.\" $NetBSD: err.3,v 1.24 2024/02/02 21:16:41 jkoshy Exp $
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.\" Copyright (c) 1993
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.\"	@(#)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