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Section 7.19.7.11 of C99 defines {{ungetc()}} as follows: \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]

The ungetc function pushes the character specified by c (converted to an unsigned char) back onto the input stream pointed to by stream. Pushed-back characters will 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, fsetpos, or rewind) discards any pushed-back characters for the stream. The external storage corresponding to the stream is unchanged.

One character of pushback is guaranteed.

The ungetc() function pushes a character onto an input stream. This pushed character can then be read by subsequent calls to functions that read from that stream. However, the ungetc() function has serious limitations. A call to a file positioning function, such as fseek(), will discard any character pushed back by ungetc(). Also, the C standard only guarantees that the pushing back of one character will succeed. Consequently, subsequent calls to ungetc() must be separated by a call to a read function or a file-positioning function (which will discard any data pushed by ungetc()).

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