C Program to Redeclaration of global variable

In C programming, variable redeclaration refers to declaring a variable more than once in the same scope. The behavior of redeclaration varies between global and local variables, and differs between C and C++. Understanding these differences is crucial for avoiding compilation errors.

Syntax

// Global variable redeclaration
int var;
int var;  // Allowed in C without initialization

// Local variable redeclaration  
int main() {
    int var;
    int var;  // Not allowed in C
}

Global Variable Redeclaration Without Initialization

C allows redeclaration of global variables when they are not initialized −

#include <stdio.h>

int var;
int var;  /* Redeclaration allowed in C */

int main() {
    printf("Var = %d
", var);
    return 0;
}

Var = 0

Global Variable Redeclaration With Initialization

When one declaration includes initialization, C allows this pattern −

#include <stdio.h>

int var;      /* First declaration without initialization */
int var = 10; /* Second declaration with initialization */

int main() {
    printf("Var = %d
", var);
    return 0;
}

Var = 10

Local Variable Redeclaration

C does not allow redeclaration of local variables in the same scope −

#include <stdio.h>

int main() {
    int var;
    int var; /* Error: redeclaration not allowed */
    printf("Var = %d
", var);
    return 0;
}

error: redeclaration of 'var' with no linkage

Comparison Table

Key Points

• C follows tentative definition rules for global variables − multiple declarations are merged into one definition.
• Global variables without explicit initialization are automatically initialized to zero.
• Local variables have block scope and cannot be redeclared in the same scope.
• Only one initialization is allowed even for global variable redeclarations.

Conclusion

C allows global variable redeclaration due to tentative definitions, but prohibits local variable redeclaration. This flexibility in global scope helps with separate compilation units while maintaining strict rules within functions.