#include <stdio.h> int ungetc(int c, FILE *stream);
The ungetc() function shall push the byte specified by c (converted to an unsigned char) back onto the input stream pointed to by stream. The pushed-back bytes shall be returned by subsequent reads on that stream in the reverse order of their pushing. A successful intervening call (with the stream pointed to by stream) to a file-positioning function (fseek(), fseeko(), fsetpos(), or rewind()) or fflush() shall discard any pushed-back bytes for the stream. The external storage corresponding to the stream shall be unchanged.
One byte of push-back shall be provided. If ungetc() is called too many times on the same stream without an intervening read or file-positioning operation on that stream, the operation may fail.
If the value of c equals that of the macro EOF, the operation shall fail and the input stream shall be left unchanged.
A successful call to ungetc() shall clear the end-of-file indicator for the stream. The value of the file-position indicator for the stream after all pushed-back bytes have been read, or discarded by calling fseek(), fseeko(), fsetpos(), or rewind() (but not fflush()), shall be the same as it was before the bytes were pushed back. The file-position indicator is decremented by each successful call to ungetc(); if its value was 0 before a call, its value is unspecified after the call.
The Base Definitions volume of POSIX.1-2008, <stdio.h>
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