Section: Linux Programmer's Manual (3)
daemon - run in the background
int daemon(int nochdir, int noclose);
Feature Test Macro Requirements for glibc (see
Since glibc 2.21:
In glibc 2.19 and 2.20:
_DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
Up to and including glibc 2.19:
_BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
function is for programs wishing to detach themselves from the
controlling terminal and run in the background as system daemons.
changes the process's current working directory
to the root directory ("/");
otherwise, the current working directory is left unchanged.
redirects standard input, standard output and standard error
otherwise, no changes are made to these file descriptors.
(This function forks, and if the
succeeds, the parent calls
so that further errors are seen by the child only.)
If an error occurs,
returns -1 and sets
to any of the errors specified for the
For an explanation of the terms used in this section, see
Not in POSIX.1.
A similar function appears on the BSDs.
function first appeared in 4.4BSD.
The glibc implementation can also return -1 when
exists but is not a character device with the expected
major and minor numbers.
In this case,
need not be set.
The GNU C library implementation of this function was taken from BSD,
and does not employ the double-fork technique (i.e.,
that is necessary to ensure that the resulting daemon process is
not a session leader.
Instead, the resulting daemon
a session leader.
On systems that follow System V semantics (e.g., Linux),
this means that if the daemon opens a terminal that is not
already a controlling terminal for another session,
then that terminal will inadvertently become
the controlling terminal for the daemon.
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