int ungetc(int c, FILE *stream);
IEEE Std 1003.1-2017
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 (
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
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.
Upon successful completion,
ungetc() shall return the byte pushed back after conversion. Otherwise, it shall return
No errors are defined.