ungetc, ungetwc

Article
04/22/2008

Pushes a character back onto the stream.

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

Parameters

c
Character to be pushed.
stream
Pointer to FILE structure.

Return Value

If successful, each of these functions returns the character argument c*.* If c cannot be pushed back or if no character has been read, the input stream is unchanged and ungetc returns EOF; ungetwc returns WEOF. If stream is NULL, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, EOF or WEOF is returned and errno is set to EINVAL.

For information on these and other error codes, see _doserrno, errno, _sys_errlist, and _sys_nerr.

Remarks

The ungetc function pushes the character c back onto stream and clears the end-of-file indicator. The stream must be open for reading. A subsequent read operation on stream starts with c*.* An attempt to push EOF onto the stream using ungetc is ignored.

Characters placed on the stream by ungetc may be erased if fflush, fseek, fsetpos, or rewind is called before the character is read from the stream. The file-position indicator will have the value it had before the characters were pushed back. The external storage corresponding to the stream is unchanged. On a successful ungetc call against a text stream, the file-position indicator is unspecified until all the pushed-back characters are read or discarded. On each successful ungetc call against a binary stream, the file-position indicator is decremented; if its value was 0 before a call, the value is undefined after the call.

Results are unpredictable if ungetc is called twice without a read or file-positioning operation between the two calls. After a call to fscanf, a call to ungetc may fail unless another read operation (such as getc) has been performed. This is because fscanf itself calls ungetc.

ungetwc is a wide-character version of ungetc. However, on each successful ungetwc call against a text or binary stream, the value of the file-position indicator is unspecified until all pushed-back characters are read or discarded.

These functions are thread-safe and lock sensitive data during execution. For a non-locking version, see _ungetc_nolock, _ungetwc_nolock.

Generic-Text Routine Mappings

TCHAR.H routine	_UNICODE & _MBCS not defined	_MBCS defined	_UNICODE defined
_ungettc	ungetc	ungetc	ungetwc

Requirements

Routine	Required header
ungetc	<stdio.h>
ungetwc	<stdio.h> or <wchar.h>

For additional compatibility information, see Compatibility in the Introduction.

Example

// crt_ungetc.c
// This program first converts a character
// representation of an unsigned integer to an integer. If
// the program encounters a character that is not a digit,
// the program uses ungetc to replace it in the  stream.
//

#include <stdio.h>
#include <ctype.h>

int main( void )
{
   int ch;
   int result = 0;

   // Read in and convert number:
   while( ((ch = getchar()) != EOF) && isdigit( ch ) )
      result = result * 10 + ch - '0';    // Use digit.
   if( ch != EOF )
      ungetc( ch, stdin );                // Put nondigit back.
   printf( "Number = %d\nNext character in stream = '%c'", 
            result, getchar() );
}

521a

Number = 521
Next character in stream = 'a'

.NET Framework Equivalent

Not applicable. To call the standard C function, use PInvoke. For more information, see Platform Invoke Examples.

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