ERR(3C) Standard C Library Functions ERR(3C)


err, verr, errx, verrx, warn, vwarn, warnx, vwarnx - formatted error


#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);


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.
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 eval.


Example 1: Display the current errno information string and terminate with

status indicating failure.

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

Example 2: Display an error message and terminate with status indicating


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

Example 3: 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);


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(3C), as the resulting string can 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);


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Committed |
|MT-Level | Safe with Exceptions |

These functions are safe to use in multithreaded applications as long as
setlocale(3C) is not being called to change the locale.


exit(3C), getexecname(3C), setlocale(3C), strerror(3C), attributes(7)

November 24, 2014 ERR(3C)