim McDaniel, Evan Manning, Mark Moraes, > Francois Pinard, randall@virginia, Pat Rankin, Rich Salz, Chip > Salzenberg, Paul Sand, Doug Schmidt, Patricia Shanahan, Peter da Silva, > Joshua Simons, Henry Spencer, Erik Talvola, Clarke Thatcher, Chris > Torek, Ed Vielmetti, Larry Virden, Freek Wiedijk, and Dave Wolverton, > who have contributed, directly or indirectly, to this article. Steve Summit scs@adam.mit.edu scs%adam.mit.edu@mit.edu mit-eddie!adam!scs
scs@adam.mit.edu (Steve Summit) (05/01/91)
It's the comp.lang.c FAQ list's first birthday, and practically all it got this month was deletions, sad to say. (Worry not; the seven elided questions were of secondary importance, and the other deleted text was mostly redundant or tangential. Through the magic of RCS and *roff "conditional compilation," all of the deleted text is preserved, and will eventually reappear in a "long form" list, to be available on request.) The deletions pull the list back under 100K in size, to avoid difficulties with some mailers. Here is the usual set of diffs (edited for readability, and unsuitable for the patch program) between the previous posting (April 2) and the new one. (Do _not_ worry if you have not seen the new one yet; it's coming up next.) < [Last modified April 2, 1991 by scs.] --- < [Last modified April 29, 1991 by scs.] ========== < This article does not, and cannot, provide an exhaustive discussion of < every subtle point and counterargument which could be mentioned with < respect to these topics. Cross-references to standard C publications < have been provided, for further study by the interested and dedicated < reader. ========== < some of the myths which this article attempts to refute. Several < noteworthy books on C are listed in this article's bibliography. --- > noteworthy books on C are listed in this article's bibliography. Many > of the questions and answers are cross-referenced to these books, for > further study by the interested and dedicated reader. ========== When function prototypes are in scope, argument passing becomes an < "assignment context," and casts may safely be omitted, since the < prototype tells the compiler that a pointer is required, and of --- > "assignment context," and most casts may safely be omitted, since > the prototype tells the compiler that a pointer is required, and of ========== < 3. But aren't pointers the same as ints? < < A: Not since the early days. Attempting to turn pointers into < integers, or to build pointers out of integers, has always been < machine-dependent and unportable, and doing so is strongly < discouraged. (Any object pointer may be cast to the "universal" < pointer type void *, or char * under a pre-ANSI compiler, when < heterogeneous pointers must be passed around.) It is no longer < guaranteed that a pointer can be cast to a "suitably capacious" < integer and back, unchanged. ========== Nevertheless, ANSI C allows the alternate #define NULL (void *)0 definition for NULL. Besides helping incorrect programs to work < (but only on machines with all pointers the same, thus questionably --- > (but only on machines with homogeneous pointers, thus questionably valid assistance) this definition may catch programs which use NULL ========== < A: No. Although preprocessor macros are often used in place of --- > A: No. Although symbolic constants are often used in place of numbers because the numbers might change, this is _not_ the reason that NULL is used in place of 0. Once again, the language guarantees ========== < 11. I once used a compiler that wouldn't work unless NULL was used. < < A: Unless the code being compiled was nonportable (see question 6), < that compiler was probably broken. In general, making decisions < about a language based on the behavior of one particular compiler < is likely to be counterproductive. ========== most machines, as zero invites unwarranted assumptions. The use of a preprocessor macro (NULL) suggests that the value might change < later, or on some weird machine. The construct "if(p == 0)" is < easily misread as calling for conversion of p to an integral type, < rather than 0 to a pointer type, before the comparison. Finally, < the distinction between the several uses of the term "null" (listed < above) is often overlooked. --- > later, or on some weird machine. Finally, the distinction between > the several uses of the term "null" (listed above) is often > overlooked. ========== A: "Certain Prime computers use a value different from all- bits-0 to encode the null pointer. Also, some large Honeywell-Bull machines use the bit pattern 06000 to encode < the null pointer. On such machines, the assignment of 0 to < a pointer yields the special bit pattern that designates the < null pointer." --- > the null pointer." -- Portable C, by H. Rabinowitz and Chaim Schaap, Prentice-Hall, 1990, page 147. ========== 19. Then why are array and pointer declarations interchangeable as function formal parameters? A: Since arrays decay immediately into pointers, an array is never < actually passed to a function. Allowing pointer parameters