.\" Copyright (c) 1994 Michael Haardt (michael@moria.de), 1994-06-04
.\" Copyright (c) 1995 Michael Haardt
.\"      (michael@cantor.informatik.rwth-aachen.de), 1995-03-16
.\" Copyright (c) 1996 Andries Brouwer (aeb@cwi.nl), 1996-01-13
.\"
.\" This is free documentation; you can redistribute it and/or
.\" modify it under the terms of the GNU General Public License as
.\" published by the Free Software Foundation; either version 2 of
.\" the License, or (at your option) any later version.
.\"
.\" The GNU General Public License's references to "object code"
.\" and "executables" are to be interpreted as the output of any
.\" document formatting or typesetting system, including
.\" intermediate and printed output.
.\"
.\" This manual is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\" GNU General Public License for more details.
.\"
.\" You should have received a copy of the GNU General Public
.\" License along with this manual; if not, write to the Free
.\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111,
.\" USA.
.\"
.\" 1996-01-13 aeb: merged in some text contributed by Melvin Smith
.\"   (msmith@falcon.mercer.peachnet.edu) and various other changes.
.\" Modified 1996-05-16 by Martin Schulze (joey@infodrom.north.de)
.\"
.TH PERROR 3 2007-07-26 "" "Linux Programmer's Manual"
.SH NAME
perror \- print a system error message
.SH SYNOPSIS
.B #include <stdio.h>
.sp
.BI "void perror(const char " *s );
.sp
.B #include <errno.h>
.sp
.BI "const char *" sys_errlist [];
.br
.BI "int " sys_nerr ;
.br
.BI "int " errno ;
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.IR sys_errlist ,
.IR sys_nerr :
_BSD_SOURCE
.SH DESCRIPTION
The routine
.BR perror ()
produces a message on the standard error output, describing the last
error encountered during a call to a system or library function.
First (if
.I s
is not NULL and
.RI * s
is not a null byte ('\\0')) the argument string
.I s
is printed, followed by a colon and a blank.
Then the message and a new-line.

To be of most use, the argument string should include the name
of the function that incurred the error.
The error number is taken from
the external variable
.IR errno ,
which is set when errors occur but not
cleared when non-erroneous calls are made.

The global error list
.IR sys_errlist "[]"
indexed by
.I errno
can be used to obtain the error message without the newline.
The largest message number provided in the table is
.IR sys_nerr " \-1."
Be careful when directly accessing this list because new error values
may not have been added to
.IR sys_errlist "[]."

When a system call fails, it usually returns \-1 and sets the
variable
.I errno
to a value describing what went wrong.
(These values can be found in
.IR "<errno.h>" .)
Many library functions do likewise.
The function
.BR perror ()
serves to translate this error code into human-readable form.
Note that
.I errno
is undefined after a successful library call:
this call may well change this variable, even though it succeeds,
for example because it internally used some other library function that failed.
Thus, if a failing call is not immediately followed by a call to
.BR perror (),
the value of
.I errno
should be saved.
.SH "CONFORMING TO"
The function
.BR perror ()
and the external
.I errno
(see
.BR errno (3))
conform to C89, C99, 4.3BSD, POSIX.1-2001.
The externals
.I sys_nerr
and
.I sys_errlist
conform to BSD.
.SH NOTES
The externals
.I sys_nerr
and
.I sys_errlist
are defined by glibc, but in
.IR <stdio.h> .
.\" and only when _BSD_SOURCE is defined.
.\" When
.\" .B _GNU_SOURCE
.\" is defined, the symbols
.\" .I _sys_nerr
.\" and
.\" .I _sys_errlist
.\" are provided.
.SH "SEE ALSO"
.BR err (3),
.BR errno (3),
.BR error (3),
.BR strerror (3)