assert_perror(3)

💅 assert_perror — the manual page, bestie edition

okay so this is assert_perror(3), living in the Library Functions Manual, and it's one of those little GNU-only gremlins. buckle up 🎀

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✨ NAME ✨

assert_perror — test errnum and abort 🫶

📚 LIBRARY 📚

Standard C library (libc, -lc) — yeah she's built in, no extra linking drama ✨

🌸 SYNOPSIS 🌸

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <assert.h>

void assert_perror(int errnum);

💖 DESCRIPTION 💖

okay so here's the whole tea, and it's a conditional situation so pay attention bestie 👑

if the macro NDEBUG was defined at the exact moment <assert.h> was last included, then the macro assert_perror() generates 😶 no code and therefore does literally nothing at all. gone. ghosted. not me existing. 💀

otherwise — so if NDEBUG was not defined — the macro assert_perror() does two things when errnum is nonzero:

• prints an error message to standard error, and
• terminates the program by calling abort(3). yes. she kills the whole process. main character behavior, honestly 😭

and the message it prints? it contains the filename, the function name, and the line number of the macro call, plus the output of strerror(errnum). so you know exactly where it went down. iconic information delivery fr fr 🌈

🦋 RETURN VALUE 🦋

No value is returned. nothing. nada. it either does nothing or it nukes your program, there is no in-between and no return value 💅

⭐ ATTRIBUTES ⭐

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

• Interface: assert_perror()
• Attribute: Thread safety
• Value: MT-Safe ✨

so yeah, MT-Safe, she's fine in threads, no cap.

👑 STANDARDS 👑

GNU. that's it, that's the standard. GNU-only, iykyk 💜

💀 BUGS 💀

and here comes the part where the manpage literally yells at you, so let's listen 🙈

the purpose of the assert macros is to help programmers find bugs in their programs — things that cannot happen unless there was a coding mistake. periodt.

however, with system or library calls the situation is rather different. error returns can happen, and will happen, and should be tested for. 🔥

just... not by an assert, okay bestie? because with an assert the test goes away when NDEBUG is defined. which is the opposite of what you want for a real error. use proper error handling code instead.

Never use this macro. 🚫 that's not me being dramatic, that is the actual manpage saying it. the whole BUGS section is basically a warning label. respect it.

🌈 SEE ALSO 🌈

• abort(3)
• assert(3)
• exit(3)
• strerror(3)

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and that's the whole page, bestie! this was Linux man-pages 6.20, dated 2026-02-08, for assert_perror(3) — so if you cite it in your bug report or your essay or your group chat, now you know exactly where it came from 🎀✨🫶