to be < declared as arrays is a simply a way of making it look as though < the array was being passed. Some programmers prefer, as a matter < of style, to use this syntax to indicate that the pointer parameter < is expected to point to the start of an array rather than to some < single value. < < Since functions can never receive arrays as parameters, any < parameter declarations which "look like" arrays, e.g. --- A: Since arrays decay immediately into pointers, an array is never > actually passed to a function. Therefore, any parameter > declarations which "look like" arrays, e.g. ========== < To repeat, however, this conversion holds only within function < formal parameter declarations, nowhere else. If this conversion < bothers you, don't use it; many people have concluded that the < confusion it causes outweighs the small advantage of having the < declaration "look like" the call and/or the uses within the < function. --- > This conversion holds only within function formal parameter > declarations, nowhere else. If this conversion bothers you, avoid > it; many people have concluded that the confusion it causes > outweighs the small advantage of having the declaration "look like" > the call and/or the uses within the function. ========== multidimensional arrays, if at all. (See question 22 above.) When people speak casually of a pointer to an array, they usually mean a < pointer to its first element; the type of this latter pointer is < generally more useful. ========== (See also question 63.) If the size of the array is unknown, N can be omitted, but the resulting type, "pointer to array of unknown < size," is almost completely useless. --- > size," is useless. ========== < (In "real" code, of course, each return value from malloc should be < checked.) --- > (In "real" code, of course, malloc should be declared correctly, > and each return value checked.) ========== < The order of other embedded side effects is similarly undefined. < For example, the expression i + (i = 2) does not necessarily yield < 4. ========== The behavior of code which contains ambiguous or undefined side < effects has always been undefined. (Note, too, that a compiler's --- > effects (including ambiguous embedded assignments) has always been > undefined. (Note, too, that a compiler's choice, especially under ========== < A: Two programs, protoize and unprotoize, are being written to convert < back and forth between prototyped and "old style" function < definitions and declarations. (These programs are _not_ expected < to handle full-blown conversion between "Classic" C and ANSI C.) < When available, these programs will exist as patches to the FSF GNU < C compiler, gcc. --- > A: Two programs, protoize and unprotoize, convert back and forth > between prototyped and "old style" function definitions and > declarations. (These programs do _not_ handle full-blown > translation between "Classic" C and ANSI C.) These programs exist > as patches to the FSF GNU C compiler, gcc. Look for the file > protoize-1.39.0 in pub/gnu at prep.ai.mit.edu (18.71.0.38), or at > several other FSF archive sites. ========== The noalias keyword was not backed up by any "prior art," and it was introduced late in the review and approval process. It was phenomenally difficult to define precisely and explain coherently, < and sparked widespread, acrimonious debate, including a scathing < pan by Dennis Ritchie. It had far-ranging implications, --- > and sparked widespread, acrimonious debate. It had far-ranging ========== < implications, must be understood.) The need for a mechanism to < support parallel implementation of non-overlapping operations < remains unfilled (although the C Numerical Extensions Working Group < is examining the problem). --- > implications, must be understood.) The need for an explicit > mechanism to support parallel implementation of non-overlapping > operations remains unfilled (although the C Numerical Extensions > Working Group is examining the problem). ========== 36. How can I write a generic macro to swap two values? A: There is no good answer to this question. If the values are integers, a well-known trick using exclusive-OR could perhaps be used, but it will not work for floating-point values or pointers, < (and the "obvious" supercompressed implementation for integral < types a^=b^=a^=b is, strictly speaking, illegal due to multiple < side-effects; and it will not work if the two values are the same < variable, and...). If the macro is intended to be used on values < of arbitrary type (the usual goal), it cannot use a temporary, < since it does not know what type of temporary it needs, and < standard C does not provide a typeof operator. (GNU C does.) --- > (and it will not work if the two values are the same variable, and > the "obvious" supercompressed implementation for integral types > a^=b^=a^=b is, strictly speaking, illegal due to multiple side- > effects, and...). If the macro is intended to be used on values of > arbitrary type (the usual goal), it cannot use a temporary, since > it does not know what type of