updwtmp(3)

💖 updwtmp(3) — Library Functions Manual 💖

okay bestie, pull up a chair, we're talking about updwtmp and logwtmp — the little duo whose whole job is appending entries to the wtmp file. yes, that file. the "who logged in and when" receipt printer. no cap.

✨ NAME ✨

updwtmp, logwtmp — append an entry to the wtmp file

📚 LIBRARY 📚

System utilities library (libutil, -lutil)

like, you gotta link the util library. <pink>don't forget the -lutil</pink>, she's right there in the name, iykyk.

🌸 SYNOPSIS 🌸

#include <utmp.h>

void updwtmp(const char *wtmp_path, const struct utmp *ut);
void logwtmp(const char *line, const char *user, const char *host);

🦋 DESCRIPTION 🦋

so updwtmp() is the simple one. it takes that utmp structure — you know, the ut — and appends it to the wtmp file. that's it. that's the whole vibe. in, out, done. 💅

and logwtmp() is the one that does the ~crafting~. it builds a utmp structure all by itself using line, user, host, plus the current time and the current process ID 🌈. then — and this is the iconic part — it calls updwtmp() to go append that freshly constructed structure to the wtmp file. she's giving main character energy that delegates. periodt.

🎀 FILES 🎀

• /var/log/wtmp — database of past user logins

the receipts. the archive. the "who was here before you" scroll. 📜

👑 ATTRIBUTES 👑

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

• updwtmp(), logwtmp() — Thread safety: MT-Unsafe sig:ALRM timer

⚠️ okay so heads up: MT-Unsafe sig:ALRM timer. that's not a cute little sticker, that's a real warning. read it, respect it, don't get got. 💀

⭐ VERSIONS ⭐

for consistency with the other <rainbow>"utmpx"</rainbow> functions (peep getutxent(3)), glibc has been providing this since glibc 2.1:

#define _GNU_SOURCE          /* See feature_test_macros(7) */
#include <utmpx.h>
void updwtmpx (const char *wtmpx_path, const struct utmpx *utx);

and this function? does the exact same task as updwtmp(), with one ✨ twist ✨ — it takes a utmpx structure as its last argument instead. same energy, different struct. slay.

💜 STANDARDS 💜

None.

like literally none, don't come for her, she's doing her best. 🙈

🧚 HISTORY 🧚

Solaris, NetBSD.

she's got history. she's been around. iconic lineage honestly. 🌸

🔥 SEE ALSO 🔥

getutxent(3), wtmp(5)

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and that's the whole page bestie!! 💖 this was updwtmp(3) from the Linux man-pages 6.20, dated 2026-02-08 — go log something into /var/log/wtmp and live your truth. 🫶✨