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.\" Copyright (c) 1980, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" the American National Standards Committee X3, on Information
.\" Processing Systems.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)strerror.3	8.1 (Berkeley) 6/9/93
.\" $FreeBSD$
.\"
.Dd June 9, 1993
.Dt STRERROR 3
.Os BSD 4
.Sh NAME
.Nm perror ,
.Nm strerror ,
.Nm sys_errlist ,
.Nm sys_nerr
.Nd system error messages
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include <stdio.h>
.Ft void
.Fn perror "const char *string"
.Vt extern const char * const sys_errlist[];
.Vt extern const int sys_nerr;
.Fd #include <string.h>
.Ft char *
.Fn strerror "int errnum"
.Sh DESCRIPTION
The
.Fn strerror
and
.Fn perror
functions look up the error message string corresponding to an
error number.
.Pp
The
.Fn strerror
function accepts an error number argument
.Fa errnum
and
returns a pointer to the corresponding
message string.
.Pp
The
.Fn perror
function finds the error message corresponding to the current
value of the global variable
.Va errno
.Pq Xr intro 2
and writes it, followed by a newline, to the
standard error file descriptor.
If the argument
.Fa string
is
.Pf non- Dv 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
.Pq Ql \&:\ \& ;
otherwise, only the error message string is printed.
.Pp
If
.Fa errnum
is not a recognized error number,
the error message string will contain
.Dq Li "Unknown error:\0
followed by the error number in decimal.
.Pp
The message strings can be accessed directly using the external
array
.Va sys_errlist .
The external value
.Va sys_nerr
contains a count of the messages in
.Va sys_errlist .
The use of these variables is deprecated;
.Fn strerror
should be used instead.
.Sh SEE ALSO
.Xr intro 2 ,
.Xr psignal 3
.Sh HISTORY
The
.Fn strerror
and
.Fn perror
functions first appeared in
.Bx 4.4 .
.Sh BUGS
For unknown error numbers, the
.Fn strerror
function will return its result in a static buffer which
may be overwritten by subsequent calls.
.Pp
Programs that use the deprecated
.Va sys_errlist
variable often fail to compile because they declare it
inconsistently.