.\"	$OpenBSD: err.3,v 1.20 2014/04/23 16:26:33 jmc 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.
.\"
.Dd $Mdocdate: April 23 2014 $
.Dt ERR 3
.Os
.Sh NAME
.Nm err ,
.Nm verr ,
.Nm errc ,
.Nm verrc ,
.Nm errx ,
.Nm verrx ,
.Nm warn ,
.Nm vwarn ,
.Nm warnc ,
.Nm vwarnc ,
.Nm warnx ,
.Nm vwarnx
.Nd formatted error messages
.Sh SYNOPSIS
.In err.h
.Ft void
.Fn err "int eval" "const char *fmt" "..."
.Ft void
.Fn verr "int eval" "const char *fmt" "va_list args"
.Ft void
.Fn errc "int eval" "int code" "const char *fmt" "..."
.Ft void
.Fn verrc "int eval" "int code" "const char *fmt" "va_list args"
.Ft void
.Fn errx "int eval" "const char *fmt" "..."
.Ft void
.Fn verrx "int eval" "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 warnc "int code" "const char *fmt" "..."
.Ft void
.Fn vwarnc "int code" "const char *fmt" "va_list args"
.Ft void
.Fn warnx "const char *fmt" "..."
.Ft void
.Fn vwarnx "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.
In all cases, the last component of the program name, followed by
a colon
.Pq Sq \&:
character and a space, are output.
The text that follows depends on the function being called.
The
.Fa fmt
specification (and associated arguments) may be any format allowed by
.Xr printf 3
or
.Dv NULL .
If the
.Fa fmt
argument is not
.Dv NULL ,
the formatted error message is output.
.Pp
In the case of the
.Fn errx ,
.Fn verrx ,
.Fn warnx ,
and
.Fn vwarnx
functions only, no additional text is output,
so the output looks like the following:
.Bd -literal -offset indent
progname: fmt
.Ed
.Pp
The other functions all output an error message string affiliated with
an error value (see
.Xr strerror 3 ) ,
preceded by a colon character and a space if
.Fa fmt
is not
.Dv NULL .
That is, the output is as follows:
.Bd -literal -offset indent
progname: fmt: error message string
.Ed
.Pp
if
.Fa fmt
is not
.Dv NULL ,
or:
.Bd -literal -offset indent
progname: error message string
.Ed
.Pp
if it is.
.Pp
In the case of the
.Fn err ,
.Fn verr ,
.Fn warn ,
and
.Fn vwarn
functions, the error value used is the current value of the global variable
.Va errno ,
while for the
.Fn errc ,
.Fn verrc ,
.Fn warnc ,
and
.Fn vwarnc
function the argument
.Fa code
is used.
.Pp
In all cases, the output is followed by a newline character.
.Pp
The
.Fn err ,
.Fn verr ,
.Fn errc ,
.Fn verrc ,
.Fn errx ,
and
.Fn verrx
functions do not return, but exit with the value of the argument
.Fa eval .
.Sh EXAMPLES
Display the current
.Va errno
information string and exit:
.Bd -literal -offset indent
if ((p = malloc(size)) == NULL)
	err(1, NULL);
if ((fd = open(file_name, O_RDONLY, 0)) == -1)
	err(1, "%s", file_name);
.Ed
.Pp
Display an error message and exit:
.Bd -literal -offset indent
if (tm.tm_hour < START_TIME)
	errx(1, "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)
	err(1, "%s", block_device);
.Ed
.Sh SEE ALSO
.Xr exit 3 ,
.Xr perror 3 ,
.Xr printf 3 ,
.Xr strerror 3
.Sh HISTORY
The functions
.Fn err ,
.Fn errx ,
.Fn verr ,
.Fn verrx ,
.Fn warn ,
.Fn warnx ,
.Fn vwarn ,
and
.Fn vwarnx
first appeared in
.Bx 4.4 .
The functions
.Fn errc ,
.Fn verrc ,
.Fn warnc ,
and
.Fn vwarnc
first appeared in
.Fx 3.0
and were ported to
.Ox 5.6 .
.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 the stack,
leading to a possible security hole.
This holds true even if the string has been built
.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