ERRC(3bsd)

NAME
errc, verrc, warnc, vwarnc - formatted error messages

LIBRARY
libbsd

SYNOPSIS
#include <err.h>
(See libbsd(7) for include usage.)
void
errc(int status, int code, const char *fmt, ...);
void
verrc(int status, int code, const char *fmt, va_list args);
void
warnc(int code, const char *fmt, ...);
void
vwarnc(int code, 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) or NULL. If the fmt argument is not NULL, the formatted error message is output.

The functions all output an error message string affiliated with an error value (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 argument code is used as the error value instead of the current value of the global variable errno.

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

The errc(), and verrc() functions do not return, but exit with the value of the argument status.

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);

SEE ALSO
err(3) exit(3), perror(3), printf(3), strerror(3)

HISTORY
The functions errc(), verrc(), warnc(), and vwarnc() first appeared in FreeBSD 3.0, NetBSD 7.0 and OpenBSD 5.6.

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() family of functions.

Always be sure to use the proper secure idiom:

    errc(1, 0, "%s", string);