#include <stdio.h>
#include <stdlib.h>

void foo(int* a_n) {  // set value *at* a_n to 5
    *a_n = 5;
    // Okay for foo(…) to write to main(…)'s stack frame
    // i.e., if main passes the address of one if its local
    // variables.  However, if foo(…) returned the address
    // of something in its stack frame, main(…) should
    // not touch it.
}

int main(int argc, char* argv[]) {
    int n = 0;
    printf("Before:  n == %d\n", n);

    foo(&n);  // pass the address of n to foo(…)

    printf("After:   n == %d\n", n);
    /* Before:  n == 0
     * After:   n == 5
     */

    return EXIT_SUCCESS;
}
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