What is the best way to accomplish the following in C?

#include <stdio.h>

struct A
{
    int x;
};

struct A createA(int x)
{
    struct A a;

    a.x = x;

    return a;
}

struct A a = createA(42);

int main(int argc, char** argv)
{
    printf("%d\n", a.x);

    return 0;
}

When I try to compile the above code, the compiler reports the following error:

"initializer element is not constant"

The bad line is this one:

struct A a = createA(42);

Can someone explain what is wrong? I'm not very experienced in C. Thanks!