The GNU library provides ways for you to define additional kinds of streams that do not necessarily correspond to an open file.
One such type of stream takes input from or writes output to a string.
These kinds of streams are used internally to implement the
sprintf and sscanf functions. You can also create such a
stream explicitly, using the functions described in String Streams.
More generally, you can define streams that do input/output to arbitrary objects using functions supplied by your program. This protocol is discussed in Custom Streams.
Portability Note: The facilities described in this section are specific to GNU. Other systems or C implementations might or might not provide equivalent functionality.