daemon — run in the background
daemon() function is for
programs wishing to detach themselves from the controlling
terminal and run in the background as system daemons.
daemon() changes the
process's current working directory to the root directory
("/"); otherwise, the current working directory is left
standard input, standard output and standard error to
/dev/null; otherwise, no
changes are made to these file descriptors.
(This function forks, and if the fork(2) succeeds, the
parent calls _exit(2), so that further
errors are seen by the child only.) On success
daemon() returns zero. If an error occurs,
daemon() returns −1 and
errno to any of the errors
specified for the fork(2) and setsid(2).
For an explanation of the terms used in this section, see attributes(7).
Not in POSIX.1. A similar function appears on the BSDs.
daemon() function first
appeared in 4.4BSD.
The glibc implementation can also return −1 when
/dev/null exists but is not a
character device with the expected major and minor numbers.
In this case,
errno need not be
The GNU C library implementation of this function was
taken from BSD, and does not employ the double-fork technique
(i.e., fork(2), setsid(2), fork(2)) that is necessary
to ensure that the resulting daemon process is not a session
leader. Instead, the resulting daemon
is 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
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(#)daemon.3 8.1 (Berkeley) 6/9/93
Added mentioning of glibc weirdness wrt unistd.h. 5/11/98, Al Viro