Extern/Global Variables¶

Variables: Declaration and Definition ¶

Functions

Functions are complex
⟶ usually not written on one line
Readability ⟶ separate declaration and definition
… even when defined and called inside the same source file

Variables

Usually written on one line ⟶ declaration and definition
⟶ no need for a declaration
But: how does one declare a variable (make it known to the compiler without allocating memory), and define it in a different file?

Variables: Separating Declaration from Definition (1)¶

main.c

extern int g_lobal;
void print_g(void);

void main(void)
{
    g_lobal = 100;
    print_g();
}

g.c

#include <stdio.h>

int g_lobal;

void print_g(void)
{
    printf("%d\n",
           g_lobal);
}

Variables: Separating Declaration from Definition (2)¶

Compiler and linker work together

extern variable declaration ⟶ explicitly marked as declaration
Compiler does not set aside memory
There is no address yet ⟶ Compiler cannot insert address where variable is used
⟶ Inserts a reference, to be resolved by the linker