Multiple Definition of Symbol

linker

multiple definition of 'globalVar'

Description

The same symbol (variable or function) is defined in multiple source files.

Common Causes

• Defining a global variable in a header file without extern
• Including a source file instead of a header file
• Defining the same function in multiple source files

Example of Error

Error Code

// In globals.h
int globalVar = 10;  // Definition in header

// In file1.c
#include "globals.h"

// In file2.c
#include "globals.h"  // globalVar is defined again

Solution

Use extern for declarations in header files and define variables only once.

Corrected Code

// In globals.h
extern int globalVar;  // Declaration only

// In globals.c
int globalVar = 10;  // Definition in one source file

// In file1.c and file2.c
#include "globals.h"  // Only gets the declaration

Additional Tips

• Use extern for global variables in header files
• Define global variables in exactly one source file
• Consider using static for variables that don't need to be shared