Check support for variable length array in C

Question

Is there a standard macro to check support of variable length arrays in C code? It it enough to check for c99 (__STDC_VERSION__ >= 199901L) in all widely used compilers?

Since you've tagged C99, you are guaranteed to have VLAs if your compiler is C99 compliant. — Christian Gibbons
– Christian Gibbons, Commented Apr 23, 2018 at 19:24
The non-compliant MSVC does not need a macro. It doesn't compile. — Weather Vane
– Weather Vane, Commented Apr 23, 2018 at 19:32
You would only need a macro in C11 when VLAs became optional. Therefore they are not standard so as @i486 says - don't use them. — Weather Vane
– Weather Vane, Commented Apr 23, 2018 at 19:34
@abelenky: In some cases, it may be possible to write code which will work with or without VLA support. For example use either int foo[x]; or int *foo = malloc(x * sizeof (int)); (and later free the storage), depending upon whether VLA suport exists. If VLA support exists, and a programmer is willing to chance a stack overflow, the VLA approach may be faster, but code should be usable (but perhaps not as fast) even without VLA support. — supercat
– supercat, Commented Apr 24, 2018 at 15:49

user3386109 · Accepted Answer · 2018-04-23 19:42:22Z

8

From the C11 specification §6.10.8.3

The following macro names are conditionally defined by the implementation:
[...]

__STDC_NO_VLA__ The integer constant 1, intended to indicate that the implementation does not support variable length arrays or variably modified types.

So if __STDC_VERSION__ > 201000L you need to check __STDC_NO_VLA__.

Otherwise, if __STDC_VERSION__ >= 199901L VLAs should work, but you'll get a compile time error if the compiler is non-compliant.

answered Apr 23, 2018 at 19:42

user3386109

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Seleznev Anton Over a year ago

Thanks. It looks like what I was looking for. I'll examine it.

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