Variable declaration in a header file [duplicate]

You should declare the variable in a header file:

extern int x;

and then define it in one C file:

int x;

In C, the difference between a definition and a declaration is that the definition reserves space for the variable, whereas the declaration merely introduces the variable into the symbol table (and will cause the linker to go looking for it when it comes to link time).

You can (should) declare it as extern in a header file, and define it in exactly 1 .c file.

Note that that .c file should also use the header and that the standard pattern looks like:

// file.hextern int x;  // declaration// file.c#include "file.h"int x = 1;    // definition and re-declaration

If you declare it like

int x;

in a header file which is then included in multiple places, you'll end up with multiple instances of x (and potentially compile or link problems).

The correct way to approach this is to have the header file say

extern int x; /* declared in foo.c */

and then in foo.c you can say

int x; /* exported in foo.h */

THen you can include your header file in as many places as you like.