ungetc(S)                      6 January 1993                      ungetc(S)


 Name

    ungetc - push character back into input stream

 Syntax


    cc ... -lc


    #include <stdio.h>

    int ungetc (c, stream)
    int c;
    FILE *stream;


 Description

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

    One character of pushback is guaranteed.  If the ungetc function 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 fails and
    the input stream is unchanged.

    A successful call to the ungetc function clears the end-of-file indicator
    for the stream.  The value of the file-position indicator for the stream
    after reading or discarding all pushed-back characters is the same as it
    was before the characters were pushed back.  One character of pushback is
    guaranteed, provided something has already been read from the stream and
    the stream is actually buffered.  In the case that stream is stdin, and
    the stream is buffered, one character may be pushed back onto the buffer
    without a previous read statement.  For a stream, the value of its file-
    position indicator after a successful call to the ungetc function is
    unspecified until all pushed-back characters are read or discarded.

 Diagnostics

    ungetc returns EOF if it cannot insert the character.

 Notes

    When stream is stdin, one character may be pushed back onto the buffer
    without a previous read statement.

 See also

    fseek(S), getc(S), setbuf(S), stdio(S)

 Standards conformance

    ungetc is conformant with:
    AT&T SVID Issue 2;
    X/Open Portability Guide, Issue 3, 1989;
    ANSI X3.159-1989 Programming Language -- C;
    Intel386 Binary Compatibility Specification, Edition 2 (iBCSe2);
    IEEE POSIX Std 1003.1-1990 System Application Program Interface (API) [C
    Language] (ISO/IEC 9945-1);
    and NIST FIPS 151-1.