ERR(3)

NAME
err, verr, errx, verrx, warn, vwarn, warnx, vwarnx - 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
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, a colon character, and a space are output. If the fmt argument is not NULL, the formatted error message is output. In the case of the err(), verr(), warn(), and vwarn() functions, the 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. In all cases, the output is followed by a newline character. The errx(), verrx(), warnx(), and vwarnx() functions will not output this error message string.

The err(), verr(), 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), strerror(3)

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 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);