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ftell

Gets the current position of a file pointer.

long ftell( 
   FILE *stream 
);

Parameter

stream
Target FILE structure.

Return Value

ftell returns the current file position. The value returned by ftell may not reflect the physical byte offset for streams opened in text mode, because text mode causes carriage return–linefeed translation. Use ftell with fseek to return to file locations correctly. On error, ftell returns –1L and errno is set to one of two constants, defined in ERRNO.H. The EBADF constant means the stream argument is not a valid file pointer value or does not refer to an open file. EINVAL means an invalid stream argument was passed to the function. On devices incapable of seeking (such as terminals and printers), or when stream does not refer to an open file, the return value is undefined.

See _doserrno, errno, _sys_errlist, and _sys_nerr for more information on these, and other, return codes.

Remarks

The ftell function gets the current position of the file pointer (if any) associated with stream. The position is expressed as an offset relative to the beginning of the stream.

Note that when a file is opened for appending data, the current file position is determined by the last I/O operation, not by where the next write would occur. For example, if a file is opened for an append and the last operation was a read, the file position is the point where the next read operation would start, not where the next write would start. (When a file is opened for appending, the file position is moved to end of file before any write operation.) If no I/O operation has yet occurred on a file opened for appending, the file position is the beginning of the file.

In text mode, CTRL+Z is interpreted as an end-of-file character on input. In files opened for reading/writing, fopen and all related routines check for a CTRL+Z at the end of the file and remove it if possible. This is done because using ftell and fseek to move within a file that ends with a CTRL+Z may cause ftell to behave improperly near the end of the file.

Requirements

Function Required header Optional headers Compatibility
ftell <stdio.h> <errno.h> ANSI, Win 98, Win Me, Win NT, Win 2000, Win XP

For additional compatibility information, see Compatibility in the Introduction.

Libraries

All versions of the C run-time libraries.

Example

// crt_ftell.c
/* This program opens a file named CRT_FTELL.C
 * for reading and tries to read 100 characters. It
 * then uses ftell to determine the position of the
 * file pointer and displays this position.
 */

#include <stdio.h>

FILE *stream;

int main( void )
{
   long position;
   char list[100];
   if( (stream = fopen( "crt_ftell.c", "rb" )) != NULL )
   {
      /* Move the pointer by reading data: */
      fread( list, sizeof( char ), 100, stream );
      /* Get position after read: */
      position = ftell( stream );
      printf( "Position after trying to read 100 bytes: %ld\n",
              position );
      fclose( stream );
   }
}

Output

Position after trying to read 100 bytes: 100

See Also

Stream I/O Routines | fgetpos | fseek | _lseek | _tell | Run-Time Routines and .NET Framework Equivalents

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