NAME
daemon – run in the background
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
int
daemon(int nochdir, int noclose);
int
daemonfd(int chdirfd, int nullfd);
DESCRIPTION
The daemon() function is for programs wishing to detach themselves from the controlling terminal and run in the background as system daemons. The fork(2) system call is used; see CAVEATS below about the environment after a fork() (without a corresponding call to one of the exec routines).
If the argument nochdir is zero, daemon() changes the current working directory to the root (/).
If the argument noclose is zero, daemon() redirects standard input, standard output, and standard error to /dev/null.
The daemonfd() function is equivalent to the daemon() function except that arguments are the descriptors for the current working directory and to the descriptor to /dev/null.
If chdirfd is equal to (-1) the current working directory is not changed.
If nullfd is equals to (-1) the redirection of standard input, standard output, and standard error is not closed.
RETURN VALUES
The daemon() and daemonfd() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
ERRORS
The daemon() and daemonfd() function may fail and set errno for any of the errors specified for the library functions fork(2), open(2), and setsid(2).
SEE ALSO
HISTORY
The daemon() function first appeared in 4.4BSD. The daemonfd() function first appeared in FreeBSD 12.0.
CAVEATS
In multithreaded programs, the child process after fork() inherits copies of all mutexes and other synchronization state, but only the calling thread survives. Any locks held by other threads at the time of the call will remain permanently acquired in the child, causing deadlocks in any code that attempts to acquire them. Until one of the exec(3) functions is called, the child should restrict itself to async-signal safe operations (see sigaction(2)).
Unless the noclose argument is non-zero, daemon() will close the first three file descriptors and redirect them to /dev/null. Normally, these correspond to standard input, standard output, and standard error. However, if any of those file descriptors refer to something else, they will still be closed, resulting in incorrect behavior of the calling program. This can happen if any of standard input, standard output, or standard error have been closed before the program was run. Programs using daemon() should therefore either call daemon() before opening any files or sockets, or verify that any file descriptors obtained have values greater than 2.
The daemon() function temporarily ignores SIGHUP while calling setsid(2) to prevent a parent session group leader's calls to fork(2) and then _exit(2) from prematurely terminating the child process.