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

// for caching the original fopen implementation
FILE * (*original_fopen) (const char *, const char *) = NULL;

// our fopen override implementation
FILE * fopen(const char * filename, const char * mode) {
    // if we haven’t already, retrieve the original fopen implementation
    if (!original_fopen)
        original_fopen = dlsym(RTLD_NEXT, "fopen");

    // do our own processing; in this case just print the parameters
    printf("You're lucky, we just print a comment and not executing nasty commands == fopen: {%s,%s} ==\n", filename, mode);
    
    // call the original fopen with the same arguments
    FILE* f = original_fopen(filename, mode);
    
    // return the result
    return f;
}