The warn() function
shall display a formatted error message on the standard error stream.
The output shall consist of 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 the formatted message, a
colon character, and a space are written to
Finally, the error message
string affiliated with the current value of the global variable
errno shall be
stderr, followed by a newline character.