C passes arguments by value, including pointers. To modify a variable belonging to the calling function, pass its address to a function and use a pointer parameter to access the original object.

#include <stdio.h>

void changeValue(int *ptr)
{
    *ptr = 100;
}

int main(void)
{
    int value = 10;

    printf("Before function call: %d\n", value);

    changeValue(&value);

    printf("After function call: %d\n", value);

    return 0;
}

Example Output #

Before function call: 10
After function call: 100

Explanation #

The function call passes the address of value:

changeValue(&value);

The address is copied into the pointer parameter ptr:

void changeValue(int *ptr)

The dereference expression:

*ptr = 100;

modifies the integer object at the address stored in ptr. Since that address refers to value, the original variable changes to 100.

How the Pointer Works #

A function can modify an original variable through a pointer, provided the pointer refers to a valid, writable object. This is commonly used when a function needs to update caller-owned data or return multiple results through output parameters.