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++? — walnut
– walnut, Commented Sep 15, 2019 at 16:19
It's one of the more inscrutable bits of the standard, but C11 §6.7.6.2 Array Declarators ¶4 defines variable length arrays. Just for once, this isn't a GCC extension. — Jonathan Leffler
– Jonathan Leffler, Commented Sep 15, 2019 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
– Maifee Ul Asad, Commented Sep 15, 2019 at 16:42

Jonathan Leffler · Accepted Answer · 2019-09-15 16:24:31Z

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, 2019 at 16:24

Jonathan Leffler

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answered Sep 15, 2019 at 15:28

Sourav Ghosh

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2 Comments

S.S. Anne Over a year ago

You might add a note that VC only supports c90.

Clifford Over a year ago

@JL2210 : ... plus those parts of C11 that are also valid C++ (but not VLAs because they are not).

Clifford · Accepted Answer · 2019-09-15 16:42:26Z

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.

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