geoff@locus.com (Geoff Kuenning) (02/05/91)
The following little program demonstrates a problem found in the XSend() routine of XlibInt.c: main () { int i = 0; while (i < 3) { int j = 4; printf ("i = %d, j = %d\n", i, j); i++; j++; } } What does this print? On the machine I'm using, it prints the same value for j (j = 4) three times. On the other hand, the programmer of XSend() clearly expected three different values for j. I just checked the ANSI C spec on this, and found it unclear. It is explicitly stated that automatics are initialized on every entry to a compound statement. However, it is not made clear whether the construct: while (<expression>) <statement> is considered to *re-enter* the compound statement every time or not. My instinct says yes, the statement is re-entered, but obviously this interpretation is open to discussion. In terms of usefulness, I could argue that either option is useful to programmers. What do people think the spec should do about this question? (BTW, the spec explicitly says that implementations get to decide whether automatics are initialized upon jumps into a compound statement, so perhaps this issue should also be implementation-defined). I'm cross-posting to comp.windows.x so that X-Windows users will be made aware of a potential bug (the problem will show up only under certain very rare conditions, so it could be lurking in your system without your knowledge). -- Geoff Kuenning geoff@la.locus.com geoff@ITcorp.com
gwyn@smoke.brl.mil (Doug Gwyn) (02/06/91)
In article <1991Feb5.023809.389086@locus.com> geoff@locus.com (Geoff Kuenning) writes:
- while (i < 3)
- {
- int j = 4;
- printf ("i = %d, j = %d\n", i, j);
- i++;
- j++;
- }
-What does this print? On the machine I'm using, it prints the same value
-for j (j = 4) three times. On the other hand, the programmer of XSend()
-clearly expected three different values for j.
The programmer clearly made a mistake. j has the value 4 for each
execution of the printf(). I was unable to tell from the code
snippet just what the programmer intended; merely changing the
storage class of j to "static" would probably not be correct either.
-I just checked the ANSI C spec on this, and found it unclear. It is
-explicitly stated that automatics are initialized on every entry to a
-compound statement. However, it is not made clear whether the construct:
- while (<expression>)
- <statement>
-is considered to *re-enter* the compound statement every time or not.
There is no question whatsoever about this. The compound statement is
completely evaluated for each iteration; this includes the auto
initialization.
tom@flood.com (Tom Chatt) (02/12/91)
In article <1991Feb5.023809.389086@locus.com> geoff@locus.com (Geoff Kuenning) writes (paraphrases in brackets): > [ In the following example (drawn from actual Xlib code): ] > > main () > { > int i = 0; > while (i < 3) > { > int j = 4; > printf ("i = %d, j = %d\n", i, j); > i++; > j++; > } > } > > [ Does the initialization of j=4 occur at the start of each iteration > of the compound statement, or only the first iteration? Mr. Kuenning's > compiler sets j=4 each time; the X programmer evidently expected otherwise. > The ANSI C standard says that automatics are initialized on every entry > to a compound statement, but does the above example count as three > entries to the compound statement, or only one? ] WRT entering the compound statement, it seems pretty clear to me that the compound statement is re-entered at each iteration, and that the initialization (as per spec) should occur each time. I say this because the test "i < 3" is clearly outside of the compound statement, and this executes at each iteration, thus you must have exited the compound statement. Though a compiler *may* optimize this out, it would be perfectly correct for a compiler to *allocate* the automatic variable at each entry to the compound statement. In the example above, the programmer has some expectation about the value of the automatic variable persisting outside of its scope (i.e., between iterations). This practice is extremely dubious and inadvisable. If a programmer expects a variable's value to persist outside of its scope, he should declare it as static. -- Tom Chatt \ Don't take offense, take action. Internet: tom@flood.com \ Speak up. When we remain silent, UUCP: ...!uunet!flood!tom / \ we oppress ourselves.