_lseek, _lseeki64

Article
04/22/2008

Moves a file pointer to the specified location.

long _lseek(
   int fd,
   long offset,
   int origin 
);
__int64 _lseeki64(
   int fd,
   __int64 offset,
   int origin 
);

Parameters

fd
File descriptor referring to an open file.
offset
Number of bytes from origin.
origin
Initial position.

Return Value

_lseek returns the offset, in bytes, of the new position from the beginning of the file. _lseeki64 returns the offset in a 64-bit integer. The function returns –1L to indicate an error. If passed an invalid parameter, such as a bad file descriptor, or the value for origin is invalid or the position specified by offset is before the beginning of the file, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, these functions set errno to EBADF and return -1L. On devices incapable of seeking (such as terminals and printers), the return value is undefined.

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

Remarks

The _lseek function moves the file pointer associated with fd to a new location that is offset bytes from origin. The next operation on the file occurs at the new location. The origin argument must be one of the following constants, which are defined in Stdio.h.

SEEK_SET
Beginning of the file.
SEEK_CUR
Current position of the file pointer.
SEEK_END
End of file.

You can use _lseek to reposition the pointer anywhere in a file or beyond the end of the file.

Requirements

Routine	Required header
_lseek	<io.h>
_lseeki64	<io.h>

For more compatibility information, see Compatibility in the Introduction.

Libraries

All versions of the C run-time libraries.

Example

// crt_lseek.c
/* This program first opens a file named lseek.txt.
 * It then uses _lseek to find the beginning of the file,
 * to find the current position in the file, and to find
 * the end of the file.
 */

#include <io.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <share.h>

int main( void )
{
   int fh;
   long pos;               /* Position of file pointer */
   char buffer[10];

   _sopen_s( &fh, "crt_lseek.c_input", _O_RDONLY, _SH_DENYNO, 0 );

   /* Seek the beginning of the file: */
   pos = _lseek( fh, 0L, SEEK_SET );
   if( pos == -1L )
      perror( "_lseek to beginning failed" );
   else
      printf( "Position for beginning of file seek = %ld\n", pos );

   /* Move file pointer a little */
    _read( fh, buffer, 10 );

   /* Find current position: */
   pos = _lseek( fh, 0L, SEEK_CUR );
   if( pos == -1L )
      perror( "_lseek to current position failed" );
   else
      printf( "Position for current position seek = %ld\n", pos );

   /* Set the end of the file: */
   pos = _lseek( fh, 0L, SEEK_END );
   if( pos == -1L )
      perror( "_lseek to end failed" );
   else
      printf( "Position for end of file seek = %ld\n", pos );

   _close( fh );
}

Input: crt_lseek.c_input

Line one.
Line two.
Line three.
Line four.
Line five.

Output

Position for beginning of file seek = 0
Position for current position seek = 10
Position for end of file seek = 57

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