Example of hooking pthread_create

thread_create_hook.c

// TO COMPILE:
// gcc -shared -fPIC -o thread_create_hook.so thread_create_hook.c -ldl
// TO USE (needs an absolute path):
// LD_PRELOAD=`pwd`/thread_create_hook.so ./application_to_hook

#define _GNU_SOURCE
#include <dlfcn.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

typedef void* (*StartRoutine)(void*);

typedef int (*PthreadCreateFunction)(pthread_t*, const pthread_attr_t *,
    StartRoutine, void*);

typedef struct {
  StartRoutine function;
  void *arg;
} OriginalStartRoutine;

void *StartRoutineHook(void *arg) {
  StartRoutine original_function = NULL;
  void *original_arg = NULL;
  OriginalStartRoutine *original = (OriginalStartRoutine *) arg;
  // The OriginalStartRoutine struct was allocated when our pthread_create hook was called, so
  // copy the contents and free it here.
  original_function = original->function;
  original_arg = original->arg;
  free(original);
  printf("In new thread! Orig=%p, arg=%p\n", original_function, original_arg);
  return original_function(original_arg);
}

int pthread_create(pthread_t *thread, const pthread_attr_t *attrs,
    void* (*start_routine)(void*), void *arg) {
  OriginalStartRoutine *start_info = (OriginalStartRoutine *) malloc(sizeof(OriginalStartRoutine));
  if (!start_info) {
    printf("malloc failed\n");
    return 1;
  }
  PthreadCreateFunction original = dlsym(RTLD_NEXT, "pthread_create");
  if (!original) {
    printf("Failed looking up original pthread_create.\n");
    return 1;
  }
  printf("Creating thread! Start routine: %p, arg: %p\n", start_routine, arg);
  start_info->function = start_routine;
  start_info->arg = arg;
  return original(thread, attrs, StartRoutineHook, start_info);
}