Why does array allocation with a variable work?

Question

This code:

int main() {
   int size;
   scanf("%d", &size);
   int array[size]; 
}

works fine with GCC, but VC expects a constant expression for the size of the array so does not compile it (which makes more sense to me). Any idea why it works with GCC?

Possible duplicate of [Enabling VLAs (variable length arrays) in MS Visual C++?](https://stackoverflow.com/questions/5246900/enabling-vlas-variable-length-arrays-in-ms-visual-c) — walnut, Sep 15 '19 at 16:19
It's one of the more inscrutable bits of the standard, but C11 [§6.7.6.2 Array Declarators ¶4](https://port70.net/~nsz/c/c11/n1570.html#6.7.6.2p4) defines variable length arrays. Just for once, this isn't a GCC extension. — Jonathan Leffler, Sep 15 '19 at 16:28
Try adding `printf("%d",size);` just after the line `int size;`. I'm not sure, maybe it's because of the garbage value. — Maifee Ul Asad, Sep 15 '19 at 16:42

score 3 · Answer 1 · edited Sep 15 '19 at 16:24

3

Yes, because gcc supports variable length arrays.

It was added as a part of C99 standard, however, in the later standards (C11 and C18), it's an optional feature.

edited Sep 15 '19 at 16:24

Jonathan Leffler

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answered Sep 15 '19 at 15:28

Sourav Ghosh

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You might add a note that VC only supports c90. – S.S. Anne Sep 15 '19 at 15:58
1

@JL2210 : ... plus those parts of C11 that are also valid C++ (but not VLAs because they are not). – Clifford Sep 15 '19 at 16:24

score 0 · Answer 2 · answered Sep 15 '19 at 16:42

Because variable-length arrays (VLAs) are neither valid in C90 nor C++11 MSVC does not support them:

From https://learn.microsoft.com/en-us/cpp/build/reference/za-ze-disable-language-extensions?view=vs-2019:(

The C compiler is an C89/C90 compiler that, by default, enables Microsoft extensions to the C language.

VLAs may be best avoided in any event because a C11 compiler need not implement them and they are potentially unsafe. Typically an implementation will allocate VLAs on the stack. In your code, it only takes a user to enter an arbitrarily large value to break your code in someone-deterministic manner - any use of VLAs should have some constraint test to ensure the length is reasonable and in the capability of the system to support.

Why does array allocation with a variable work?

2 Answers2