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fseek, _fseeki64

Moves the file pointer to a specified location.

int fseek( 
   FILE *stream,
   long offset,
   int origin 
);
int _fseeki64( 
   FILE *stream,
   __int64 offset,
   int origin 
);

stream

Pointer to FILE structure.

offset

Number of bytes from origin.

origin

Initial position.

If successful, fseek and _fseeki64 returns 0. Otherwise, it returns a nonzero value. On devices incapable of seeking, the return value is undefined. If stream is a null pointer, or if origin is not one of allowed values described below, fseek and _fseeki64 invoke the invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, these functions set errno to EINVAL and return -1.

The fseek and _fseeki64 functions moves the file pointer (if any) associated with stream to a new location that is offset bytes from origin. The next operation on the stream takes place at the new location. On a stream open for update, the next operation can be either a read or a write. The argument origin must be one of the following constants, defined in STDIO.H:

SEEK_CUR

Current position of file pointer.

SEEK_END

End of file.

SEEK_SET

Beginning of file.

You can use fseek and _fseeki64 to reposition the pointer anywhere in a file. The pointer can also be positioned beyond the end of the file. fseek and _fseeki64clears the end-of-file indicator and negates the effect of any prior ungetc calls against stream.

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. If no I/O operation has yet occurred on a file opened for appending, the file position is the start of the file.

For streams opened in text mode, fseek and _fseeki64have limited use, because carriage return–linefeed translations can cause fseek and _fseeki64to produce unexpected results. The only fseek and _fseeki64operations guaranteed to work on streams opened in text mode are:

  • Seeking with an offset of 0 relative to any of the origin values.

  • Seeking from the beginning of the file with an offset value returned from a call to ftell when using fseekor _ftelli64when using_fseeki64.

Also 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 fseek and ftellor_fseeki64 and _ftelli64, to move within a file that ends with a CTRL+Z may cause fseek or _fseeki64 to behave improperly near the end of the file.

When the CRT opens a file that begins with a Byte Order Mark (BOM), the file pointer is positioned after the BOM (that is, at the start of the file's actual content). If you have to fseek to the beginning of the file, use ftell to get the initial position and fseek to it rather than to position 0.

This function locks out other threads during execution and is therefore thread-safe. For a non-locking version, see _fseek_nolock, _fseeki64_nolock.

Function

Required header

fseek

<stdio.h>

_fseeki64

<stdio.h>

For additional compatibility information, see Compatibility in the Introduction.

// crt_fseek.c
// This program opens the file FSEEK.OUT and
// moves the pointer to the file's beginning.
 
#include <stdio.h>

int main( void )
{
   FILE *stream;
   char line[81];
   int  result;

   if ( fopen_s( &stream, "fseek.out", "w+" ) != 0 )
   {
      printf( "The file fseek.out was not opened\n" );
      return -1;
   }
   fprintf( stream, "The fseek begins here: "
                    "This is the file 'fseek.out'.\n" );
   result = fseek( stream, 23L, SEEK_SET);
   if( result )
      perror( "Fseek failed" );
   else
   {
      printf( "File pointer is set to middle of first line.\n" );
      fgets( line, 80, stream );
      printf( "%s", line );
    }
   fclose( stream );
}
File pointer is set to middle of first line.
This is the file 'fseek.out'.
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