err
displays a formatted error message on the
standard error output. The last component of the program name, a colon
character, and a space are output. If fmt is
non-NULL, the formatted error message, a colon
character, and a space are output. The error message string affiliated
with the current value of the global variable errno
is output. The output is followed by a newline character.
err
does not return, but exits with the value of
eval.