.\" Copyright (c) 1994 Michael Haardt (michael@moria.de), Sat Jun  4 20:38:26 MET DST 1994
.\" Copyright (c) 1995 Michael Haardt (michael@cantor.informatik.rwth-aachen.de), Thu Mar 16 18:46:23 MET 1995
.\" Copyright (c) 1996 Andries Brouwer (aeb@cwi.nl), Sat Jan 13 00:16:41 MET 1996
.\"
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.\" Sat Jan 13 00:16:41 MET 1996, aeb: merged in some text contributed
.\" by Melvin Smith (msmith@falcon.mercer.peachnet.edu) and various
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.\" Modified Fri May 16 23:41:15 1996 by Martin Schulze (joey@infodrom.north.de)
.\"
.TH PERROR 3 "May 16 1996" "" "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 ;
.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.  The
argument string
.I s
is printed first, then 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"
ANSI C, BSD 4.3, POSIX, X/OPEN
.SH "SEE ALSO"
.BR strerror (3)