daemon(3)

✨ daemon(3) — the "go run in the background bestie" page ✨

okay so this is daemon(3) from the Library Functions Manual, and it lives in the Standard C library, aka libc, aka -lc. she's been around forever and she's lowkey iconic. let's get into it 💅

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🌸 NAME 🌸

daemon — run in the background

📚 LIBRARY 📚

Standard C library (libc, -lc) 🫶

🎀 SYNOPSIS 🎀

#include <unistd.h>

int daemon(int nochdir, int noclose);

🦋 Feature Test Macro Requirements for glibc (see feature_test_macros(7)): 🦋

daemon():

• Since glibc 2.21:
  • _DEFAULT_SOURCE
• 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)

💖 DESCRIPTION 💖

so the daemon() function is for programs that literally want to detach themselves from the controlling terminal and go run in the background as system daemons. main character energy, no audience, just vibes. 🧚

if nochdir is zero, daemon() changes the process's current working directory to the root directory ("/"). otherwise (i.e. if nochdir isn't zero), the current working directory is left unchanged. your call, bestie.

if noclose is zero, daemon() redirects standard input, standard output, and standard error to /dev/null. otherwise, no changes are made to these file descriptors. it's giving "silence is golden" ✨

🔥 RETURN VALUE 🔥

(this function forks, and if the fork(2) succeeds, the parent calls _exit(3), so that further errors are seen by the child only.) — yes, that's a whole thing, keep reading.

• on success, daemon() returns zero. slay. ✅
• if an error occurs, daemon() returns -1 and sets errno to any of the errors specified for the fork(2) and setsid(2). 😭

👑 ATTRIBUTES 👑

for an explanation of the terms used in this section, see attributes(7).

• daemon() — Thread safety: MT-Safe

🌈 VERSIONS 🌈

a similar function appears on the BSDs. (sisters, not twins fr fr)

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 set. sneaky lil edge case, ngl. 💀

⭐ STANDARDS ⭐

None. (she's not standardised, she's just her. periodt.)

🍓 HISTORY 🍓

4.4BSD.

💜 BUGS 💜

okay bestie, this one's important, don't scroll past it 🙈

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 daemon. 💀

not me warning you that your daemon might accidentally adopt a terminal it did not ask for. that's the bug, that's the vibe, be aware.

🌸 SEE ALSO 🌸

• fork(2)
• setsid(2)
• daemon(7)
• logrotate(8)

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and that's the whole page, bestie!! this was Linux man-pages 6.20, dated 2026-06-05, and the page we just read was daemon(3) 🫶 go forth and detach responsibly 💖✨🎀