char *strerror( char *str )

	- prototype in string.h

	- str should contain your customized error message to which a system
	  error message will be appended
	- allows you to generate customized error messages, returning a
	  pointer to a null-terminated string containing an error message
	- if str is NULL, the return value contains the most recently
	  generated system error message; this string is null-terminated
	- if str is not NULL, the return value contains, a colon, a space,
	  the most recently generated system error message, and a newline;
	  with length of str being 94 characters or less
	- this function generates the error string but does NOT print it
	- for accurate error-handling, strerror should be called as soon
	  as a library routine generates an error return