man-pages/man3/perror.3

.\" 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
.\"
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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 2015-04-19 "" "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 * const " sys_errlist [];
.br
.BI "int " sys_nerr ;
.br
.BI "int " errno ";       \fR/* Not really declared this way; see errno(3) */"
.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
.BR perror ()
function produces a message on standard error describing the last
error encountered during a call to a system or library function.

First (if
.I s
is not NULL and
.I *s
is not a null byte (\(aq\\0\(aq)), the argument string
.I s
is printed, followed by a colon and a blank.
Then an error message corresponding to the current value of
.I errno
and a new-line.

To be of most use, the argument string should include the name
of the function that incurred the error.

The global error list
.IR sys_errlist "[],"
which can be indexed by
.IR 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 "[]."
The use of
.IR sys_errlist "[]"
is nowadays deprecated.

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 sysme call or library function 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 ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR perror ()
T}	Thread safety	MT-Safe race:stderr
.TE

.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)