Hook into libc's open

loudopen.c

#define _GNU_SOURCE

#include <stdio.h>
#include <dlfcn.h>
#include <sys/types.h>
#include <stdarg.h>

int open(const char* pathname, int flags, ...)
{
    int (*libc_open)(const char*, int, ...);
    printf("In our own open, opening %s\n", pathname);

    va_list ap;
    mode_t mode;

    va_start(ap, flags);
    mode = va_arg(ap, mode_t);
    va_end(ap);

    libc_open = dlsym(RTLD_NEXT, "open");
    return (*libc_open)(pathname,flags,mode);
}

/*

 build with: gcc -Wall -fPIC -shared -o loudopen.so loudopen.c -ldl
 run with: LD_PRELOAD=./loudopen.so cmd args

 $ echo "foo">bar.txt
 $ LD_PRELOAD=./loudopen.so cat bar.txt
 In our own open, opening bar.txt
 foo

*/