The err() function
shall display a formatted error message on the standard error stream.
First, err() shall write
the last component of the program name, a colon
character, and a space character.
If fmt is non-NULL, it shall be used as a
format string for the printf()
family of functions, and err() shall
write the formatted message, a colon character, and a space.
Finally, the error message
string affiliated with the current value of the global variable
errno shall be
written, followed by a newline character.
The err() function shall not return, the program shall terminate with the exit value of eval.