perror(3)

💖 perror(3) — the manual page, bestie edition 💖

okay so this is perror(3) from the Library Functions Manual and honestly? she's a little legend. she takes your cryptic error number and turns it into words a human can read. no cap. let's get into it 🫶

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✨ NAME ✨

perror — print a system error message

like… that's it. that's the whole job. and she does it flawlessly 💅

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📚 LIBRARY 📚

Standard C library (libc, -lc)

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🌸 SYNOPSIS 🌸

#include <stdio.h>

void perror(const char *s);

#include <errno.h>

int errno;  /* Not really declared this way; see errno(3) */

[[deprecated]] const char *const sys_errlist[];
[[deprecated]] int sys_nerr;

🧚 Feature Test Macro Requirements for glibc (see feature_test_macros(7)) 🧚

for sys_errlist, sys_nerr:

• From glibc 2.19 to glibc 2.31:
  • _DEFAULT_SOURCE
• glibc 2.19 and earlier:
  • _BSD_SOURCE

⚠️ heads up bestie — those two are marked [[deprecated]] for a reason. keep reading, i'll spill. 💀

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🦋 DESCRIPTION 🦋

perror() writes a message to standard error describing the last error that happened during a call to a system or library function. it's basically the translator between "errno said 2" and "ohhh that's what went wrong."

first off — if s is not NULL and *s is not a null byte ('\0') — then your argument string s gets printed, followed by a colon and a blank. then comes the error message that matches the current value of errno, and then a new-line. cute little sandwich, isn't it 🥪✨

to be of most use, the argument string should include the name of the function that incurred the error. like, that's the whole vibe — tell people where it broke.

the global error list sys_errlist[], which can be indexed by errno, can be used to obtain the error message without the newline. the largest message number provided in the table is sys_nerr-1. be careful when directly accessing this list, because new error values may not have been added to sys_errlist[]. and the use of sys_errlist[] is nowadays deprecated — use strerror(3) instead. not me being the messenger for the deprecation notice 😭

when a system call fails, it usually returns -1 and sets the variable errno to a value describing what went wrong. (these values can be found in <errno.h>.) many library functions do likewise. perror() exists to translate that error code into human-readable form.

💀 REAL TALK WARNING: errno is undefined after a successful system call or library function call. this call may well change this variable, even though it succeeds — for example because it internally used some other library function that failed. so if a failing call is not immediately followed by a call to perror(), the value of errno should be saved. save it, bestie. don't let it get away. 🫶

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🔥 ATTRIBUTES 🔥

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

• perror() — Thread safety — MT-Safe race:stderr

that's the whole table, we love a one-row moment 👑

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👑 STANDARDS 👑

• errno
• perror()
  • C11, POSIX.1-2008.
• sys_nerr
• sys_errlist
  • BSD.

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💜 HISTORY 💜

• errno
• perror()
  • POSIX.1-2001, C89, 4.3BSD.
• sys_nerr
• sys_errlist
  • Removed in glibc 2.32.

so yeah, sys_nerr and sys_errlist got ✨ yeeted ✨ in glibc 2.32. told you the deprecation was real 🙈

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🌈 SEE ALSO 🌈

err(3), errno(3), error(3), strerror(3)

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and that's the whole page, bestie! this one's from Linux man-pages 6.20, dated 2026-02-08, and it's giving exactly what it needed to give 💖✨🎀 go forth and perror() responsibly 🦋