#define _FILE_OFFSET_BITS 64
#define _LARGEFILE_SOURCE
#define _LARGEFILE64_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdint.h>
#include <error.h>
#include <errno.h>
#include <assert.h>
#include <pthread.h>
#include <semaphore.h>

#define ERR(args...) fprintf(stderr, args);

#define READSIZE 4096
#define FILENAME "./bigfile"

int nthreads = 64;

extern int errno;
int fds[1024];
pthread_mutex_t locks[1024];
uint64_t filesize;
sem_t sem;

void * thread(void *arg)
{
  char buf[READSIZE];
  off_t offset;
  int index, fd, ret;

  fd = 0;
  do {
    for(index = 0; index < nthreads; index++) {
      if(pthread_mutex_trylock(&locks[index]) == 0) {
	// Found a free file descriptor
	fd = fds[index];
	break;
      }
    }
  } while(fd == 0);

  offset = (((uint64_t)random() << 32) | random()) % (filesize - READSIZE);

  assert(offset < filesize - READSIZE);

  if(lseek(fd, offset, SEEK_SET) == -1) {
    ERR("thread(): lseek(): %s\n", strerror(errno));
    goto done;
  }
  
  if((ret = read(fd, buf, READSIZE)) != READSIZE) {
    if(ret == -1) {
      ERR("thread(): read(): %s\n", strerror(errno));
    } else if(ret == 0) {
      ERR("thread(): read(): EOF\n");
    } else {
      ERR("thread(): read(): Only read %d of %d bytes\n", ret, READSIZE);
    }
  }
  
 done:
  
  pthread_mutex_unlock(&locks[index]);
  
  sem_post(&sem);
}

int main(int argc, char **argv)
{
  pthread_attr_t attr;
  pthread_t child;
  struct stat fstat;
  int i;

  if(argc == 2) {
    nthreads = atoi(argv[1]);
    assert(nthreads < 1024);
  }

  if(pthread_attr_init(&attr) != 0) {
    ERR("main(): pthread_attr_init(): failed\n");
    exit(1);
  }

  if(pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED) != 0) {
    ERR("main(): pthread_attr_setdetachstate(): failed\n");
    exit(1);
  }

  if(stat(FILENAME, &fstat) == -1) {
    ERR("main(): stat(): %s\n", strerror(errno));
    exit(1);
  }

  filesize = fstat.st_size;

  for(i = 0; i < nthreads; i++) {
    if((fds[i] = open(FILENAME, O_RDONLY | O_LARGEFILE)) == -1) {
      ERR("main(): open(): %s\n", strerror(errno));
      exit(1);
    }
    if(pthread_mutex_init(&locks[i], NULL) != 0) {
      ERR("main(): pthread_mutex_init(): %s\n", strerror(errno));
      exit(1);
    }
  }

  if(sem_init(&sem, 0, nthreads) == -1) {
    ERR("main(): sem_init(): %s\n", strerror(errno));
    exit(1);
  }

  while(1) {
    if(sem_wait(&sem) == 0) {
      if(pthread_create(&child, &attr, thread, NULL) != 0) {
	ERR("main(): pthread_create(): failed\n");
	exit(1);
      }
    }
  }
}