mitchell@tci.UUCP (Rob Mitchell) (10/23/90)
Subj: Known Symantec Think C 4.01 bug and work-around Date: 22-Oct-1990 Auth: Rob Mitchell, Technology Concepts Inc. BUG: LLastClick, of List Manager routines, returns a long instead of a Cell type as defined in Inside Mac. BUGGY CODE: ... Cell theCell; ListHandle myList; /* compiler generates illegal * union/struct operation */ theCell = LLastClick (myList); ... SOLUTION: Stick return of LLastClick into a long and cast long to Cell type. SOLUTION CODE: ... Cell theCell; ListHandle myList; long tLong; tLong = LLastClick (myList); theCell = *(Cell *) &tLong; ... IN CLOSING: Symantec's Technial Support staff took all of 30 seconds to analayze the problem and offer the solution. I'm really impressed by the service and quality of Symantec.
steve@huxley.huxley.bitstream.com (Steve Stein) (10/23/90)
In article <514@tci.UUCP> mitchell@tci.UUCP (Rob Mitchell) writes: > Subj: Known Symantec Think C 4.01 bug and work-around > BUG: LLastClick, of List Manager routines, returns a long instead > of a Cell type as defined in Inside Mac. > > BUGGY CODE: > ... Cell theCell; > ListHandle myList; > > /* compiler generates illegal > * union/struct operation > */ > theCell = LLastClick (myList); > In all fairness to Symantec, THIS IS NOT A BUG!!! (This is a "feature" of C.) C functions may not return structs (ref: Kernighan & Ritchie's "The C Programming Language", Appendix A ("C Reference Manual") Section 8.4: "functions may not return arrays, structures, unions or functions".) The problem arises from the fact that the Mac Toolbox is designed with Pascal interfaces (which may not exactly be the case nowadays). Pascal functions may return records ("struct"s in C). Luckily for C, this "record" (Cell) is only 4 bytes long, so C can return it in a register. It's just that the C language won't allow declaring LLastClick as returning the struct type Cell. The C language requires that LLastClick be declared as returning a long. > SOLUTION: Stick return of LLastClick into a long and cast > long to Cell type. > > SOLUTION CODE: > ... > Cell theCell; > ListHandle myList; > long tLong; > > tLong = LLastClick (myList); > theCell = *(Cell *) &tLong; I find it more convenient to use: union cell_return_type { Cell c; long n; } theCell; ListHandle myList; theCell.n = LLastClick(myList); and use "theCell.c" whenever I have to refer to the Cell's values. This is a bit kludgy, but it generates better code. > IN CLOSING: > > Symantec's Technial Support staff took all of 30 seconds to > analayze the problem and offer the solution. I'm really impressed > by the service and quality of Symantec. Hear, Hear!!!!! Technical support for the THINK languages has long been a true strength of THINK C and THINK Pascal. Disclaimer: I used to work for THINK and Symantec. - Steve Stein
lim@iris.ucdavis.edu (Lloyd Lim) (10/24/90)
In article <STEVE.90Oct23100517@huxley.huxley.bitstream.com> steve@huxley.huxley.bitstream.com (Steve Stein) writes: >C functions may not return structs (ref: Kernighan & Ritchie's "The C >Programming Language", Appendix A ("C Reference Manual") Section 8.4: >"functions may not return arrays, structures, unions or functions".) Your K&R is out of date. This has changed with ANSI C. Here's a quote from K&R, Second Edition, p. 129, "The only legal operations on a structure are copying it or assigning to it as a unit, taking its address with &, and accessing its members. Copy and assignment include passing arguments to functions and returning values from functions as well." To reiterate, in Appendix C, Summary of Changes, p. 260, "Structures may be assigned, passed to functions, and returned by functions." If you look at K&R 2 and the THINK C manual, you'll notice there are quite a few differences between ANSI C and THINK C. +++ Lloyd Lim Internet: lim@iris.ucdavis.edu (128.120.57.20) Compuserve: 72647,660 US Mail: 215 Lysle Leach Hall, U.C. Davis, Davis, CA 95616
steve@huxley.huxley.bitstream.com (Steve Stein) (10/24/90)
