ungetc() - un-get character from input stream
#include <stdio.h>
int ungetc (int c, FILE *stream)
The ungetc(3) function pushes the character c (converted to an unsigned char) back onto the input stream pointed to by stream. The pushed-backed characters will be returned by subsequent reads on the stream (in reverse order). A successful intervening call, using the same stream, to one of the file positioning functions (fseek(3), fsetpos(3), or rewind(3)) will discard the pushed back characters.
One character of push-back is guaranteed, but as long as there is sufficient memory, an effectively infinite amount of pushback is allowed.
If a character is successfully pushed-back, the end-of-file indicator for the stream is cleared.
If the value of the argument c character equals EOF, the operation will fail and the stream will remain unchanged.
The ungetc(3) function returns the character pushed-back after the conversion, or EOF if the operation fails.
getc(3)
fseek(3)
setvbuf(3)