temporary it needs, and standard C > does not provide a typeof operator. ========== The best all-around solution is probably to forget about using a < macro. If you're worried about the use of an ugly temporary, and < know that your machine provides an exchange instruction, convince < your compiler vendor to recognize the standard three-assignment < swap idiom in the optimization phase. --- > macro, unless you don't mind passing in the type as a third > argument. ========== If all of the statements in the intended macro are simple < expressions, with no declarations, conditionals, or loops, another < technique is to write a single, parenthesized expression using one < or more comma operators. (This technique also allows a value to be < "returned.") --- > expressions, with no declarations or loops, another technique is to > write a single, parenthesized expression using one or more comma > operators. (This technique also allows a value to be "returned.") ========== < (If you know enough about your machine's architecture, it is < possible to pick arguments off of the stack "by hand," but there is < little reason to do so, since portable mechanisms exist. If you < know how to access arguments "by hand," but have access to neither < <stdarg.h> nor <varargs.h>, you could as easily implement < <stdarg.h> yourself, leaving your code portable.) ========== < 44. How can I write a function analogous to scanf? < < A: Unfortunately, vscanf and the like are not standard. You're on < your own. ========== A: This information is not available to a portable program. Some < systems have a nonstandard nargs() function available, but its use < is questionable, since it typically returns the number of words --- > systems provide a nonstandard nargs() function, but its use is > questionable, since it typically returns the number of words ========== < 46. How can I write a function which takes a variable number of < arguments and passes them to some other function (which takes a < variable number of arguments)? < < A: In general, you cannot. You must provide a version of that other < function which accepts a va_list pointer, as does vfprintf in the < example above. If the arguments must be passed directly as actual < arguments (not indirectly through a va_list pointer) to another < function which is itself variadic (for which you do not have the < option of creating an alternate, va_list-accepting version) no < portable solution is possible. (The problem can be solved by < resorting to machine-specific assembly language.) ========== < That the "other half," better error detection, was deferred to < lint, was a fairly deliberate decision on the part of the earliest < Unix C compiler authors, but is inexcusable (in the absence of a < supplied, consistent lint, or equivalent error checking) in a < modern compiler. ========== < 50. Don't ANSI function prototypes render lint obsolete? < < A: Not really. First of all, prototypes work well only if the < programmer works assiduously to maintain them, and the effort to do < so (plus the extra recompilations required by numerous, more- < frequently-modified header files) can rival the toil of keeping < function calls correct manually. Secondly, an independent program < like lint will probably always be more scrupulous at enforcing < compatible, portable coding practices than will any particular, < implementation-specific, feature- and extension-laden compiler. < (Some vendors seem to introduce incompatible extensions < deliberately, perhaps to lock in market share.) ========== A: The pointer variable "answer," which is handed to the gets function as the location into which the response should be stored, has not < been set to point to any valid storage. It is an uninitialized < variable, just as is the variable i in < < int i; < printf("i = %d\n", i); < < That is, we cannot say where the pointer "answer" points. (Since --- > been set to point to any valid storage. That is, we cannot say > where the pointer "answer" points. (Since local variables are not ========== < overly-long line. (Unfortunately, fgets does not automatically < delete the trailing \n, as gets would.) It would also be possible --- > overly-long line. (Unfortunately for this example, fgets does not > automatically delete the trailing \n, as gets would.) It would ========== < Since strcat returns the value of its first argument, the s3 < variable is superfluous. --- > Since strcat returns the value of its first argument (s1, in this > case), the s3 variable is superfluous. ========== < invocation used by the caller. In particular, many routines accept < pointers (e.g. to structs or strings), and the caller usually < passes the address of some object (a struct, or an array -- see --- > invocation used by the caller. In particular, many routines which > accept pointers (e.g. to structs or strings), are usually called > with the address of some object (a struct, or an array -- see ========== A: No. Some early man pages for malloc stated that the contents of < freed memory was "left undisturbed;" this ill-advised guarantee