// gcc x.c
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>

#define free myfree

void myfree(void *ptr);

void myfree(void *ptr){
        printf("my free called\n");
}
void foo() {
    char *s = (char *)malloc(100);
    sprintf(s, "%s", "hello there");
    printf("'%s'\n", s);
    free(s);
}
void main(void){ foo(); }