The below code snippet works and compiles in C:
const int n=10;
int main(void)
{
int a[n];
return 0;
}
However, when the array is declared in global scope, it throws a compilation error.
const int n=10;
int a[n];
int main(void)
{
return 0;
}
Why doesn't it throw an error inside main?
Despite n being marked as const, it's not a compile time constant1. As such the array a is a variable length array2.
VLA's cannot have static storage duration. Placing it at file scope makes it a static variable, and your program is ill-formed because of it3.
An integer constant expression117) shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, _Alignof expressions, and floating constants that are the immediate operands of casts. Cast operators in an integer constant expression shall only convert arithmetic types to integer types, except as part of an operand to the sizeof or _Alignof operator.
If the size is not present, the array type is an incomplete type. If the size is * instead of being an expression, the array type is a variable length array type of unspecified size, which can only be used in declarations or type names with function prototype scope;143) such arrays are nonetheless complete types. 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.
If an identifier is declared as having a variably modified type, it shall be an ordinary identifier (as defined in 6.2.3), have no linkage, and have either block scope or function prototype scope. If an identifier is declared to be an object with static or thread storage duration, it shall not have a variable length array type.
int (*array)[foo] = malloc(sizeof(*array));. The restriction is an explicit restriction to deal with the fact that there is no defined execution order of global/thread storage declaration statements in C. Consequently, there is no notion of "this variable has that value at this VLA declaring statement" for globals.
