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.\" 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.
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.\" 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
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.\" 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 2001-12-14 "" "Library functions"
.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 ;
.SH DESCRIPTION
The routine
.B 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 NUL) 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
.B 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
.B perror()
and the external
.I errno
(see
.BR errno (3))
conform to ANSI C, 4.3BSD, POSIX, X/OPEN.
The externals
.I sys_nerr
and
.I sys_errlist
conform to BSD.
.SH NOTE
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 _GNU_SOURCE is defined, the symbols
.\" .I _sys_nerr
.\" and
.\" .I _sys_errlist
.\" are provided.
.SH "SEE ALSO"
.BR errno (3),
.BR strerror (3)