initializing static variable with a function call gives compilation error?

Question

#include <stdio.h>
int foo(){
    return 1;
}
int main(void) {
    static int q = foo(); 
    return 0;
}

Here is a link for the same. This is a C code and not C++. It compiles and run fine in C++ but not C.

This code was getting compilation error. Can someone please explain why is it getting error? Can static members only be initialized by constant values ? In C++ we need to DEFINE static members after declaring them , why is it not required in C ? I couldn't find any thread with similar query or a good answer.

Answer 1

Global and static variables can only be initialized with constant expressions known at compile time. Calling your foo() function does not constitute using a constant expression. Further, the order in which global and static variables are initialized is not specified. Generally, calling foo() would mean that there must be a certain order, because the function can reasonably expect some other variables to be already initialized.

IOW, in C, neither of your code is executed before main().

In C++ there are ways around it, but not in C.

Answer 2

All the static variables are compile time and the function is giving the output at run time so you are initializing a compile time variable with a run time variable which is not possible so it is giving error.

Another example may be as follows

int main()
{
int p=9;
static int x=p;
}

the above code is also gives you compile time error,The cause is same as above.

Answer 3

If you are doing this in C rather than C++ you can only assign static variables values that are available during compilation. So the use of foo() is not permitted due to its value not being determined until runtime.