gcc runtime declaration of static and auto variables

Question

I am compiling my C code using the gcc compiler

My code is

main()
{
    int i;
    scanf("%d",&i);
    int a[i];
}

... and it executes it without any warning. However, if I use:

main()
{
    int i;
    scanf("%d",&i);
    static int a[i];
}

... I get an error message saying the size of array is not constant.

If execution of program starts at main function it should not give such error. and if static data is allocated first before start of main(), how is the compiler able to generate such errors during during compile time?

Technically, the first shouldn't be allowed either. The fact that it is is a gcc extension, I believe. — dlf, Jun 11 '14 at 12:15
you are using VLAs, which are not allowed to be static or extern — mch, Jun 11 '14 at 12:17

Shafik Yaghmour · Accepted Answer · 2014-06-11T12:26:54.543

1

Variables length arrays are not allowed to be static from the draft C99 standard section 6.7.5.2 Array declarators paragraph 2 says (emphasis mine):

An ordinary identifier (as defined in 6.2.3) that has a variably modified type shall have either block scope and no linkage or function prototype scope. If an identifier is declared to be an object with static storage duration, it shall not have a variable length array type.

In C++ variable length arrays are not part of the standard and are a compiler extension but gcc should be be following the C99 rules there as well.

edited Jun 11 '14 at 12:26

answered Jun 11 '14 at 12:19

Shafik Yaghmour

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Does this means that VLA is always consumes stack portion? – Vagish Jun 11 '14 at 12:22
@Vagish that standard does not specify that but in practice yes, they are allocated on the stack. – Shafik Yaghmour Jun 11 '14 at 12:27

gcc runtime declaration of static and auto variables

1 Answers1