ERR(3)

NAME
err, verr, errx, verrx, errc, verrc, warn, vwarn, warnx, vwarnx, warnc, vwarnc - formatted error messages

LIBRARY
libc

SYNOPSIS
#include <err.h>
void
err(int status, const char *fmt, ...);
void
verr(int status, const char *fmt, va_list args);
void
errx(int status, const char *fmt, ...);
void
verrx(int status, const char *fmt, va_list args);
void
errc(int status, int code, const char *fmt, ...);
void
verrc(int status, int code, 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);
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 these functions output the last component of the program name, a colon character, and a space. If the fmt argument is not NULL, it is used as a printf(3) -like format specification for the error message.

In the case of the err(), verr(), warn(), and vwarn() functions, an additional error message string affiliated with the current value of the global variable errno is output next, preceded by a colon character and a space if fmt is not NULL. The errc(), verrc(), warnc(), and vwarnc() functions take an additional code argument to be used as the error number instead of using the global errno variable. The errx(), verrx(), warnx(), and vwarnx() functions will not output an additional error message string.

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

The err(), verr(), errc(), verrc(), errx(), and verrx() functions do not return, but instead cause the program to terminate with the status value given by the argument status. It is often appropriate to use the value EXIT_FAILURE, defined in <stdlib.h>, as the status argument given to these functions.

EXAMPLES
Display the current errno information string and terminate with status indicating failure:

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

Display an error message and terminate with status indicating failure:

    if (tm.tm_hour < START_TIME)
    	errx(EXIT_FAILURE, "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)
    	warn("%s", block_device);

SEE ALSO
exit(3), getprogname(3), printf(3), strerror(3)

HISTORY
The err() and warn() functions first appeared in 4.4BSD. The errc() and warnc() functions first appeared in FreeBSD 3.0 and NetBSD 7.0.

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 your stack, leading to a possible security hole. This holds true even if you have built the string “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);