Why can’t I create an array with size determined by a global variable?

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In C99, 6.7.8/3:

The type of the entity to be
initialized shall be an array of
unknown size or an object type that is
not a variable length array type.

6.6/2:

A constant expression can be evaluated
during translation rather than runtime

6.6/6:

An integer constant expression
shall have integer type and shall only
have operands that are integer
constants, enumeration constants,
character constants, sizeof
expressions whose results are integer
constants, and floating constants that
are the immediate operands of casts.

6.7.5.2/4:

If the size is an integer constant
expression and the element type has a
known constant size, the array type is
not a variable length array type;
otherwise, the array type is a
variable length array type.

a has variable length array type, because size is not an integer constant expression. Thus, it cannot have an initializer list.

In C90, there are no VLAs, so the code is illegal for that reason.

In C++ there are also no VLAs, but you could make size a const int. That’s because in C++ you can use const int variables in ICEs. In C you can’t.

Presumably you didn’t intend a to have variable length, so what you need is:

#define size 5

If you actually did intend a to have variable length, I suppose you could do something like this:

int a[size];
int initlen = size;
if (initlen > 5) initlen = 5;
memcpy(a, (int[]){1,2,3,4,5}, initlen*sizeof(int));

Or maybe:

int a[size];
for (int i = 0; i < size && i < 5; ++i) {
    a[i] = i+1;
}

It’s difficult to say, though, what “should” happen here in the case where size != 5. It doesn’t really make sense to specify a fixed-size initial value for a variable-length array.

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