Home > Articles, Operating Systems > Implementation of a C function to convert a typical process into a background service (Daemon) – Version 2.

Implementation of a C function to convert a typical process into a background service (Daemon) – Version 2.

In a previous article I had shown to you how a typical process in GNU/Linux operating systems can be converted to a background daemon process. The source code from the previous article was taken from an old project I wrote years ago. Nowadays, I have rewritten the daemonizing function in a new project and I think it could be nice to share it. I have simplified the function and now it has a clearer and simpler API.

Some of the differences / improvements of the new implementation are:

  1. The source code unit uses a header file.
  2. The source code unit supports internationalization.
  3. The source code handles POSIX interruptible system calls (EINTR, EAGAIN error codes).
  4. The double fork design pattern is used.
  5. The PID file is not locked.
  6. There is a better handling of the inherited file descriptors.
  7. The working directory path and the PID file path can be given as parameters.
  8. New source code units are introduced for handling both the PID and the PID file.

At any time you can see the source code of the header, the implementation and the client using it.

Below, is the header introducing the new interface:

/*
 *  Copyright (C) 2014  Efstathios Chatzikyriakidis (contact@efxa.org).
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#ifndef _DAEMONIZE_UTILITY_H_
#define _DAEMONIZE_UTILITY_H_

#include "shared-types.h"

/*
 * function prototypes.
 */

bool_t daemonize (const string_t const working_directory_path,
                  const string_t const pid_file_path);

#endif // _DAEMONIZE_UTILITY_H_

Next, follows an example of a client that wants to convert its self in a daemon process:

/*
 *  Copyright (C) 2014  Efstathios Chatzikyriakidis (contact@efxa.org).
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#include "daemonize-utility.h"

/*
 * functions.
 */

int
main (int argc, const string_t const argv [])
{
  ...

  const bool_t daemon_success = daemonize ("/", "/var/run/service.pid");

  if (daemon_success)
  {
    // run code as daemon.
  }

  ...
}

Lastly, the implementation of the new daemonize function:

/*
 *  Copyright (C) 2014  Efstathios Chatzikyriakidis (contact@efxa.org).
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#include <sys/stat.h>
#include <sys/file.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

#include "interruption-wrapper-helper-macros.h"
#include "daemonize-utility.h"
#include "pid-file-utility.h"
#include "logging-utility.h"
#include "file-utility.h"
#include "i18n-macros.h"

/*
 * messages.
 */

static const string_t const failed_to_create_new_process_message =
  N_("Failed to create a new process.");

static const string_t const failed_to_create_new_session_message =
  N_("Failed to create a new session.");

static const string_t const failed_to_open_virtual_null_device_file_message =
  N_("Failed to open the virtual null device file.");

static const string_t const failed_to_redirect_standard_io_streams_message =
  N_("Failed to redirect the standard I/O streams.");

static const string_t const failed_to_change_working_directory_message =
  N_("Failed to change the working directory.");

static const string_t const failed_to_write_pid_file_message =
  N_("Failed to write the pid file.");

static const string_t const failed_to_remove_pid_file_message =
  N_("Failed to remove the pid file.");

static const string_t const failed_to_close_file_descriptor_message =
  N_("Failed to close the file descriptor:");

static const string_t const failed_to_close_virtual_null_device_file_descriptor_message =
  N_("Failed to close the descriptor of the virtual null device file.");

static const string_t const pid_file_is_not_readable_message =
  N_("The pid file is not readable.");

static const string_t const another_instance_probably_running_message =
  N_("Another instance probably is running.");

/*
 * variables.
 */

static const mode_t file_mode_creation_mask = 027;

static const string_t const virtual_null_device_file_path = "/dev/null";

/*
 * functions.
 */

bool_t
daemonize (const string_t const working_directory_path,
           const string_t const pid_file_path)
{
  // is it already a daemon process?
  if (getppid () == 1)
  {
    return true;
  }

  // try to create a new process.
  pid_t pid = HANDLE_EAGAIN(fork ());

  // failed to create a new process?
  if (pid < 0)
  {
    log_error (_(failed_to_create_new_process_message));
    return false;
  }
  // parent continues in this code section.
  else if (pid > 0)
  {
    // forget the parent process.
    exit (EXIT_SUCCESS);
  }

  /* try to make new session to ensure that
     we do not have a controlling terminal. */
  if (setsid () < 0)
  {
    log_error (_(failed_to_create_new_session_message));
    return false;
  }

  /* double fork again to ensure that we
     can't regain a controlling terminal. */
  pid = HANDLE_EAGAIN(fork ());

  // failed to create a new process?
  if (pid < 0)
  {
    log_error (_(failed_to_create_new_process_message));
    return false;
  }
  // parent continues in this code section.
  else if (pid > 0)
  {
    // forget the parent process.
    exit (EXIT_SUCCESS);
  }

  // try to change the working directory.
  if (chdir (working_directory_path) < 0)
  {
    log_error (_(failed_to_change_working_directory_message));
    return false;
  }

  // change file creation mask.
  umask (file_mode_creation_mask);

  // try to open the virtual null device file.
  const int virtual_null_device_file_descriptor =
    HANDLE_EINTR(open (virtual_null_device_file_path, O_RDWR));

  // failed to open the virtual null device file?
  if (virtual_null_device_file_descriptor < 0)
  {
    log_error (_(failed_to_open_virtual_null_device_file_message));
    return false;
  }

  // try to redirect standard I/O streams to virtual null device file.
  if (HANDLE_EINTR(dup2 (virtual_null_device_file_descriptor, STDIN_FILENO)) < 0 ||
      HANDLE_EINTR(dup2 (virtual_null_device_file_descriptor, STDOUT_FILENO)) < 0 ||
      HANDLE_EINTR(dup2 (virtual_null_device_file_descriptor, STDERR_FILENO)) < 0)
  {
    // try to close the virtual null device file descriptor.
    if (close (virtual_null_device_file_descriptor) < 0)
    {
      log_error (_(failed_to_close_virtual_null_device_file_descriptor_message));
      return false;
    }

    log_error (_(failed_to_redirect_standard_io_streams_message));
    return false;
  }

  // try to close the virtual null device file descriptor.
  if (close (virtual_null_device_file_descriptor) < 0)
  {
    log_error (_(failed_to_close_virtual_null_device_file_descriptor_message));
    return false;
  }

  // try to close all inherited file descriptors.
  for (int fd = getdtablesize() - 1; fd >= 0; fd--)
  {
    if (close (fd) < 0 && errno != EBADF)
    {
      log_error ("%s '%d'.", _(failed_to_close_file_descriptor_message), fd);
      return false;
    }
  }

  // in case the pid file already exists.
  if (file_exists (pid_file_path))
  {
    // try to check if the pid file is readable.
    if (!file_is_readable (pid_file_path))
    {
      log_error (_(pid_file_is_not_readable_message));
      return false;
    }

    // try to read the pid from the pid file.
    const pid_t pid_from_pid_file = read_pid_from_file (pid_file_path);

    // if there is a pid that really exists in the system.
    if (pid_from_pid_file != -1 && pid_exists (pid_from_pid_file))
    {
      log_error (_(another_instance_probably_running_message));
      return false;
    }

    // try to remove the pid file since it is garbage.
    if (!remove_file (pid_file_path))
    {
      log_error (_(failed_to_remove_pid_file_message));
      return false;
    }
  }

  // try to create the pid file and write the pid of the daemon to it.
  if (!write_pid_to_file (pid_file_path, getpid ()))
  {
    log_error (_(failed_to_write_pid_file_message));
    return false;
  }

  // the program runs as a daemon.
  return true;
}
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