.\" 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. 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
.\"
.Dd December 17, 2020
.Dt STRERROR 3
.Os
.Sh NAME
.Nm perror ,
.Nm strerror ,
.Nm strerror_l ,
.Nm strerror_r ,
.Nm sys_errlist ,
.Nm sys_nerr
.Nd system error messages
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft void
.Fn perror "const char *string"
.Vt extern const char * const sys_errlist[] ;
.Vt extern const int sys_nerr ;
.In string.h
.Ft "char *"
.Fn strerror "int errnum"
.Ft "char *"
.Fn strerror_l "int errnum" "locale_t"
.Ft int
.Fn strerror_r "int errnum" "char *strerrbuf" "size_t buflen"
.Sh DESCRIPTION
The
.Fn strerror ,
.Fn strerror_l ,
.Fn strerror_r ,
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
in the current locale.
.Fn strerror
is not thread-safe.
It returns a pointer to an internal static buffer that could be
overwritten by a
.Fn strerror
call from another thread.
.Pp
The
.Fn strerror_l
function accepts
.Fa errnum
error number and
.Fa locale
locale handle arguments and returns a pointer to a string
corresponding to the specified error in the given locale.
.Fn strerror_l
is thread-safe, its result can be only overwritten by
another call to
.Fn strerror_l
from the current thread.
.Pp
The
.Fn strerror_r
function renders the same result into
.Fa strerrbuf
for a maximum of
.Fa buflen
characters and returns 0 upon success.
.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 Dq Li ":\ " ;
otherwise, only the error message string is printed.
.Pp
If the error number is not recognized, these functions return an error message
string containing
.Dq Li "Unknown error:\ "
followed by the error number in decimal.
The
.Fn strerror
and
.Fn strerror_r
functions return
.Er EINVAL
as a warning.
Error numbers recognized by this implementation fall in
the range 0 <
.Fa errnum
<
.Fa sys_nerr .
The number 0 is also recognized, although applications that take advantage of
this are likely to use unspecified values of
.Va errno .
.Pp
If insufficient storage is provided in
.Fa strerrbuf
(as specified in
.Fa buflen )
to contain the error string,
.Fn strerror_r
returns
.Er ERANGE
and
.Fa strerrbuf
will contain an error message that has been truncated and
.Dv NUL
terminated to fit the length specified by
.Fa buflen .
.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 ,
.Fn strerror_l ,
or
.Fn strerror_r
should be used instead.
.Sh EXAMPLES
The following example shows how to use
.Fn perror
to report an error.
.Bd -literal -offset 2n
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>

int
main(void)
{
	int fd;

	if ((fd = open("/nonexistent", O_RDONLY)) == -1) {
		perror("open()");
		exit(1);
	}
	printf("File descriptor: %d\en", fd);
	return (0);
}
.Ed
.Pp
When executed, the program will print an error message along the lines of
.Ql "open(): No such file or directory" .
.Sh SEE ALSO
.Xr intro 2 ,
.Xr err 3 ,
.Xr psignal 3
.Sh STANDARDS
The
.Fn perror
and
.Fn strerror
functions conform to
.St -isoC-99 .
The
.Fn strerror_r
function conforms to
.St -p1003.1-2001 .
The
.Fn strerror_l
function conforms to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn strerror
and
.Fn perror
functions first appeared in
.Bx 4.4 .
The
.Fn strerror_r
function was implemented in
.Fx 4.4
by
.An Wes Peters Aq Mt wes@FreeBSD.org .
The
.Fn strerror_l
function was added in
.Fx 13.0 .
.Sh BUGS
The
.Fn strerror
function returns its result in a static buffer which
will be overwritten by subsequent calls.
.Pp
Programs that use the deprecated
.Va sys_errlist
variable often fail to compile because they declare it
inconsistently.
Size of the
.Va sys_errlist
object might increase during FreeBSD lifetime,
breaking some ABI stability guarantees.