Windows互斥锁的gcc编译错误

Question

Why does the compile give me error: initializer element is not constant for a simple creation of a mutex HANDLE ghMutex = CreateMutex( NULL, FALSE, NULL);

I've tried searching but I am totally stumped. No matter what I do it won't compile.I even tried breaking it up:

HANDLE ghMutex;
ghMutex = CreateMutex( NULL, FALSE, NULL);

And the compile complain:

test.c:90:1: error: conflicting types for 'ghMutex'
test.c:89:8: note: previous declaration of 'ghMutex' was here
 HANDLE ghMutex;
        ^
test.c:90:1: error: initializer element is not constant
 ghMutex = CreateMutex( NULL, FALSE, NULL);

I think there is something wrong with my syntax I just don't know what.

Answer 1

For variables outside of a function, in C, they are part of the .data part of the executable, wherein memory is allocated at compile time. Without a constant (which the return variable of CreateMutex is not), there is no way to know how much memory to allocate.

To circumvent this, the compiler throws an error, so that you have to put the initialisation in a function so that memory is allocated dynamically at run-time.

This error isn't specific to Mutexes either. The following code would also throw the error:

int x = 3;
int y = 5;
int z = x*y;
int main(void)
{
    return 0;
}

If you're just interested in seeing how would would get around the error, then you could do something like this:

#include <windows.h>    

HANDLE ghMutex;

int main(void)
{
    ghMutex = CreateMutex(NULL, FALSE, NULL);
    CloseHandle(ghMutex);
    return 0;
}

This works because ghMutex is being allocated at run-time, because it is in a function. As OP stated in a comment: "The problem was [that ghMutex was] outside the function."

Or if you'd like a more in-depth view, the documentation also displays this: https://msdn.microsoft.com/en-gb/library/windows/desktop/ms686927(v=vs.85).aspx