[Linux] fcntl函数的说明与实例 (文件加锁)

#include <unistd.h>
#include <fcntl.h></p> <p>int fcntl(int fd, int cmd,struct flock *plock );</p> <p>struct flock {
...
short l_type; /* Type of lock: F_RDLCK,
F_WRLCK, F_UNLCK */
short l_whence; /* How to interpret l_start:
SEEK_SET, SEEK_CUR, SEEK_END */
off_t l_start; /* Starting offset for lock */
off_t l_len; /* Number of bytes to lock */
pid_t l_pid; /* PID of process blocking our lock
(F_GETLK only) */
...
};

/* slock.c */</p> <p>#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h></p> <p>int main()
{
struct flock _lock;</p> <p> _lock.l_type = F_WRLCK;
_lock.l_whence = SEEK_SET;
_lock.l_start = 0;
_lock.l_len = 0;</p> <p> int fd = open( "/dev/shm/test",O_CREAT|O_RDWR,S_IRWXU|S_IRGRP|S_IWGRP|S_IRWXO );
if ( fd < 0 )
{
puts( "open error" );
return 0;
}</p> <p> int ret = fcntl( fd,F_SETLK,&_lock );
if ( ret < 0 )
{
puts( "fcntl error" );
close( fd );
return 0;
}</p> <p> puts( "sleep now ..." );
sleep( 100 );
puts( "exit..." );

_lock.l_type = F_UNLCK;
_lock.l_whence = SEEK_SET;
_lock.l_start = 0;
_lock.l_len = 0;</p> <p> ret = fcntl( fd,F_SETLK,&_lock );
if ( ret < 0 )
{
puts( "unlock error" );
}</p> <p> close( fd );
}

/* glock.c*/</p> <p>#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h></p> <p>int main()
{
struct flock _lock;</p> <p> _lock.l_type = F_RDLCK;
_lock.l_whence = SEEK_SET;
_lock.l_start = 0;
_lock.l_len = 0;</p> <p> int fd = open( "/dev/shm/test",O_RDWR );
if ( fd < 0 )
{
perror( "open error" );
return 0;
}</p> <p> int ret = fcntl( fd,F_GETLK,&_lock );
if ( ret < 0 )
{
perror( "fcntl error:" );
close( fd );
return 0;
}</p> <p> printf( "lock is %d\n",_lock.l_type );</p> <p> close( fd );
}

./slock
sleep now ...
./glock
lock is 1
exit...

To make use of mandatory locks, mandatory locking must be enabled bothon the filesystem that contains the file to be locked, and on the fileitself. Mandatory locking is enabled on a filesystem using the "-o
mand" option to mount(8), or the MS_MANDLOCK flag for mount(2). Mandatory locking is enabled on a file by disabling group execute permission
on the file and enabling the set-group-ID permission bit (see chmod(1)and chmod(2)).