The errx() function shall display a formatted error message on the standard error stream. The last component of the program name, a colon character, and a space shall be output. If fmt is non-NULL, it shall be used as the format string for the printf() family of functions, and the formatted error message, a colon character, and a space shall be output. The output shall be followed by a newline character.
errx() does not return, but shall exit with the value of eval.