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.Dd November 15, 2022
.Dt ERR 3C
.Os
.Sh NAME
.Nm err ,
.Nm errc ,
.Nm errx ,
.Nm warn ,
.Nm warnc ,
.Nm warnx ,
.Nm verr ,
.Nm verrc ,
.Nm verrx ,
.Nm vwarn ,
.Nm vwarnc ,
.Nm vwarnx
.Nd formatted error messages
.Sh SYNOPSIS
.In err.h
.Ft void
.Fo err
.Fa "int eval"
.Fa "const char *fmt"
.Fa "..."
.Fc
.Ft void
.Fo errc
.Fa "int eval"
.Fa "int code"
.Fa "const char *fmt"
.Fa "..."
.Fc
.Ft void
.Fo errx
.Fa "int eval"
.Fa "const char *fmt"
.Fa "..."
.Fc
.Ft void
.Fo warn
.Fa "const char *fmt"
.Fa "..."
.Fc
.Ft void
.Fo warnc
.Fa "int code"
.Fa "const char *fmt"
.Fa "..."
.Fc
.Ft void
.Fo warnx
.Fa "const char *fmt"
.Fa "..."
.Fc
.Ft void
.Fo verr
.Fa "int eval"
.Fa "const char *fmt"
.Fa "va_list args"
.Fc
.Ft void
.Fo verrc
.Fa "int eval"
.Fa "int code"
.Fa "const char *fmt"
.Fa "va_list args"
.Fc
.Ft void
.Fo verrx
.Fa "int eval"
.Fa "const char *fmt"
.Fa "va_list args"
.Fc
.Ft void
.Fo vwarn
.Fa "const char *fmt"
.Fa "va_list args"
.Fc
.Ft void
.Fo vwarnc
.Fa "int code"
.Fa "const char *fmt"
.Fa "va_list args"
.Fc
.Ft void
.Fo vwarnx
.Fa "const char *fmt"
.Fa "va_list args"
.Fc
.Sh DESCRIPTION
The
.Fn err
and
.Fn warn
family of functions display a formatted error message to standard error.
In all cases, the last component of the program name, followed by a colon
character and a space, are output.
If the
.Ar fmt
argument is not
.Dv NULL ,
the formatted error message is output.
.Pp
In the case of the
.Fn err ,
.Fn errc ,
.Fn warn ,
.Fn warnc ,
.Fn verr ,
.Fn verrc ,
.Fn vwarn
and
.Fn vwarnc
functions, an error message obtained from
.Xr strerror 3C
is output next, preceded by a colon character and a space if
.Ar fmt
is not
.Dv NULL .
The
.Fn err ,
.Fn warn ,
.Fn verr
and
.Fn vwarn
functions produce the error string affiliated with the current value of the
global variable
.Va errno .
The
.Fn errc ,
.Fn warnc ,
.Fn verrc
and
.Fn vwarnc
functions use the provided
.Ar code
value to look up the error message.
.Pp
The
.Fn errx ,
.Fn verrx ,
.Fn warnx
and
.Fn vwarnx
functions will not output this error message string.
.Pp
In all cases, the output is followed by a newline character.
.Pp
The
.Fn err ,
.Fn errc ,
.Fn errx ,
.Fn verr ,
.Fn verrc
and
.Fn verrx
functions do not return, but instead cause the program to terminate with the
status value given by the
.Ar eval
argument.
.Sh EXAMPLES
.Sy Example 1
Display the current
.Va errno
information string and terminate with status indicating failure.
.Bd -literal -offset indent
#include <err.h>
\&...
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
.Sy Example 2
Display an error message and terminate with status indicating failure.
.Bd -literal -offset indent
if (tm.tm_hour < START_TIME)
	errx(EXIT_FAILURE, "wait until %s", start_time_string);
.Ed
.Pp
.Sy Example 3
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
.Pp
.Sy Example 4
Warn of an error using a custom error code
.Bd -literal -offset indent
int error = function_returning_error_code();
if (error != 0)
	warnc(error, "%s", "function did not succeed");
.Ed
.Sh WARNINGS
It is important never to pass a string with user-supplied data as a format
without using
.Sq %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 by hand using a function
like
.Xr snprintf 3C ,
as the resulting string can 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
.Sh INTERFACE STABILITY
.Sy Committed
.Sh MT-LEVEL
.Sy MT-Safe with Exceptions
.Pp
These functions are safe to use in multithreaded applications as long as
.Xr setlocale 3C
is not being called to change the locale.
.Sh SEE ALSO
.Xr exit 3C ,
.Xr getexecname 3C ,
.Xr setlocale 3C ,
.Xr strerror 3C ,
.Xr attributes 7
.Sh STANDARDS
The functions described in this man page are
.Bx
extensions and should not be used in portable code.