.\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright (C) 2005, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
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.\" entire resulting derived work is distributed under the terms of a
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.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
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.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 18:05:30 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Fri Feb 16 14:25:17 1996 by Andries Brouwer <aeb@cwi.nl>
.\" Modified Sun Jul 21 20:55:44 1996 by Andries Brouwer <aeb@cwi.nl>
.\" Modified Mon Oct 15 21:16:25 2001 by John Levon <moz@compsoc.man.ac.uk>
.\" Modified Tue Oct 16 00:04:43 2001 by Andries Brouwer <aeb@cwi.nl>
.\" Modified Fri Jun 20 03:04:30 2003 by Andries Brouwer <aeb@cwi.nl>
.\" 2005-12-13, mtk, Substantial rewrite of strerror_r() description
.\"         Addition of extra material on portability and standards.
.\"
.TH STRERROR 3  2009-03-30 "" "Linux Programmer's Manual"
.SH NAME
strerror, strerror_r \- return string describing error number
.SH SYNOPSIS
.nf
.B #include <string.h>
.sp
.BI "char *strerror(int " errnum );
.sp
.BI "int strerror_r(int " errnum ", char *" buf ", size_t " buflen );
            /* XSI-compliant */
.sp
.BI "char *strerror_r(int " errnum ", char *" buf ", size_t " buflen );
            /* GNU-specific */
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
The XSI-compliant version of
.BR strerror_r ()
is provided if:
.br
(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) && !\ _GNU_SOURCE
.br
Otherwise, the GNU-specific version is provided.
.SH DESCRIPTION
The
.BR strerror ()
function returns a pointer to a string that describes the error
code passed in the argument \fIerrnum\fP, possibly using the
.B LC_MESSAGES
part of the current locale to select the appropriate language.
This string must not be modified by the application, but may be
modified by a subsequent call to
.BR perror (3)
or
.BR strerror ().
No library function will modify this string.

The
.BR strerror_r ()
function is similar to
.BR strerror (),
but is
thread safe.
This function is available in two versions:
an XSI-compliant version specified in POSIX.1-2001
(available since glibc 2.3.4),
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_SOURCE
is defined by default with the value
200112L, so that the XSI-compliant version of
.BR strerror_r ()
is provided by default.

The XSI-compliant
.BR strerror_r ()
is preferred for portable applications.
It returns the error string in the user-supplied buffer
.I buf
of length
.IR buflen .

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 buflen
bytes are stored (the string may be truncated if
.I buflen
is too small) and the string always includes a terminating null byte.
.SH "RETURN VALUE"
The
.BR strerror ()
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.

The XSI-compliant
.BR strerror_r ()
function returns 0 on success;
on error, \-1 is returned and
.I errno
is set to indicate the error.
.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 "CONFORMING TO"
.BR strerror ()
is specified by POSIX.1-2001, C89, C99.
.BR strerror_r ()
is specified by POSIX.1-2001.

The GNU-specific
.BR strerror_r ()
function is a non-standard extension.

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.
.SH "SEE ALSO"
.BR err (3),
.BR errno (3),
.BR error (3),
.BR perror (3),
.BR strsignal (3)
.SH COLOPHON
This page is part of release 3.22 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
