NAME

err, verr, errx, verrx, warn, vwarn, warnx, vwarnx —

formatted error messages

SYNOPSIS

#include <err.h>

void
err(int eval, const char *fmt, ...);

void
verr(int eval, const char *fmt, va_list args);

void
errx(int eval, const char *fmt, ...);

void
verrx(int eval, const char *fmt, va_list args);

void
warn(const char *fmt, ...);

void
vwarn(const char *fmt, va_list args);

void
warnx(const char *fmt, ...);

void
vwarnx(const char *fmt, va_list args);

DESCRIPTION

The err() and 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 (‘:’) character and a space, are output. The text that follows depends on the function being called. The fmt specification (and associated arguments) may be any format allowed by printf(3), a simple string, or NULL. If the fmt argument is not NULL, the formatted error message is output.

In the case of the err(), verr(), warn(), and vwarn() functions only, the error message string affiliated with the current value of the global variable errno is output (see strerror(3)), preceded by a colon character and a space if fmt is not NULL. That is, the output is as follows:

progname: fmt: error message string

if fmt is not NULL, or:

progname: error message string

if it is.

The counterpart functions, errx(), verrx(), warnx(), and vwarnx(), do not output the error message string, so the output looks like the following:

progname: fmt

In all cases, the output is followed by a newline character.

The err(), verr(), errx(), and verrx() functions do not return, but exit with the value of the argument eval.

EXAMPLES

Display the current errno information string and exit:

if ((p = malloc(size)) == NULL)
	err(1, NULL);
if ((fd = open(file_name, O_RDONLY, 0)) == -1)
	err(1, "%s", file_name);

Display an error message and exit:

if (tm.tm_hour < START_TIME)
	errx(1, "too early, wait until %s", start_time_string);

Warn of an error:

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);

HISTORY

The err() and warn() functions first appeared in 4.4BSD.

CAVEATS

It is important never to pass a string with user-supplied data as a format without using ‘%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 snprintf(), as the resulting string may still contain user-supplied conversion specifiers for later interpolation by the err() and warn() functions.

Always be sure to use the proper secure idiom:

err(1, "%s", string);