extern
- C course
- Intermediate
- Lesson 60/116
- 5 min
Learn to share global variables with extern.
The extern keyword
extern declares that a variable is defined in another file. You write extern int counter; in a header; one source file has the definition int counter = 0;. Other files include the header and use counter.
extern is a declaration, not a definition; it allocates no memory. Multiple .c files can share the same global. The definition must appear in exactly one file; otherwise the linker errors. This mechanism is the standard way to share state across modules; used for config flags, counters and error codes.
extern and definition
To share a global: put extern int counter; in the header, and int counter = 0; in one .c file. The definition file is usually the module's own .c; for example counter.c.
When each .c file does #include "counter.h", it sees and uses counter. The initial value in the definition is optional but recommended; uninitialized globals are zero in C, but writing it explicitly is clearer. Multiple definitions are a linker error; keep the one-definition rule.
Practice Use an extern global
Simulate in one file: write extern int counter; declaration, then int counter = 5; definition. In int main(void) add counter += 10; and printf("%d\n", counter);. Don't forget <stdio.h>.
Starter code
#include <stdio.h>
// Declare counter with extern
// Define counter as 5
int main(void) {
// Increment counter by 10 and print
return 0;
}Sample solution
#include <stdio.h>
extern int counter;
int counter = 5;
int main(void) {
counter += 10;
printf("%d\n", counter);
return 0;
}Expected output
15
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the extern declaration
/* config.h */
____ int debug_mode;/* config.h */
extern int debug_mode;Explanation: extern int debug_mode; leaves the definition to another file.
2 If multiple .c files share a global, what happens?
- No file can define it
- Definition goes in the header
- Each file must define it
- Only one defines it Correct answer
Explanation: One file defines; the others declare with extern.
3 Match extern terms
| Term | Meaning |
|---|---|
extern int x; |
Header declaration |
int x = 0; |
Source definition |
#include "x.h" |
Header inclusion |
linker |
Merges definitions |