.\" $NetBSD: err.3,v 1.22.16.2 2024/07/20 15:05:52 martin Exp $
.\"
.\" Copyright (c) 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" 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.
.\"
.\"	@(#)err.3	8.1 (Berkeley) 6/9/93
.\"
.Dd February 2, 2024
.Dt ERR 3
.Os
.Sh NAME
.Nm err ,
.Nm verr ,
.Nm errx ,
.Nm verrx ,
.Nm errc ,
.Nm verrc ,
.Nm warn ,
.Nm vwarn ,
.Nm warnx ,
.Nm vwarnx ,
.Nm warnc ,
.Nm vwarnc
.Nd formatted error messages
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In err.h
.Ft void
.Fn err "int status" "const char *fmt" "..."
.Ft void
.Fn verr "int status" "const char *fmt" "va_list args"
.Ft void
.Fn errx "int status" "const char *fmt" "..."
.Ft void
.Fn verrx "int status" "const char *fmt" "va_list args"
.Ft void
.Fn errc "int status" "int code" "const char *fmt" "..."
.Ft void
.Fn verrc "int status" "int code" "const char *fmt" "va_list args"
.Ft void
.Fn warn "const char *fmt" "..."
.Ft void
.Fn vwarn "const char *fmt" "va_list args"
.Ft void
.Fn warnx "const char *fmt" "..."
.Ft void
.Fn vwarnx "const char *fmt" "va_list args"
.Ft void
.Fn warnc "int code" "const char *fmt" "..."
.Ft void
.Fn vwarnc "int code" "const char *fmt" "va_list args"
.Sh DESCRIPTION
The
.Fn err
and
.Fn warn
family of functions display a formatted error message on the standard
error output.
.Pp
In all cases these functions output the last component of the program name,
a colon character, and a space.
If the
.Fa fmt
argument is not
.Dv NULL ,
it is used as a
.Xr printf 3 Ns
-like format specification for the error message.
.Pp
In the case of the
.Fn err ,
.Fn verr ,
.Fn warn ,
and
.Fn vwarn
functions, an additional error message string affiliated with the current
value of the global variable
.Va errno
is output next, preceded by a colon character and a space if
.Fa fmt
is not
.Dv NULL .
The
.Fn errc ,
.Fn verrc ,
.Fn warnc ,
and
.Fn vwarnc
functions take an additional
.Ar code
argument to be used as the error number instead of using the global
.Va errno
variable.
The
.Fn errx ,
.Fn verrx ,
.Fn warnx ,
and
.Fn vwarnx
functions will not output an additional error message string.
.Pp
In all cases, the output is terminated by a newline character.
.Pp
The
.Fn err ,
.Fn verr ,
.Fn errc ,
.Fn verrc ,
.Fn errx ,
and
.Fn verrx
functions do not return, but instead cause the program to terminate
with the status value given by the argument
.Fa status .
It is often appropriate to use the value
.Dv EXIT_FAILURE ,
defined in
.In stdlib.h ,
as the
.Fa status
argument given to these functions.
.Sh EXAMPLES
Display the current
.Va errno
information string and terminate with status indicating failure:
.Bd -literal -offset indent
if ((p = malloc(size)) == NULL)
	err(EXIT_FAILURE, NULL);
if ((fd = open(file_name, O_RDONLY, 0)) == -1)
	err(EXIT_FAILURE, "%s", file_name);
.Ed
.Pp
Display an error message and terminate with status indicating failure:
.Bd -literal -offset indent
if (tm.tm_hour < START_TIME)
	errx(EXIT_FAILURE, "too early, wait until %s",
	    start_time_string);
.Ed
.Pp
Warn of an error:
.Bd -literal -offset indent
if ((fd = open(raw_device, O_RDONLY, 0)) == -1)
	warnx("%s: %s: trying the block device",
	    raw_device, strerror(errno));
if ((fd = open(block_device, O_RDONLY, 0)) == -1)
	warn("%s", block_device);
.Ed
.Sh SEE ALSO
.Xr exit 3 ,
.Xr getprogname 3 ,
.Xr printf 3 ,
.Xr strerror 3
.Sh HISTORY
The
.Fn err
and
.Fn warn
functions first appeared in
.Bx 4.4 .
The
.Fn errc
and
.Fn warnc
functions first appeared in
.Fx 3.0
and
.Nx 7.0 .
.Sh CAVEATS
It is important never to pass a string with user-supplied data as a
format without using
.Ql %s .
An attacker can put format specifiers in the string to mangle your stack,
leading to a possible security hole.
This holds true even if you have built the string
.Dq by hand
using a function like
.Fn snprintf ,
as the resulting string may still contain user-supplied conversion specifiers
for later interpolation by the
.Fn err
and
.Fn warn
functions.
.Pp
Always be sure to use the proper secure idiom:
.Bd -literal -offset indent
err(1, "%s", string);
.Ed