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.Dd $Mdocdate: May 31 2007 $
.Dt ERR 3
.Os
.Sh NAME
.Nm err ,
.Nm verr ,
.Nm errx ,
.Nm verrx ,
.Nm warn ,
.Nm vwarn ,
.Nm warnx ,
.Nm vwarnx
.Nd formatted error messages
.Sh SYNOPSIS
.Fd #include <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 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 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 ,
a simple string, 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 err ,
.Fn verr ,
.Fn warn ,
and
.Fn vwarn
functions only, the error message string affiliated with the current value of
the global variable
.Va errno
is output (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
The counterpart functions,
.Fn errx ,
.Fn verrx ,
.Fn warnx ,
and
.Fn vwarnx ,
do not output the error message string, so the output looks like the following:
.Bd -literal -offset indent
progname: fmt
.Ed
.Pp
In all cases, the output is followed by a newline character.
.Pp
The
.Fn err ,
.Fn verr ,
.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
.Fn err
and
.Fn warn
functions first appeared in
.Bx 4.4 .
.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