sleep(3)

💤 sleep(3) — the "bestie go take a nap" function

okay so this is sleep(3), and yeah it lives in the Library Functions Manual section 3, which is where all the library-call girlies hang out. it's a function, not a command, so no shell prompt energy here — you're calling this from C, bestie. 💅

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🌸 NAME

sleep — sleep for a specified number of seconds

like. literally just nap for a bit. that's the whole job. iconic behavior honestly.

📚 LIBRARY

Standard C library (libc, -lc)

so yeah, link it with -lc like every other basic libc function. nothing exotic. 🎀

✨ SYNOPSIS

#include <unistd.h>

unsigned int sleep(unsigned int seconds);

one include, one call, one lil unsigned int in and one unsigned int out. we love a minimal API. 🦋

🫶 DESCRIPTION

okay so sleep() makes the calling thread go night-night 🛌 — it sleeps either until the number of real-time seconds you passed in have actually elapsed, or until a signal shows up that isn't being ignored.

that's it. that's the tea. also heads up — it's the calling thread that snoozes, not the whole process, so your other thread besties keep vibing. 💖

🎀 RETURN VALUE

• 0 — the requested time fully elapsed, you got your full nap, slay 😌
• the number of seconds left to sleep — the call got interrupted by a signal handler, so it hands you back the leftover time like "here's what you still owe the sandman" ⏳

💀 ATTRIBUTES

for the explanation of the terms used in this section, go peek at attributes(7). it's giving required reading, ngl.

• Interface: sleep()
• Attribute: Thread safety
• Value: MT-Unsafe sig:SIGCHLD/linux

no cap, that MT-Unsafe with the sig:SIGCHLD/linux tag is the part you screenshot and remember. 😭

🌈 VERSIONS

on Linux, sleep() is implemented via nanosleep(2). so if you wanna know about the clock it's using, the nanosleep(2) man page has the whole discussion — go read it, i'll wait 🫶

on some systems, sleep() might be implemented with alarm(2) and SIGALRM instead — and yeah, POSIX.1 permits that lil switcheroo. which means mixing calls to alarm(2) and sleep() is a bad idea. like, genuinely bad. don't do it. 💀

⭐ STANDARDS

POSIX.1-2008. certified, official, legitimate. 👑

🦋 HISTORY

POSIX.1-2001. so it's been around, it's a legacy queen, it's been doing this since forever. 💜

😭 CAVEATS

listen. this is the scary part. danger energy only:

using longjmp(3) from a signal handler or modifying the handling of SIGALRM while you're sleeping will cause undefined results. undefined!! as in the universe makes no promises!! do not do this!! 🙈

fr fr, undefined behavior means nobody can tell you what happens. not even the kernel. not even your most knowledgeable mutual. just don't.

👑 SEE ALSO

sleep(1), alarm(2), nanosleep(2), signal(2), signal(7)

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and that's the page, bestie!! this was pulled straight from Linux man-pages 6.20, dated 2026-02-08 🩷 sleep well, ily, go drink some water ✨🧚💖