is < not universal and is not required by ANSI. --- > freed memory was "left undisturbed;" this ill-advised guarantee was > never universal and is not required by ANSI. ========== A: alloca allocates memory which is automatically freed when the < function from which alloca was called returns. That is, memory --- > function which called alloca returns. That is, memory allocated ========== alignment of later fields correct). A field-by-field comparison would require unacceptable amounts of repetitive, in-line code for < large structures. Either method would not necessarily "do the < right thing" with pointer fields: oftentimes, equality should be < judged by equality of the things pointed to rather than strict < equality of the pointers themselves. --- > large structures. ========== < function to do so. C++ (among other languages) would let you < arrange for the == operator to map to your function. --- > function to do so. C++ would let you arrange for the == operator > to map to your function. ========== structures), use short. Otherwise, use int. If well-defined overflow characteristics are important and/or negative values are < not, use unsigned. (But beware mixtures of signed and unsigned.) --- > not, use the corresponding unsigned types. (But beware mixtures of > signed and unsigned.) ========== < Similar arguments operate when deciding between float and double. --- > Similar arguments apply when deciding between float and double. ========== < 67. I can't seem to define a linked list node which contains a pointer < to itself. I tried --- > 62. I can't seem to define a linked list successfully. I tried ========== A: Structs in C can certainly contain pointers to themselves; the discussion and example in section 6.5 of K&R make this clear. The < problem is that the example above attempts to hide the struct < pointer behind a typedef, which is not complete at the time it is < used. First, rewrite it without a typedef: < < struct node < { < char *item; < struct node *next; < }; < < Then, if you wish to use typedefs, define them after the fact: < < typedef struct node NODE, *NODEPTR; < < Alternatively, define the typedefs first (using the line just < above) and follow it with the full definition of struct node, which < can then use the NODEPTR typedef for the "next" field. --- > problem with this example is that the NODEPTR typedef is not > complete when the "next" field is declared. You will have to give > the structure a tag ("struct node"), and declare the "next" field > as "struct node next;". > > A similar problem, with a similar solution, can arise when > attempting to declare a pair of typedef'ed mutually recursive > structures. ========== < 68. How can I define a pair of mutually referential structures? I < tried < < typedef struct < { < int structafield; < STRUCTB *bpointer; < } STRUCTA; < < typedef struct < { < int structbfield; < STRUCTA *apointer; < } STRUCTB; < < but the compiler doesn't know about STRUCTB when it is used in < struct a. < < A: Again, the problem lies not in the pointers but the typedefs. < First, define the two structures without using typedefs: < < struct a < { < int structafield; < struct b *bpointer; < }; < < struct b < { < int structbfield; < struct a *apointer; < }; < < The compiler can accept the field declaration struct b *bpointer < within struct a, even though it has not yet heard of struct b. < Occasionally it is necessary to precede this couplet with the empty < declaration < < struct b; < < to mask the declarations (if in an inner scope) from a different < struct b in an outer scope. < < Again, the typedefs could also be defined before, and then used < within, the definitions for struct a and struct b. Problems arise < only when an attempt is made to define and use a typedef within the < same declaration. ========== < 2. Build it up in stages, using typedefs: --- > 2. Build the declaration up in stages, using typedefs: ========== A: Several public-domain versions are available. One is in volume 14 < of comp.sources.unix . (Commercial versions may also be available, < at least one of which was shamelessly lifted from the public domain < copy submitted by Graham Ross, one of cdecl's originators.) See < question 96. --- > of comp.sources.unix . (See question 89.) ========== < cannot be standardized by the C language. If you are using curses, < use its cbreak() function. Under UNIX, use ioctl to play with the --- > cannot be standardized by the C language. Some versions of curses > have a cbreak() function which does what you want. Under UNIX, use ========== Operating system specific questions are not appropriate for comp.lang.c . Many common questions are answered in frequently- asked questions postings in such groups as comp.unix.questions and < comp.os.msdos.programmer . Note that the answers are often not < unique even across different variants of Unix. Bear in mind when --- > comp.sys.ibm.pc.misc . Note that the answers are often not unique > even across different variants of a system. Bear in mind when ========== A: In general, it cannot. Different operating