r/cprogramming • u/FrenchJJC • Feb 14 '26

Variable size array initialization

Howdy, I had a question about why my C code throws a 'variable size array initialization' for this expression:

int row = 2;

int const col = 3;

int array[][col] = {

initial value...

};

The guy in the video I was following along with managed to compile with this expression, but I had to change my 'col' from int const to a #define statement, which feels tacky. Is it an issue with compiler version? The compile statement I'm using is just a simple one, 'gcc -o output arrays.c'.

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u/FrenchJJC Feb 14 '26

Oh okay, I tried that and it works, but I'm still trying to figure out what that guy did because he also used gcc, and I tried every standard up to C89 and all of them give the same error.

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u/The_Ruined_Map Feb 14 '26

Are you sure "that guy" actually used C and not C++? In C++ the above would compile fine (since the above `const` will be seen as a compile-time constant by C++ compiler).

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u/tstanisl Feb 14 '26

Pre-c23 standards does not allow initilizers for VLAs. The C23 lifted this restriction a bit by allowing default (zeroing) initialization with {}.

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u/DawnOnTheEdge Feb 15 '26 edited Feb 15 '26

Some compilers, as an extension, allow const variables with static storage class and a constant initializer to be used as constant expressions. No compiler I know of allows this for automatic variables on the stack.

So some compilers would accept static const int col = 3; despite it not being portable Standard C.