Upon successful completion a pointer is returned;
this pointer becomes invalid after the next
(whether successful or not)
or as soon as the stream is closed.
does not distinguish between end-of-file and error; the routines
must be used
to determine which occurred.
If an error occurs, the global variable
is set to indicate the error.
The end-of-file condition is remembered, even on a terminal, and all
subsequent attempts to read will return
until the condition is
The text to which the returned pointer points may be modified,
provided that no changes are made beyond the returned size.
These changes are lost as soon as the pointer becomes invalid.