ungetc(3)ungetc(3)NAME
ungetc, ungetwc - Push a byte or wide-character code back into the
input stream
SYNOPSIS
#include <stdio.h>
int ungetc(
int c,
FILE *stream ); #include <wchar.h>
wint_t ungetwc(
wint_t wchar,
FILE *stream );
For the ungetwc() function, application developers may want to specify
an #include statement for <stdio.h> before the one for <wchar.h> if
programs are being developed for multiple platforms. The additional
#include statement is not required on Tru64 UNIX systems or by ISO or
X/Open standards, but may be required on other vendors' systems that
conform to these standards.
LIBRARY
Standard C Library (libc)
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
ungetc(), ungetwc(): ISO C, XPG4, XPG4-UNIX
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
Specifies a byte to be inserted into the input stream. Specifies the
input stream. Specifies a wide-character code.
DESCRIPTION
The ungetc() function converts the byte specified by the c parameter
into an unsigned char and inserts it into the buffer associated with
the input stream specified by the stream parameter. This causes the
next call to the getc() function to return c.
If the c parameter has a value equal to EOF, the ungetc() function does
not place anything in the buffer and the input stream is unchanged.
The ungetwc() function inserts the wide character specified by wchar
into the buffer associated with the input stream. The wide character
may consist of one or more bytes. This causes the next call to the
getwc() function to return the value of the wchar parameter.
If the ungetwc() wchar parameter is WEOF, the ungetwc() function does
not place anything in the buffer and the input stream is unchanged.
A call to one of the file-positioning functions (fseek(), fsetpos(), or
rewind()), if it uses the same stream and intervenes between a call to
ungetc() or ungetwc() and getc() or getwc(), discards any pushed back
bytes for the stream. The value of the file-position indicator after
reading or discarding pushed-back bytes will be the same as it was
before the bytes were pushed back.
A successful call to ungetc() or ungetwc() clears the end-of-file indi‐
cator and decrements the file-position indicator for the stream. If the
value of the file-position indicator is zero before the call, the value
is indeterminate after the call. These functions do not have any effect
on the external storage corresponding to the stream.
One character of push back is guaranteed (this corresponds to one byte
for ungetc()) and one or more bytes for ungetwc()); however, if one of
these functions is called too many times on the same stream without an
intervening read or file-positioning operation on that stream, industry
standards specify that the functions may fail. (Applications do not
encounter this failure on Tru64 UNIX systems. However, results are
unpredictable if applications intermix calls to ungetc() and ungetwc()
on the same stream.)
RETURN VALUES
On successful insertion of the converted byte into the stream, the
ungetc() function returns the value of c. Otherwise, the function
returns EOF.
On successful insertion of the converted wide character into the
stream, the ungetwc() function returns the value of wchar. Otherwise,
the function returns WEOF.
ERRORS
If the following condition occurs, the ungetwc() function sets errno to
the corresponding value: An invalid byte sequence is detected, or a
wide-character code does not correspond to a valid single-byte or
multibyte character in the current locale.
SEE ALSO
Functions: fseek(3), getc(3), getwc(3), setbuf(3)
Standards: standards(5)ungetc(3)
