NAME
err, verr,
errx, verrx,
warn, vwarn,
warnx, vwarnx —
formatted error messages
SYNOPSIS
#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);
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), a simple string, or NULL. If the
fmt argument is not NULL, the
formatted error message is output.
In the case of the
err(),
verr(),
warn(), and
vwarn()
functions only, the error message string affiliated with the current value
of the global variable errno is output (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 counterpart functions,
errx(),
verrx(),
warnx(),
and
vwarnx(),
do not output the error message string, so the output looks like the
following:
progname: fmt
In all cases, the output is followed by a newline character.
The
err(),
verr(),
errx(), and verrx()
functions do not return, but exit with the value of the argument
eval.
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
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 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);