The function __backspace() is used by the scanf family of functions, and must be re-implemented if you retarget the stdio arrangements at the fgetc() level.

The syntax is:

int __backspace(FILE *stream);

__backspace(stream) must only be called after reading a character from the stream. You must not call it after a write, a seek, or immediately after opening the file, for example. It returns to the stream the last character that was read from the stream, so that the same character can be read from the stream again by the next read operation. This means that a character that was read from the stream by scanf but that is not required (that is, it terminates the scanf operation) is read correctly by the next function that reads from the stream.

__backspace is separate from ungetc(). This is to guarantee that a single character can be pushed back after the scanf family of functions has finished.

The value returned by __backspace() is either 0 (success) or EOF (failure). It returns EOF only if used incorrectly, for example, if no characters have been read from the stream. When used correctly, __backspace() must always return 0, because the scanf family of functions do not check the error return.

The interaction between __backspace() and ungetc() is: