Variable Sized Arrays in C

Question

I guess my question is whether the following is valid C

int main(void) {
  int r = 3;
  int k[r];
  return 0;
}

If so, would some one care to explain why it does not work in Microsoft's C compiler, but in GCC, and when it was added to the C standard.

Thank you

Answer 1

It is in C99. MSVC only supports C89.

Answer 2

The C99 standard added variable-length arrays, but other vendors such as GCC added them much earlier.

Answer 3

It is a GCC extension that the current MSVC does not support. You can replace it in MSVC fairly easily with an _alloca (a stack allocation that requires no manual deallocation on the part of the programmer):

#include <malloc.h>

...

int *k = (int *)_alloca(sizeof(*k)*r);

Answer 4

I'm sorry this is not an answer, but I'd like to point out a potential problem with using variable-length arrays. Most of the code that I have come across looks like this.

void foo(int n)
{
    int bar[n];
    .
    .
}

There is no explicit error checking here. A large n can easily cause problems.

Answer 5

It is a GCC extension that the current MSVC does not support. You can replace it in MSVC fairly easily with an _alloca (a stack allocation that requires no manual deallocation on the part of the programmer)

yes but it is limited to 1mb