systems implement < name/value functionality similar to the Unix environment in many --- > name/value functionality similar to the Unix environment in ========== different ways. Whether the "environment" can be usefully altered < by a running program, and if so, how, is entirely system-dependent. --- > by a running program, and if so, how, is system-dependent. ========== provide setenv() and/or putenv() functions to do this), and the modified environment is usually passed on to any child processes, < but it is _not_ propagated back to the parent process. (The < environment of the parent process can only be altered if the parent < is explicitly set up to listen for some kind of change requests. < The conventional execution of the BSD "tset" program in .profile < and .login files effects such a scheme.) --- > but it is _not_ propagated back to the parent process. ========== stdout is not a terminal. Although the output operation goes on to < complete successfully, errno still contains ENOTTY. This behavior < can be mildly confusing, but it is not strictly incorrect, because < it is only meaningful for a program to inspect the contents of < errno after an error has occurred (that is, after a library < function that sets errno on error has returned an error code). --- > complete successfully, errno still contains ENOTTY. ========== A: scanf() was designed for free-format input, which is seldom what you want when reading from the keyboard. In particular, "\n" in a format string does not mean "expect a newline", it means "discard < all whitespace". But the only way to discard all whitespace is to < continue reading the stream until a non-whitespace character is < seen (which is then left in the buffer for the next input), so the < effect is that it keeps going until it sees a nonblank line. --- > all whitespace". > > It is usually better to fgets() to read a whole line, and then use > sscanf() or other string functions to parse the line buffer. ========== < 84. So what should I use instead? < < A: You could use a "%c" format, which will read one character that you < can then manually compare against a newline; or "%*c" and no < variable if you're willing to trust the user to hit a newline; or < "%*[^\n]%*c" to discard everything up to and including the newline. < Usually the best solution is to use fgets() to read a whole line, < and then use sscanf() or other string functions to parse the line < buffer. ========== A: This problem is, in general, insoluble. Under Unix, for instance, a scan of the entire disk, (perhaps requiring special permissions) < would be required, and would fail if the file descriptor were a < pipe (and could give a misleading answer for a file with multiple --- > would theoretically be required, and would fail if the file > descriptor was a pipe or referred to a deleted file (and could give > a misleading answer for a file with multiple links). It is best to ========== < A: Variables (and arrays) with "static" duration (that is, those < declared outside of functions, and those declared with the storage < class static), are guaranteed initialized to zero, as if the --- > A: Variables with "static" duration (that is, those declared outside > of functions, and those declared with the storage class static), > are guaranteed initialized to zero, as if the programmer had typed ========== A: The best solution is to use text files (usually ASCII), written with fprintf and read with fscanf or the like. (Similar advice < also applies to network protocols.) Be very skeptical of arguments --- > also applies to network protocols.) Be skeptical of arguments ========== < If the binary format is being imposed on you by an existing < program, first see if you can get that program changed to use a < more portable format. ========== > A PL/M to C converter was posted to alt.sources in April, 1991. ========== < Lexeme Corporation < Richard Cox < 4 Station Square, #250 < Commerce Court < Pittsburgh, PA 15219-1119 USA < (+1) 412 281 5454 ========== < The comp.sources.unix archives also contain converters between < "K&R" C and ANSI C. --- > See also question 29. ========== < 97. Where can I get the winners of old Obfuscated C Contests? When < will the next contest be held? < < A: Send mail to {pacbell,uunet,utzoo}!hoptoad!obfuscate . The contest < is usually announced in March, with entries due in May. Contest < announcements are posted in several obvious places. The winning < entries are archived on uunet (see question 96). --- > 90. When will the next International Obfuscated C Contest (IOCCC) be > held? How can I get a copy of the current and previous winning > entries? > > A: The contest typically runs from early March through mid-May. To > obtain a current copy of the rules, send email to: > > {pacbell,uunet,utzoo}!hoptoad!judges or judges@toad.com > > Contest winners are first announced at the Summer Usenix Conference > in mid-June, and posted to the net in July. Previous winners are > available on uunet (see question 89) under the directory > ~/pub/ioccc. ========== The character sequences /* and */ are not special within double- quoted strings, and do not therefore introduce comments, because a program (particularly one which is generating C code as output) < might want to print them. It is hard to imagine why anyone would < want or need to place a comment inside a quoted string. It is easy < to imagine a program needing to print "/*". --- > might want to print them. ========== For the small fraction of code that is time-critical, it is vital < to pick a good algorithm; it is much less important to < "microoptimize" the coding details. Many of the "efficient coding --- > to pick a good algorithm; it is less important to "microoptimize" > the coding details. Many of the "efficient coding tricks" which ========== < degraded. If the performance of your code is so important that you < are willing to invest programming time in source-level < optimizations, you would be better served by buying the best < optimizing compiler you can afford (compilers can perform < optimizations that are impossible at the source level). ========== < It is not the intent here to suggest that efficiency can be < completely ignored. Most of the time, however, by simply paying < attention to good algorithm choices, implementing them clearly and < obviously, and avoiding obviously inefficient blunders (i.e. shun < O(n**3) implementations of O(n**2) algorithms), perfectly < acceptable results can be achieved. --- > For more discussion of efficiency tradeoffs, as well as good advice > on how to increase efficiency when it is important, see chapter 7 > of Kernighan and Plaugher's The Elements of Programming Style, and > Jon Bentley's Writing Efficient Programs. ========== Function calls, though obviously incrementally slower than in-line code, contribute so much to modularity and code clarity that there < is rarely good reason to avoid them. (Actually, by reducing bulk, < functions can improve performance.) --- > is rarely good reason to avoid them. ========== Among other things, the associative and distributive laws do not hold completely (i.e. order of operation may be important, repeated < addition is not necessarily equivalent to multiplication, and < underflow or cumulative precision loss is often a problem). --- > addition is not necessarily equivalent to multiplication). > Underflow or cumulative precision loss is often a problem. ========== < programming text should cover the basics. (Beware, though, that < subtle problems can occupy numerical analysts for years.) Do make < sure that you have #included <math.h>, and correctly declared other --- > programming text should cover the basics. Do make sure that you > have #included <math.h>, and correctly declared other functions ========== including code to handle %e, %f, and %g. It happens that Turbo C's heuristics for determining whether the program uses floating point are occasionally insufficient, and the programmer must sometimes < insert one dummy explicit floating-point operation to force loading < of floating-point support. --- > insert a dummy explicit floating-point call to force loading of > floating-point support. ========== arrays. Many systems have fixed-size stacks, and those which < perform dynamic stack allocation automatically (e.g. Unix) are < often confused when the stack tries to grow by a huge chunk all at --- > perform dynamic stack allocation automatically (e.g. Unix) can be > confused when the stack tries to grow by a huge chunk all at once. > > (See also question 56.) ========== frequently come up in. You can find lots of information in the net.announce.newusers frequently-asked questions postings, the "jargon file" (also published as _The Hacker's Dictionary_), and < the official Usenet ASCII pronunciation list, maintained by Maarten < Litmaath. (The pronunciation list also appears in the jargon file < under ASCII, as well as in the comp.unix frequently-asked questions < list.) --- > the Usenet ASCII pronunciation list. ========== < available for anonymous ftp, or via a mailserver. (Note that the < size of the list is monotonically increasing; older copies are < obsolete and don't contain much, except the occasional typo, that < the current list doesn't.) --- > available for anonymous ftp, or via a mailserver. > > This list is an evolving document, not just a collection of this > month's interesting questions. Older copies are obsolete and don't > contain much, except the occasional typo, that the current list > doesn't. ========== > Jon Louis Bentley, Writing Efficient Programs, Prentice-Hall, > 1982, ISBN 0-13-970244-X. Steve Summit scs@adam.mit.edu
scs@adam.mit.edu (Steve Summit) (06/03/91)
No changes this month except for one typo correction; I've been busy. (If you've sent suggestions, they're in the queue.) ========== < [Last modified April 29, 1991 by scs.] --- > [Last modified June 2, 1991 by scs.] ========== complete when the "next" field is declared. You will have to give the structure a tag ("struct node"), and declare the "next" field < as "struct node next;". --- > as "struct node *next;". ========== Steve Summit scs@adam.mit.edu