daemon()

Updated: May 06, 2022

Run a process in the background

Synopsis:

#include <stdlib.h>

int daemon( int nochdir, 
            int noclose );

Arguments:

nochdir
If this argument is 0, the current working directory is changed to the root directory (/).
noclose
If this argument is 0, standard input, standard output, and standard error are redirected to /dev/null.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The daemon() function allows programs to detach themselves from the controlling terminal and run in the background as system daemons.

This function calls fork() and setsid().

If daemon() is called with a noclose value of 0 and fails, then the state of file descriptors 0, 1, and 2 (STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO) is undefined.

Note: The controlling terminal behaves as in Unix System V, Release 4. An open() on a terminal device not already associated with another session causes the device to become the controlling terminal for that process.

The High Availability Manager can see death messages only from self-attached entities, processes that terminate abnormally, and tasks that are running in session 1. The daemon() function doesn't put the caller into that session; either make your process into a self-attached entity, or use procmgr_daemon() instead if you want to use your application with the HAM.

The HAM automatically switches to monitoring the new process that daemon() creates, if the original process was a self-attached entity. For more information, see the High Availability Framework Developer's Guide.

Returns:

Zero for success, or -1 if an error occurs (errno is set).

Classification:

Legacy Unix

Safety:  
Cancellation point No
Interrupt handler No
Signal handler Yes
Thread Yes