From a project I worked on back in 2004

double_fork.cc

#include "double_fork.hh"
#include <cstdlib>
#include <cerrno>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <fcntl.h>

int double_fork()/*{{{*/
{
  pid_t pid;

  // First fork
  pid = fork();

  if (pid == 0)
  {
    // Child: second fork
    pid = fork();
    
    if (pid == 0)
    {
      // Child
      return 0;
    }
    else if (pid > 0)
    {
      // Parent
      exit(0);
    }
    else
    {
      // Error
      exit(1);
    }
    
  }
  else if (pid > 0)
  {
    // Parent: wait for child to exit
    int status;
    for (;;)
    {
      int result = waitpid(pid, &status, 0);

      if (result < 0)
      {
        // Error
        if (errno != EINTR)
          return result;
      }
      else
        break;
    }

    if (WIFEXITED(status) && WEXITSTATUS(status) == 0)
      // The child exited with status 0 -- success!
      return 1;
    else
      return -1;
  }
  else
  {
    // Error
    return pid;
  }

}/*}}}*/

bool double_fork_daemon()
{
  bool success = false;

  int result = double_fork();
  if (result > 0)
  {
    // Parent
    exit(0);
  }
  else if (result == 0)
  {
    // Child
    success = true;

    umask(0);
    setsid();
    chdir("/");

    // Open these first, so they don't get STD(IN|OUT|ERR)_FILENO
    int in  = open("/dev/null", O_RDONLY, 0);
    int out = open("/dev/null", O_WRONLY, 0);

    // Close these
    close(STDIN_FILENO);
    close(STDOUT_FILENO);
    close(STDERR_FILENO);

    // Replace with /dev/null
    dup2(in , STDIN_FILENO);
    dup2(out, STDOUT_FILENO);
    dup2(out, STDERR_FILENO);

    close(in);
    close(out);
  }

  // Error: fall through

  return success;
}

double_fork.hh

#ifndef double_fork_hh
#define double_fork_hh

int double_fork();
bool double_fork_daemon();

#endif