In article <7865@ucdavis.ucdavis.edu> lim@iris.ucdavis.edu (Lloyd Lim) writes: >In article <STEVE.90Oct23100517@huxley.huxley.bitstream.com> >steve@huxley.huxley.bitstream.com (Steve Stein) writes: >>C functions may not return structs (ref: Kernighan & Ritchie's "The C >>Programming Language", Appendix A ("C Reference Manual") Section 8.4: >>"functions may not return arrays, structures, unions or functions".) > >Your K&R is out of date. This has changed with ANSI C. Here's a quote from >K&R, Second Edition, p. 129, "The only legal operations on a structure are >copying it or assigning to it as a unit, taking its address with &, and >accessing its members. Copy and assignment include passing arguments to >functions and returning values from functions as well." > >To reiterate, in Appendix C, Summary of Changes, p. 260, "Structures may be >assigned, passed to functions, and returned by functions." > ANSI C allows functions to return a struct. ANSI C allows functions to return a struct. ANSI C allows functions to return a struct. ANSI C allows functions to return a struct. ANSI C allows functions to return a struct. ANSI C allows functions to return a struct. ANSI C allows functions to return a struct. ANSI C allows functions to return a struct. ANSI C allows functions to return a struct. ... Thanks Lloyd, and to those who wrote to update my knowledge of C. (I would reply by eMail, but my eMail is bouncing.) - Steve Stein
siegel@endor.uucp (Rich Siegel) (10/25/90)
In article <STEVE.90Oct23100517@huxley.huxley.bitstream.com> steve@huxley.huxley.bitstream.com (Steve Stein) writes: >In article <514@tci.UUCP> mitchell@tci.UUCP (Rob Mitchell) writes: > >> Subj: Known Symantec Think C 4.01 bug and work-around >> BUG: LLastClick, of List Manager routines, returns a long instead >> of a Cell type as defined in Inside Mac. This is not a bug. The reason that LLastClick returns a long is because functions declared 'pascal' (such as all of the Toolbox routines) in THINK C may not return structures or any simple type larger than 4 bytes (such as a result-type of 'double'). The behavior of 'pascal'-declared functions is not standardized, and is pretty much implementation-dependent. (A future release of THINK C may support the returning of structured function results by 'pascal' functions. One way to deal with LLastClick is to cast the result using a pointer cast; the compiler will recognize this trick for what it is and generate a simple move. If you feel more comfortable doing it explicitly, you can try this: { extern LLastClick(); asm { clr.l -(a7) ; function result move.l myList, -(a7) ; argument jsr LLastClick ; call it move.l (a7)+, theCell ; pop result } } R. Rich Siegel Software Engineer Symantec Languages Group Internet: siegel@endor.harvard.edu UUCP: ..harvard!endor!siegel If you have telekinetic powers, raise my hand.
phils@chaos.cs.brandeis.edu (Phil Shapiro) (10/25/90)
Not to knock a compliment, but the 'bug' in LLastClick() is described
on p. 122 of the User's Manual. Perhaps an easier workaround to this
problem would be:
main()
{
Cell c;
ListHandle list;
*(long *)&c = LLastClick(list);
}
-phil
--
Phil Shapiro Technical Support Analyst
Language Products Group Symantec Corporation
Internet: phils@chaos.cs.brandeis.eduwillcox@urbana.mcd.mot.com (David A Willcox) (10/25/90)
In article <STEVE.90Oct23100517@huxley.huxley.bitstream.com> steve@huxley.huxley.bitstream.com (Steve Stein) writes: >In all fairness to Symantec, THIS IS NOT A BUG!!! (This is a "feature" of C.) >C functions may not return structs (ref: Kernighan & Ritchie's "The C >Programming Language", Appendix A ("C Reference Manual") Section 8.4: >"functions may not return arrays, structures, unions or functions".) In all fairness to C, your statement is false. C functions have been able to return structures for quite some time - I think that every UNIX compiler I've used since Version 6 days has allowed it. You're right, K&R does say that it is illegal, but it is a very common extension to allow it. Also, to quote the ANSI C standard (subclause 3.7.1): The return type of a function shall be void or an object type other than array. In other words, in ANSI C, returning a structure definitely IS legal.