NAME
daemon —
run in the background
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include
<stdlib.h>
int
daemon(int
nochdir, int
noclose);
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). On Mac OS X, the
use of this API is discouraged in favor of using
launchd(8).
Unless the argument nochdir is
non-zero,
daemon()
changes the current working directory to the root
(/).
Unless the argument noclose is
non-zero,
daemon()
will redirect standard input, standard output, and standard error to
/dev/null.
RETURN VALUES
The daemon() function returns 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() function may fail and set
errno for any of the errors specified for the library
functions fork(2) and setsid(2).
SEE ALSO
HISTORY
The daemon() function first appeared in
4.4BSD.
CAVEATS
There are limits to what you can do in the child process. To be
totally safe you should restrict yourself to only executing async-signal
safe operations (see
sigaction(2)) until such time as one of the exec functions is called.
All APIs, including global data symbols, in any framework or library should
be assumed to be unsafe after a fork() unless
explicitly documented to be safe or async-signal safe. If you need to use
these frameworks in the child process, you must exec. In this situation it
is reasonable to exec yourself.
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.