Why does the "initializer element is not constant" error appear

Look at the following code:

#include <stdio.h>
int a = 1;
int b = 2;
int c = a+b;

int main () {
    printf ("C is%d\n", c);
    return 0;
}

Gcc-o Test test.c Compile error: initializer element is not constant

-----

Cause: The value of global variable C cannot be determined at compile time, to be determined in execution (compilation principle: ）

Workaround:

#include <stdio.h>
int a = 1;
int b = 2;
int C; DECLARE only

int main () {
	c = a + B;//Assign a value in the main function
	printf ("C is%d\n", c);
	return 0;
}