如何锁定文件

_locking

Locks or unlocks bytes of a file.



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



Routine Required Header Optional Headers Compatibility 

_locking <io.h> and <sys/locking.h> <errno.h> Win 95, Win NT 





For additional compatibility information, see Compatibility in the Introduction.



Libraries



LIBC.LIB Single thread static library, retail version 

LIBCMT.LIB Multithread static library, retail version 

MSVCRT.LIB Import library for MSVCRT.DLL, retail version 





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