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ERR(3) Library Functions Manual ERR(3)


err, verr, errc, verrc, errx, verrx, warn, vwarn, warnc, vwarnc, warnx, vwarnxformatted error messages


#include <err.h>
err(int eval, const char *fmt, ...);
verr(int eval, const char *fmt, va_list args);
errc(int eval, int code, const char *fmt, ...);
verrc(int eval, int code, const char *fmt, va_list args);
errx(int eval, const char *fmt, ...);
verrx(int eval, const char *fmt, va_list args);
warn(const char *fmt, ...);
vwarn(const char *fmt, va_list args);
warnc(int code, const char *fmt, ...);
vwarnc(int code, const char *fmt, va_list args);
warnx(const char *fmt, ...);
vwarnx(const char *fmt, va_list args);


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.
In the case of the errx(), verrx(), warnx(), and vwarnx() functions only, no additional text is output, so the output looks like the following:
progname: fmt
The other 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.
In the case of the err(), verr(), warn(), and vwarn() functions, the error value used is the current value of the global variable errno, while for the errc(), verrc(), warnc(), and vwarnc() function the argument code is used.
In all cases, the output is followed by a newline character.
The err(), verr(), errc(), verrc(), errx(), and verrx() functions do not return, but exit with the value of the argument eval.


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);


exit(3), perror(3), printf(3), strerror(3)


The functions err(), errx(), verr(), verrx(), warn(), warnx(), vwarn(), and vwarnx() first appeared in 4.4BSD. The functions errc(), verrc(), warnc(), and vwarnc() first appeared in FreeBSD 3.0 and were ported to OpenBSD 5.6.


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);
April 23, 2014 OpenBSD-current