'\" t
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH strerror 3 2025-06-28 "Linux man-pages (unreleased)"
.SH NAME
strerror, strerrorname_np, strerrordesc_np, strerror_r, strerror_l \-
return string describing error number
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <string.h>
.P
.BI "char *strerror(int " errnum );
.BI "const char *strerrorname_np(int " errnum );
.BI "const char *strerrordesc_np(int " errnum );
.P
.BR "int strerror_r(" "size_t size;"
.BI "               int " errnum ", char " buf [ size "], size_t " size );
               /* XSI-compliant */
.P
.BR "char *strerror_r(" "size_t size;"
.BI "               int " errnum ", char " buf [ size "], size_t " size );
               /* GNU-specific */
.P
.BI "char *strerror_l(int " errnum ", locale_t " locale );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR strerrorname_np (),
.BR strerrordesc_np ():
.nf
    _GNU_SOURCE
.fi
.P
.BR strerror_r ():
.nf
    The XSI-compliant version is provided if:
        (_POSIX_C_SOURCE >= 200112L) && ! _GNU_SOURCE
    Otherwise, the GNU-specific version is provided.
.fi
.SH DESCRIPTION
The
.BR strerror ()
function returns a pointer to a string that describes the error
code passed in the argument
.IR errnum ,
possibly using the
.B LC_MESSAGES
part of the current locale to select the appropriate language.
(For example, if
.I errnum
is
.BR 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
.BR strerror ()
or
.BR strerror_l (),
or if the thread that obtained the string exits.
No other library function, including
.BR perror (3),
will modify this string.
.P
Like
.BR strerror (),
the
.BR strerrordesc_np ()
function returns a pointer to a string that describes the error
code passed in the argument
.IR errnum ,
with the difference that the returned string is not translated
according to the current locale.
.P
The
.BR strerrorname_np ()
function returns a pointer to a string containing the name of the error
code passed in the argument
.IR errnum .
For example, given
.B EPERM
as an argument, this function returns a pointer to the string "EPERM".
Given
.B 0
as an argument,
this function returns a pointer to the string "0".
.\"
.SS strerror_r()
.BR strerror_r ()
is like
.BR strerror (),
but might use the supplied buffer
.I 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)
.B _POSIX_C_SOURCE
is defined by default with the value
200112L, so that the XSI-compliant version of
.BR strerror_r ()
is provided by default.
.P
The XSI-compliant
.BR strerror_r ()
is preferred for portable applications.
It returns the error string in the user-supplied buffer
.I buf
of size
.IR size .
.P
The GNU-specific
.BR 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
.IR buf ,
or a pointer to some (immutable) static string
(in which case
.I buf
is unused).
If the function stores a string in
.IR buf ,
then at most
.I size
bytes are stored (the string may be truncated if
.I size
is too small and
.I errnum
is unknown).
The string always includes a terminating null byte (\[aq]\[rs]0\[aq]).
.\"
.SS strerror_l()
.BR strerror_l ()
is like
.BR strerror (),
but maps
.I errnum
to a locale-dependent error message in the locale specified by
.IR locale .
The behavior of
.BR strerror_l ()
is undefined if
.I locale
is the special locale object
.B LC_GLOBAL_LOCALE
or is not a valid locale object handle.
.SH RETURN VALUE
The
.BR strerror (),
.BR strerror_l (),
and the GNU-specific
.BR strerror_r ()
functions return
the appropriate error description string,
or an "Unknown error nnn" message if the error number is unknown.
.P
On success,
.BR strerrorname_np ()
and
.BR strerrordesc_np ()
return the appropriate error description string.
If
.I errnum
is an invalid error number, these functions return NULL.
.P
The XSI-compliant
.BR strerror_r ()
function returns 0 on success.
On error,
a (positive) error number is returned (since glibc 2.13),
or \-1 is returned and
.I errno
is set to indicate the error (before glibc 2.13).
.P
POSIX.1-2001 and POSIX.1-2008 require that a successful call to
.BR strerror ()
or
.BR strerror_l ()
shall leave
.I 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
.I errno
to zero before the call,
and then check
.I errno
after the call.
.SH ERRORS
.TP
.B EINVAL
The value of
.I errnum
is not a valid error number.
.TP
.B ERANGE
Insufficient storage was supplied to contain the error description string.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR strerror ()
T}	Thread safety	T{
.na
.nh
MT-Safe
T}
T{
.na
.nh
.BR strerrorname_np (),
.BR strerrordesc_np ()
T}	Thread safety	MT-Safe
T{
.na
.nh
.BR strerror_r (),
.BR strerror_l ()
T}	Thread safety	MT-Safe
.TE
.P
Before glibc 2.32,
.BR strerror ()
is not MT-Safe.
.SH STANDARDS
.TP
.BR strerror ()
C11, POSIX.1-2008.
.TP
.BR strerror_r ()
.\" FIXME . for later review when Issue 8 is one day released...
.\" A future POSIX.1 may remove strerror_r()
.\" http://austingroupbugs.net/tag_view_page.php?tag_id=8
.\" http://austingroupbugs.net/view.php?id=508
.TQ
.BR strerror_l ()
POSIX.1-2008.
.TP
.BR strerrorname_np ()
.TQ
.BR strerrordesc_np ()
GNU.
.P
POSIX.1-2001 permits
.BR strerror ()
to set
.I 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,
.\" e.g., Solaris 8, HP-UX 11
.BR strerror ()
returns NULL if the error number is unknown.
On other systems,
.\" e.g., FreeBSD 5.4, Tru64 5.1B
.BR strerror ()
returns a string something like "Error nnn occurred" and sets
.I errno
to
.B EINVAL
if the error number is unknown.
C99 and POSIX.1-2008 require the return value to be non-NULL.
.SH HISTORY
.TP
.BR strerror ()
POSIX.1-2001, C89.
.TP
.BR strerror_r ()
POSIX.1-2001.
.TP
.BR strerror_l ()
glibc 2.6.
POSIX.1-2008.
.TP
.BR strerrorname_np ()
.TQ
.BR strerrordesc_np ()
glibc 2.32.
.SH NOTES
.BR strerrorname_np ()
and
.BR strerrordesc_np ()
are thread-safe and async-signal-safe.
.SH SEE ALSO
.BR err (3),
.BR errno (3),
.BR error (3),
.BR perror (3),
.BR strsignal (3),
.BR locale (7),
.BR signal-safety (7)