Passing by pointer to change a caller variable

Simulating pass-by-reference in C, returning multiple values, and modifying pointers themselves.

11 min read · Lesson 4 of 6 · Pro

C has only pass-by-value

There is no pass-by-reference in C. Every argument is copied. What people call "pass by reference in C" is really "pass a copy of an address" — and because the copy of the address still points at the original, the function can reach it.

Hold that sentence. It explains everything below.

#include <stdio.h>

void broken(int x)   { x = 99; }
void works(int *x)   { *x = 99; }

int main(void)
{
    int a = 1, b = 1;
    broken(a);
    works(&b);
    printf("a = %d, b = %d\n", a, b);   /* a = 1, b = 99 */
    return 0;
}

The rest of this lesson is Pro

The free lessons of Pointers and Memory in C finish what they start — read those first if you have not. This one goes further, and it is part of the paid half.

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