err(3)

🌸 NAME 🌸

okay so bestie, gather round, this is the err(3) page from the Library Functions Manual 💖 and it's all about formatted error messages, no cap.

• err, verr, errx, verrx, warn, vwarn, warnx, vwarnx — formatted error messages ✨

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📚 LIBRARY 📚

Standard C library (libc, -lc) — it's giving classic, it's giving foundational, it's literally the C library 💅

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🎀 SYNOPSIS 🎀

#include <err.h>

[[noreturn]] void err(int eval, const char *fmt, ...);
[[noreturn]] void errx(int eval, const char *fmt, ...);

void warn(const char *fmt, ...);
void warnx(const char *fmt, ...);

#include <stdarg.h>

[[noreturn]] void verr(int eval, const char *fmt, va_list args);
[[noreturn]] void verrx(int eval, const char *fmt, va_list args);

void vwarn(const char *fmt, va_list args);
void vwarnx(const char *fmt, va_list args);

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💜 DESCRIPTION 💜

so the err() and warn() family of functions are the ones that display a formatted error message on the standard error output. every single time, no exceptions, they print the last component of the program name, then a colon character, then a space. then, if the fmt argument is not NULL, they output the printf(3)-like formatted error message. and then the output is terminated by a newline character. slay 💅

• the err(), verr(), warn(), and vwarn() functions append an error message obtained from strerror(3) based on the global variable errno, and that message is preceded by another colon and space — unless the fmt argument is NULL. 😌
• the errx() and warnx() functions do not append an error message. nope, not doing it, not today 🚫
• the err(), verr(), errx(), and verrx() functions do not return, but exit with the value of the argument eval. like, they're gone, they left the chat, periodt 👑

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✨ ATTRIBUTES ✨

for an explanation of the terms used in this section, see attributes(7). here's the vibe check on thread safety, bestie:

• Interface: err(), errx(), warn(), warnx(), verr(), verrx(), vwarn(), vwarnx()
• Attribute: Thread safety
• Value: MT-Safe locale

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🌈 STANDARDS 🌈

BSD. iconic behaviour honestly 🫶

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🦋 HISTORY 🦋

• err()
• warn() — 4.4BSD

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🍓 EXAMPLES 🍓

Display the current errno information string and exit:

p = malloc(size);
if (p == NULL)
    err(EXIT_FAILURE, NULL);
fd = open(file_name, O_RDONLY, 0);
if (fd == -1)
    err(EXIT_FAILURE, "%s", file_name);

Display an error message and exit:

if (tm.tm_hour < START_TIME)
    errx(EXIT_FAILURE, "too early, wait until %s",
            start_time_string);

Warn of an error:

fd = open(raw_device, O_RDONLY, 0);
if (fd == -1)
    warnx("%s: %s: trying the block device",
            raw_device, strerror(errno));
fd = open(block_device, O_RDONLY, 0);
if (fd == -1)
    err(EXIT_FAILURE, "%s", block_device);

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👑 SEE ALSO 👑

• error(3)
• exit(3)
• perror(3)
• printf(3)
• strerror(3)

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and that's the whole page bestie, you ate that 🧚💖

this was err(3) from Linux man-pages 6.20, dated 2025-05-17 — go forth and print pretty errors, ily 🌸🦋💜