---
name: "strerror"
section: "3"
platform: "macos"
package: "Libc"
version: "1752.120.2"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/strerror"
---

# strerror(3)

system error messages

## NAME

**perror**, **strerror**, **strerror\_r**, **sys\_errlist**, **sys\_nerr** – system error messages

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<stdio.h\>**

*void*\

**perror**(*const char \*s*);

*extern* *const* *char* *\** *const* *sys\_errlist\[\]*; *extern* *const* *int* *sys\_nerr*;

**#include \<string.h\>**

*char \**\

**strerror**(*int errnum*);

*int*\

**strerror\_r**(*int errnum*, *char \*strerrbuf*, *size\_t buflen*);

## DESCRIPTION

The **strerror**(), **strerror\_r**(), and **perror**() functions look up the error message string corresponding to an error number.

The **strerror**() function accepts an error number argument *errnum* and returns a pointer to the corresponding message string.

The **strerror\_r**() function renders the same result into *strerrbuf* for a maximum of *buflen* characters and returns 0 upon success.

The **perror**() function finds the error message corresponding to the current value of the global variable *errno* ([intro(2)](/intro/2.md)) and writes it, followed by a newline, to the standard error file descriptor. If the argument *s* is non-NULL and does not point to the null character, this string is prepended to the message string and separated from it by a colon and space (“: ”); otherwise, only the error message string is printed.

If the error number is not recognized, these functions return an error message string containing “Unknown error: ” followed by the error number in decimal. The **strerror**() and **strerror\_r**() functions return EINVAL as a warning. Error numbers recognized by this implementation fall in the range 0 \<= *errnum* \< *sys\_nerr*.

If insufficient storage is provided in *strerrbuf* (as specified in *buflen*) to contain the error string, **strerror\_r**() returns ERANGE and *strerrbuf* will contain an error message that has been truncated and NUL terminated to fit the length specified by *buflen*.

The message strings can be accessed directly using the external array *sys\_errlist*. The external value *sys\_nerr* contains a count of the messages in *sys\_errlist*. The use of these variables is deprecated; **strerror**() or **strerror\_r**() should be used instead.

## SEE ALSO

[intro(2)](/intro/2.md), [psignal(3)](/psignal/3.md)

## STANDARDS

The **perror**() and **strerror**() functions conform to ISO/IEC 9899:1999 (“ISO C99”). The **strerror\_r**() function conforms to IEEE Std 1003.1-2001 (“POSIX.1”).

## HISTORY

The **strerror**() and **perror**() functions first appeared in 4.4BSD. The **strerror\_r**() function was implemented in FreeBSD 4.4 by Wes Peters ⟨wes@FreeBSD.org⟩.

## BUGS

For unknown error numbers, the **strerror**() function will return its result in a static buffer which may be overwritten by subsequent calls.

The return type for **strerror**() is missing a type-qualifier; it should actually be *const* *char* *\**.

Programs that use the deprecated *sys\_errlist* variable often fail to compile because they declare it inconsistently.