ftell and _ftelli64 return the current file position. The value returned by ftell and _ftelli64 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 or _ftelli64 with _fseeki64 to return to file locations correctly. On error, ftell and _ftelli64 invoke the invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, these functions return –1L and set errno 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.

The ftell and _ftelli64 functions retrieve 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 the combination of ftell and fseek or _ftelli64 and _fseeki64, to move within a file that ends with a CTRL+Z may cause ftell or _ftelli64 to behave improperly near the end of the file.

This function locks the calling thread during execution and is therefore thread-safe. For a non-locking version, see _ftell_nolock.

For additional compatibility information, see Compatibility in the Introduction.

// 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( fopen_s( &stream, "crt_ftell.c", "rb" ) == 0 )
   {
      // 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

System::IO::FileStream::Position

Reference