ungetc
To Do UnreadingThe function to unread a character is called ungetc
, because it
reverses the action of getc
.
int
ungetc (int c, FILE *stream)
¶Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt | See POSIX Safety Concepts.
The ungetc
function pushes back the character c onto the
input stream stream. So the next input from stream will
read c before anything else.
If c is EOF
, ungetc
does nothing and just returns
EOF
. This lets you call ungetc
with the return value of
getc
without needing to check for an error from getc
.
The character that you push back doesn’t have to be the same as the last
character that was actually read from the stream. In fact, it isn’t
necessary to actually read any characters from the stream before
unreading them with ungetc
! But that is a strange way to write a
program; usually ungetc
is used only to unread a character that
was just read from the same stream. The GNU C Library supports this
even on files opened in binary mode, but other systems might not.
The GNU C Library only supports one character of pushback—in other
words, it does not work to call ungetc
twice without doing input
in between. Other systems might let you push back multiple characters;
then reading from the stream retrieves the characters in the reverse
order that they were pushed.
Pushing back characters doesn’t alter the file; only the internal
buffering for the stream is affected. If a file positioning function
(such as fseek
, fseeko
or rewind
; see File Positioning) is called, any pending pushed-back characters are
discarded.
Unreading a character on a stream that is at end of file clears the end-of-file indicator for the stream, because it makes the character of input available. After you read that character, trying to read again will encounter end of file.
wint_t
ungetwc (wint_t wc, FILE *stream)
¶Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt | See POSIX Safety Concepts.
The ungetwc
function behaves just like ungetc
just that it
pushes back a wide character.
Here is an example showing the use of getc
and ungetc
to
skip over whitespace characters. When this function reaches a
non-whitespace character, it unreads that character to be seen again on
the next read operation on the stream.
#include <stdio.h> #include <ctype.h> void skip_whitespace (FILE *stream) { int c; do /* No need to check forEOF
because it is notisspace
, andungetc
ignoresEOF
. */ c = getc (stream); while (isspace (c)); ungetc (c, stream); }