_locking

Locks or unlocks bytes of a file.

int _locking( int handle, int mode, long nbytes );

For additional compatibility information, see Compatibility in the Introduction.

Libraries

Return Value

_locking returns 0 if successful. A return value of –1 indicates failure, in which case errno is set to one of the following values:

EACCES

Locking violation (file already locked or unlocked).

EBADF

Invalid file handle.

EDEADLOCK

Locking violation. Returned when the _LK_LOCK or _LK_RLCK flag is specified and the file cannot be locked after 10 attempts.

EINVAL

An invalid argument was given to _locking.

Parameters

handle

File handle

mode

Locking action to perform

nbytes

Number of bytes to lock

Remarks

The _locking function locks or unlocks nbytes bytes of the file specified by handle. Locking bytes in a file prevents access to those bytes by other processes. All locking or unlocking begins at the current position of the file pointer and proceeds for the next nbytes bytes. It is possible to lock bytes past end of file.

mode must be one of the following manifest constants, which are defined in LOCKING.H:

_LK_LOCK

Locks the specified bytes. If the bytes cannot be locked, the program immediately tries again after 1 second. If, after 10 attempts, the bytes cannot be locked, the constant returns an error.

_LK_NBLCK

Locks the specified bytes. If the bytes cannot be locked, the constant returns an error.

_LK_NBRLCK

Same as _LK_NBLCK.

_LK_RLCK

Same as _LK_LOCK.

_LK_UNLCK

Unlocks the specified bytes, which must have been previously locked.

Multiple regions of a file that do not overlap can be locked. A region being unlocked must have been previously locked. _locking does not merge adjacent regions; if two locked regions are adjacent, each region must be unlocked separately. Regions should be locked only briefly and should be unlocked before closing a file or exiting the program.

Example

/* LOCKING.C: This program opens a file with sharing. It locks
 * some bytes before reading them, then unlocks them. Note that the
 * program works correctly only if the file exists.
 */

#include <io.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/locking.h>
#include <share.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>

void main( void )
{
   int  fh, numread;
   char buffer[40];

   /* Quit if can't open file or system doesn't 
    * support sharing. 
    */
   fh = _sopen( "locking.c", _O_RDWR, _SH_DENYNO, 
                 _S_IREAD | _S_IWRITE );
   if( fh == -1 )
      exit( 1 );

   /* Lock some bytes and read them. Then unlock. */
   if( _locking( fh, LK_NBLCK, 30L ) != -1 )
   {
      printf( "No one can change these bytes while I'm reading them\n" );
      numread = _read( fh, buffer, 30 );
      printf( "%d bytes read: %.30s\n", numread, buffer );
      lseek( fh, 0L, SEEK_SET );
     _locking( fh, LK_UNLCK, 30L );
      printf( "Now I'm done. Do what you will with them\n" );
   }
   else
      perror( "Locking failed\n" );

   _close( fh );
}

Output

No one can change these bytes while I'm reading them
30 bytes read: /* LOCKING.C: This program ope
Now I'm done. Do what you will with them

File Handling Routines

See Also   _creat, _open