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#include <stdio.h>
// for: int printf(const char *, ...);

void
foo(int *a)
{
    *a = 123;
}

int
main(void)
{
    int a = 42;

    printf("before calling foo(): a = %d\n", a);
    foo(&a);
    printf("after calling foo(): a = %d\n", a);
}