Section (3) daemon
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_zsingle_quotesz_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