_locking
The latest version of this topic can be found at _locking.
Locks or unlocks bytes of a file.
Syntax
int _locking(
int fd,
int mode,
long nbytes
);
Parameters
fd
File descriptor.
mode
Locking action to perform.
nbytes
Number of bytes to lock.
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 descriptor.
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.
If the failure is due to a bad parameter, such as an invalid file descriptor, the invalid parameter handler is invoked, as described in Parameter Validation.
Remarks
The _locking function locks or unlocks nbytes bytes of the file specified by fd. 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.
Requirements
| Routine | Required header | Optional header |
|---|---|---|
_locking |
<io.h> and <sys/locking.h> | <errno.h> |
For more compatibility information, see Compatibility in the Introduction.
Libraries
All versions of the C run-time libraries.
Example
// crt_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 <sys/types.h>
#include <sys/stat.h>
#include <sys/locking.h>
#include <share.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <io.h>
int main( void )
{
int fh, numread;
char buffer[40];
/* Quit if can't open file or system doesn't
* support sharing.
*/
errno_t err = _sopen_s( &fh, "crt_locking.txt", _O_RDONLY, _SH_DENYNO,
_S_IREAD | _S_IWRITE );
printf( "%d %d\n", err, fh );
if( err != 0 )
exit( 1 );
/* Lock some bytes and read them. Then unlock. */
if( _locking( fh, LK_NBLCK, 30L ) != -1 )
{
long lseek_ret;
printf( "No one can change these bytes while I'm reading them\n" );
numread = _read( fh, buffer, 30 );
buffer[30] = '\0';
printf( "%d bytes read: %.30s\n", numread, buffer );
lseek_ret = _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 );
}
Input: crt_locking.txt
The first thirty bytes of this file will be locked.
Sample Output
No one can change these bytes while I'm reading them
30 bytes read: The first thirty bytes of this
Now I'm done. Do what you will with them