Nav_Panel on A nice, little known C feature: Static array indices in parameter declarations

Comment on A nice, little known C feature: Static array indices in parameter declarations

Doesn't seem to work for me using GCC 4.7.2.

flags: -g -Wall -Wextra -std=c99 -pedantic

The following code compiles and runs (for me at least) with no errors. Tried with both stack and heap allocated arrays of various sizes.

    #include <stdlib.h>
    
    void foo(int array[static 10]) {
      (void) array; // suppress unused var compiler warning
      return;
    }
    
    int main () {
      int *x = calloc(10, sizeof(int));
      int *y = calloc(9, sizeof(int));
      int *z = calloc(11, sizeof(int));
      foo(x);
      foo(y);
      foo(z);
      foo(NULL);
      int a[9];
      int b[4];
      int c[11];
      foo(a);
      foo(b);
      foo(c);
      return 0;
    }

ambrop7 13y

I've tried this, and gcc indeed doesn't produce warnings even with -Wall -Wextra. Clang does however without needing any flags.

qznc 13y

My clang (3.0-6ubuntu3) does not.

jrajav 13y

I can confirm. Perhaps this is a bug? As per the C99 standard [Clause 6.7.5.3: Function Declarators, point 7]:

A declaration of a parameter as "array of type" shall be adjusted to "qualified pointer to type", where the type qualifiers (if any) are those specified within the [ and ] of the array type derivation. If the keyword static also appears within the [ and ] of the array type derivation, then for each call to the function, the value of the corresponding actual argument shall provide access to the first element of an array with at least as many elements as specified by the size expression.

Since that's a 'shall' declaration, shouldn't it at least throw out a warning?

quasive 13y

This "shall" is not listed under "Constraints", so violation of it is undefined behavior and does not require a diagnostic. See 4 (Conformance): If a ‘‘shall’’ or ‘‘shall not’’ requirement that appears outside of a constraint is violated, the behavior is undefined.

This is an area where a compiler can (sometimes) see violations, though, so I think gcc should diagnose when possible, in the same vein as format specifier mismatches in printf().

onedognight 13y

> The information provided by static in parameter array declarators is not used for optimization.

This is not a reason to avoid the construct, but as yet gcc[1] doesn't optimize it either.

[1] http://gcc.gnu.org/c99status.html

niggler 13y

Not in front of a computer, but have you tried -std=gnu99

DiabloD3 13y

Thats a bad way of writing code, to be honest. On projects I run, code has to be warning free with -Wall -Wextra -pedantic -std=c99

kelnos 13y

It's not bad at all, at least if you've made the conscious decision to write GNU C and not std C, and accept that non-gcc compilers (except maybe clang) may not be able to compile your code.

Unfortunately, though, I believe one of the -std=gnuXX variants is the default, so most people don't make that a conscious decision.

niggler 13y

gnu89 is the default. gnu99 will be the default when C99 is fully implemented (from the manpage, search for /-std=$/)

simias 13y

C99 is still not fully implemented in GCC? What's missing?

TorKlingberg 13y

There is a list at http://gcc.gnu.org/c99status.html

Nav_Panel 13y

Just tried it now. Still compiles/runs without errors.

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