locking

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Header File

io.h, sys\locking.h

Category

Input/output Routines

Prototype

int locking(int handle, int cmd, long length);

Description

Sets or resets file-sharing locks.

locking provides an interface to the operating system file-sharing mechanism. The file to be locked or unlocked is the open file specified by handle. The region to be locked or unlocked starts at the current file position, and is length bytes long.

Locks can be placed on arbitrary, nonoverlapping regions of any file. A program attempting to read or write into a locked region will retry the operation three times. If all three retries fail, the call fails with an error.

The cmd specifies the action to be taken (the values are defined in sys\locking.h):

LK_LOCK	Lock the region. If the lock is unsuccessful, try once a second for 10 seconds before giving up.
LK_RLCK	Same as LK_LOCK.
LK_NBLCK	Lock the region. If the lock if unsuccessful, give up immediately.
LK_NBRLCK	Same as LK_NBLCK.
LK_UNLCK	Unlock the region, which must have been previously locked.

Return Value

On successful operations, locking returns 0. Otherwise, it returns -1, and the global variable errno is set to one of the following values:

EACCES	File already locked or unlocked
EBADF	Bad file number
EDEADLOCK	File cannot be locked after 10 retries (cmd is LK_LOCK or LK_RLCK)
EINVAL	Invalid cmd, or SHARE.EXE not loaded

Example

#include <io.h>
#include <fcntl.h>
#include <process.h>
#include <share.h>
#include <stdio.h>
#include <sys\locking.h>
int main(void)
{
  int handle, status;
  long length;
  handle = sopen("c:\\autoexec.bat", O_RDONLY,SH_DENYNO);
  if (handle < 0) {
    printf("sopen failed\n");
    exit(1);
  }
  length = filelength(handle);
  status = locking(handle,LK_LOCK,length/2);
  if (status == 0)
    printf("lock succeeded\n");
  else
    perror("lock failed");
  status = locking(handle,LK_UNLCK,length/2);
  if (status == 0)
    printf("unlock succeeded\n");
  else
    perror("unlock failed");
  close(handle);
  return 0;
}

Portability

POSIX	Win32	ANSI C	ANSI C++
	+