Provided by: manpages-dev_6.15-1_all 
NAME
strerror, strerrorname_np, strerrordesc_np, strerror_r, strerror_l - return string describing error
number
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <string.h>
char *strerror(int errnum);
const char *strerrorname_np(int errnum);
const char *strerrordesc_np(int errnum);
int strerror_r(size_t size;
int errnum, char buf[size], size_t size);
/* XSI-compliant */
char *strerror_r(size_t size;
int errnum, char buf[size], size_t size);
/* GNU-specific */
char *strerror_l(int errnum, locale_t locale);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
strerrorname_np(), strerrordesc_np():
_GNU_SOURCE
strerror_r():
The XSI-compliant version is provided if:
(_POSIX_C_SOURCE >= 200112L) && ! _GNU_SOURCE
Otherwise, the GNU-specific version is provided.
DESCRIPTION
The strerror() function returns a pointer to a string that describes the error code passed in the
argument errnum, possibly using the LC_MESSAGES part of the current locale to select the appropriate
language. (For example, if errnum is EINVAL, the returned description will be "Invalid argument".) This
string must not be modified by the application, and the returned pointer will be invalidated on a
subsequent call to strerror() or strerror_l(), or if the thread that obtained the string exits. No other
library function, including perror(3), will modify this string.
Like strerror(), the strerrordesc_np() function returns a pointer to a string that describes the error
code passed in the argument errnum, with the difference that the returned string is not translated
according to the current locale.
The strerrorname_np() function returns a pointer to a string containing the name of the error code passed
in the argument errnum. For example, given EPERM as an argument, this function returns a pointer to the
string "EPERM". Given 0 as an argument, this function returns a pointer to the string "0".
strerror_r()
strerror_r() is like strerror(), but might use the supplied buffer buf instead of allocating one
internally. This function is available in two versions: an XSI-compliant version specified in
POSIX.1-2001 (available since glibc 2.3.4, but not POSIX-compliant until glibc 2.13), and a GNU-specific
version (available since glibc 2.0). The XSI-compliant version is provided with the feature test macros
settings shown in the SYNOPSIS; otherwise the GNU-specific version is provided. If no feature test
macros are explicitly defined, then (since glibc 2.4) _POSIX_C_SOURCE is defined by default with the
value 200112L, so that the XSI-compliant version of strerror_r() is provided by default.
The XSI-compliant strerror_r() is preferred for portable applications. It returns the error string in
the user-supplied buffer buf of size size.
The GNU-specific strerror_r() returns a pointer to a string containing the error message. This may be
either a pointer to a string that the function stores in buf, or a pointer to some (immutable) static
string (in which case buf is unused). If the function stores a string in buf, then at most size bytes
are stored (the string may be truncated if size is too small and errnum is unknown). The string always
includes a terminating null byte ('\0').
strerror_l()
strerror_l() is like strerror(), but maps errnum to a locale-dependent error message in the locale
specified by locale. The behavior of strerror_l() is undefined if locale is the special locale object
LC_GLOBAL_LOCALE or is not a valid locale object handle.
RETURN VALUE
The strerror(), strerror_l(), and the GNU-specific strerror_r() functions return the appropriate error
description string, or an "Unknown error nnn" message if the error number is unknown.
On success, strerrorname_np() and strerrordesc_np() return the appropriate error description string. If
errnum is an invalid error number, these functions return NULL.
The XSI-compliant strerror_r() function returns 0 on success. On error, a (positive) error number is
returned (since glibc 2.13), or -1 is returned and errno is set to indicate the error (before glibc
2.13).
POSIX.1-2001 and POSIX.1-2008 require that a successful call to strerror() or strerror_l() shall leave
errno unchanged, and note that, since no function return value is reserved to indicate an error, an
application that wishes to check for errors should initialize errno to zero before the call, and then
check errno after the call.
ERRORS
EINVAL The value of errnum is not a valid error number.
ERANGE Insufficient storage was supplied to contain the error description string.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────┬───────────────┬──────────────────────────────────────────────────────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────┼───────────────┼──────────────────────────────────────────────────────────┤
│ strerror() │ Thread safety │ MT-Safe │
├────────────────────────────┼───────────────┼──────────────────────────────────────────────────────────┤
│ strerrorname_np(), │ Thread safety │ MT-Safe │
│ strerrordesc_np() │ │ │
├────────────────────────────┼───────────────┼──────────────────────────────────────────────────────────┤
│ strerror_r(), strerror_l() │ Thread safety │ MT-Safe │
└────────────────────────────┴───────────────┴──────────────────────────────────────────────────────────┘
Before glibc 2.32, strerror() is not MT-Safe.
STANDARDS
strerror()
C11, POSIX.1-2008.
strerror_r()
strerror_l()
POSIX.1-2008.
strerrorname_np()
strerrordesc_np()
GNU.
POSIX.1-2001 permits strerror() to set errno if the call encounters an error, but does not specify what
value should be returned as the function result in the event of an error. On some systems, strerror()
returns NULL if the error number is unknown. On other systems, strerror() returns a string something
like "Error nnn occurred" and sets errno to EINVAL if the error number is unknown. C99 and POSIX.1-2008
require the return value to be non-NULL.
HISTORY
strerror()
POSIX.1-2001, C89.
strerror_r()
POSIX.1-2001.
strerror_l()
glibc 2.6. POSIX.1-2008.
strerrorname_np()
strerrordesc_np()
glibc 2.32.
NOTES
strerrorname_np() and strerrordesc_np() are thread-safe and async-signal-safe.
SEE ALSO
err(3), errno(3), error(3), perror(3), strsignal(3), locale(7), signal-safety(7)
Linux man-pages 6.15 2025-06-28 strerror(3)