syslog(3)

💖 syslog(3) 💖 — it's the Library Functions Manual page, bestie, and it's all about screaming into the system logger

okay so picture this: your program has ✨thoughts✨ and it needs to tell the system about them. That's this whole page. We're talking closelog, openlog, syslog, vsyslog — the little squad that sends messages to the system logger. No cap.

🌸 NAME 🌸

closelog, openlog, syslog, vsyslog — send messages to the system logger 💌

📚 LIBRARY 📚

Standard C library (libc, -lc). You know the vibe.

✨ SYNOPSIS ✨

#include <syslog.h>

void openlog(const char *ident, int option, int facility);
void syslog(int priority, const char *format, ...);
void closelog(void);

void vsyslog(int priority, const char *format, va_list ap);

🧚 Feature Test Macro Requirements for glibc (peep feature_test_macros(7) for the deets) 🧚

• vsyslog():
  • Since glibc 2.19: _DEFAULT_SOURCE
  • glibc 2.19 and earlier: _BSD_SOURCE

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🫶 DESCRIPTION 🫶

🌈 openlog() 🌈

openlog() opens a connection to the system logger for a program. Slay, now you're connected.

The string pointed to by ident is prepended to every message, and is typically set to the program name. If ident is NULL, the program name is used. (POSIX.1-2008 does not specify the behavior when ident is NULL — so like, that part is giving mystery. 🙈)

The option argument specifies flags which control the operation of openlog() and subsequent calls to syslog(). The facility argument establishes a default to be used if none is specified in subsequent calls to syslog(). The values that may be specified for option and facility are described below. Down we go, bestie. ⬇️

The use of openlog() is optional; it will automatically be called by syslog() if necessary, in which case ident will default to NULL. Iconic behavior honestly, it just handles it. 💅

🦋 syslog() and vsyslog() 🦋

syslog() generates a log message, which will be distributed by syslogd(8).

The priority argument is formed by ORing together a facility value and a level value (described below). If no facility value is ORed into priority, then the default value set by openlog() is used, or, if there was no preceding openlog() call, a default of LOG_USER is employed. She has a fallback. Always. 👑

The remaining arguments are a format, as in printf(3), and any arguments required by the format, except that the two-character sequence %m will be replaced by the error message string strerror(errno). The format string need not include a terminating newline character. So you don't gotta add the \n, and honestly? We love a low-maintenance format string. 😌

The function vsyslog() performs the same task as syslog() with the difference that it takes a set of arguments which have been obtained using the stdarg(3) variable argument list macros. Same energy, different ✨packaging✨.

🎀 closelog() 🎀

closelog() closes the file descriptor being used to write to the system logger. The use of closelog() is optional. Byeee, connection. 🫡

🍓 Values for option 🍓

The option argument to openlog() is a bit mask constructed by ORing together any of the following values:

• LOG_CONS — Write directly to the system console if there is an error while sending to the system logger. Backup plan activated. 🚨
• LOG_NDELAY — Open the connection immediately (normally, the connection is opened when the first message is logged). This may be useful, for example, if a subsequent chroot(2) would make the pathname used internally by the logging facility unreachable. Basically: do it NOW before the paths get weird. 🔥
• LOG_NOWAIT — Don't wait for child processes that may have been created while logging the message. (The GNU C library does not create a child process, so this option has no effect on Linux.) Cute flag, does nothing here. 💀
• LOG_ODELAY — The converse of LOG_NDELAY; opening of the connection is delayed until syslog() is called. (This is the default, and need not be specified.) The chill option. You probably don't even need to type it.
• LOG_PERROR — (Not in POSIX.1-2001 or POSIX.1-2008.) Also log the message to stderr. Double posting, but make it intentional.
• LOG_PID — Include the caller's PID with each message. Who dis? Now we know. 🕵️

👑 Values for facility 👑

The facility argument is used to specify what type of program is logging the message. This lets the configuration file specify that messages from different facilities will be handled differently. Sorting hat behavior. 🎩

• LOG_AUTH — security/authorization messages
• LOG_AUTHPRIV — security/authorization messages (private)
• LOG_CRON — clock daemon (cron and at)
• LOG_DAEMON — system daemons without separate facility value
• LOG_FTP — ftp daemon
• LOG_KERN — kernel messages (these can't be generated from user processes)
• LOG_LOCAL0 through LOG_LOCAL7 — reserved for local use
• LOG_LPR — line printer subsystem
• LOG_MAIL — mail subsystem
• LOG_NEWS — USENET news subsystem
• LOG_SYSLOG — messages generated internally by syslogd(8)
• LOG_USER (default) — generic user-level messages
• LOG_UUCP — UUCP subsystem

💜 Values for level 💜

This determines the importance of the message. The levels are, in order of decreasing importance:

• LOG_EMERG — system is unusable. 🚩
• LOG_ALERT — action must be taken immediately
• LOG_CRIT — critical conditions
• LOG_ERR — error conditions
• LOG_WARNING — warning conditions
• LOG_NOTICE — normal, but significant, condition
• LOG_INFO — informational message
• LOG_DEBUG — debug-level message

The function setlogmask(3) can be used to restrict logging to specified levels only. Picky logging, we respect it. 💅

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🌟 ATTRIBUTES 🌟

For an explanation of the terms used in this section, see attributes(7). (Original was a table, but tables are not the vibe here, so bestie, here it is as a list, facts fully intact. ✨)

• openlog(), closelog() — Thread safety — MT-Safe
• syslog(), vsyslog() — Thread safety — MT-Safe env locale

📜 STANDARDS 📜

• syslog()
• openlog()
• closelog()

POSIX.1-2008.

• vsyslog()

None.

🕰️ HISTORY 🕰️

• syslog() — 4.2BSD, SUSv2, POSIX.1-2001.
• openlog(), closelog() — 4.3BSD, SUSv2, POSIX.1-2001.
• vsyslog() — 4.3BSD-Reno.

POSIX.1-2001 specifies only the LOG_USER and LOG_LOCAL* values for facility. However, with the exception of LOG_AUTHPRIV and LOG_FTP, the other facility values appear on most UNIX systems.

The LOG_PERROR value for option is not specified by POSIX.1-2001 or POSIX.1-2008, but is available in most versions of UNIX.

🧠 NOTES 🧠

The argument ident in the call of openlog() is probably stored as-is. Thus, if the string it points to is changed, syslog() may start prepending the changed string, and if the string it points to ceases to exist, the results are undefined. Most portable is to use a string constant. Deadass — just use a constant, bestie. Don't let your ident go off and become something else mid-run. 🙈

⚠️ Never pass a string with user-supplied data as a format. Like, never ever. Use the following instead:

syslog(priority, "%s", string);

That's the whole safety tip. It's giving "don't hand random strangers the keys to your format string." 🚫

👀 SEE ALSO 👀

journalctl(1), logger(1), setlogmask(3), syslog.conf(5), syslogd(8)

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and that's the page, bestie!! 💖✨🎀 this came to you from Linux man-pages 6.20, dated 2026-02-08, and it's syslog(3) through and through. go log something iconic. 🦋👑🌸