DAEMON(3) Library Functions Manual DAEMON(3)

daemonrun in the background

Standard C Library (libc, -lc)

#include <stdlib.h>

daemon(int nochdir, int noclose);

The () 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 () (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, () changes the current working directory to the root (/).

Unless the argument noclose is non-zero, () will redirect standard input, standard output, and standard error to /dev/null.

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.

The daemon() function may fail and set errno for any of the errors specified for the library functions fork(2) and setsid(2).

fork(2), launchd(8), setsid(2), sigaction(2)

The daemon() function first appeared in 4.4BSD.

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.

June 9, 1993 macOS 14.4