xxxxxxxxxx
The err() function produces a message that consists of:
the last component of the program name,
followed by a colon and a space.
the formatted message,
followed by a colon and a space,
if the fmt argument isn't NULL.
the string associated with the current value of errno. a newline character.
xxxxxxxxxx
The err() and warn() family of functions display a formatted error message on the standard error output.
In all cases, the last component of the program name,
a colon character, and a space are output.
If the fmt argument is not NULL, the printf(3)-like formatted error message is output.