The err() function 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 the fmt argument is not 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.
The err() function does not return, but exits with the value of the argument eval.