Example (Appendix A-4, "Intercepting arbitrary functions ..."), tested on OSX 10.9

buildrun.sh

#!/usr/bin/env bash

# see also https://stackoverflow.com/questions/617554/override-a-function-call-in-c
# this script builds and runs Appendix A-4 example in:
# "Intercepting arbitrary functions on Windows, UNIX, and Macintosh OS X platforms", by Daniel S. Myers and Adam L. Bazinet, University of Maryland (2004)

THIS_SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"

set -x
cd "$THIS_SCRIPT_DIR"

gcc -g -fno-common -c fopenwrap.c
gcc -dynamiclib -o libwrapper.dylib fopenwrap.o
gcc -g fopen.c -o fopentest.exe -L. -lwrapper

# create a file to be opened by fopentest.exe
cat > mytestfile.txt <<'EOF'
=== this is mytestfile.txt ===
Second line here...
Third line here...
EOF

DYLD_BIND_AT_LAUNCH=YES ./fopentest.exe ./mytestfile.txt

fopen.c

#include <stdio.h>
int main(int argc, char** argv) {
  char buf[80];
  FILE* fileptr = fopen(argv[1], "r");

  do {
    fgets(buf, 80, fileptr);
    if (feof(fileptr)) {
      break;
    }
    printf("%s", buf);
  } while(1);

  fclose(fileptr);
  return 0;
}

// "Finally, we compile the test code and link it with the wrapper library:
// gcc fopen.c -o fopentest -L. -lwrapper
// Remember to set the DYLD_BIND_AT_LAUNCH environment variable when running the test code:
// DYLD_BIND_AT_LAUNCH=YES ./fopentest"

fopenwrap.c

// "Once again, we wrap the fopen function. This example was tested on OS X 10.3.
// We use the same test code as before, in the file fopen.c.  For the wrapper, we put the following in fopenwrap.c:"

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

FILE* fopen(const char *path, const char *mode)
{
  printf("This is a wrapper function for fopen.\n");
  FILE* (*real_fopen)(const char*, const char*) =
    (FILE* (*)(const char*, const char*)) dlsym(RTLD_NEXT, "fopen");
  return real_fopen (path, mode);
}

// "To compile the code, we issue the following two commands:
// gcc -fno-common -c fopenwrap.c
// gcc -dynamiclib -o libwrapper.dylib fopenwrap.o"