Advanced Programming in the UNIX® Environment

ptg10805159

Section 14.3 RecordLocking 489

Example — Requesting and Releasing a Lock

To save ourselves from having to allocate anflockstructureand fill in all the elements

each time, the functionlock_regin Figure14.5 handles all these details.

#include "apue.h"

#include <fcntl.h>

int

lock_reg(int fd, int cmd, int type, off_t offset, int whence, off_t len)

{

struct flock lock;

lock.l_type = type; /* F_RDLCK, F_WRLCK, F_UNLCK */

lock.l_start = offset; /* byte offset, relative to l_whence */

lock.l_whence = whence; /* SEEK_SET, SEEK_CUR, SEEK_END */

lock.l_len = len; /* #bytes (0 means to EOF) */

return(fcntl(fd, cmd, &lock));

}

Figure 14.5 Function to lock or unlock a region of a file

Since most locking calls are to lock or unlock a region (the commandF_GETLKis rarely

used), we normally use one of the following five macros, which aredefined inapue.h

(Appendix B).

#define read_lock(fd, offset, whence, len) \

lock_reg((fd), F_SETLK, F_RDLCK, (offset), (whence), (len))

#define readw_lock(fd, offset, whence, len) \

lock_reg((fd), F_SETLKW, F_RDLCK, (offset), (whence), (len))

#define write_lock(fd, offset, whence, len) \

lock_reg((fd), F_SETLK, F_WRLCK, (offset), (whence), (len))

#define writew_lock(fd, offset, whence, len) \

lock_reg((fd), F_SETLKW, F_WRLCK, (offset), (whence), (len))

#define un_lock(fd, offset, whence, len) \

lock_reg((fd), F_SETLK, F_UNLCK, (offset), (whence), (len))

We have purposely defined the first three arguments to these macros in the same order

as thelseekfunction.

Example — Testing for a Lock

Figure14.6 defines the functionlock_testthat we’ll use to test for a lock.

#include "apue.h"

#include <fcntl.h>

pid_t

lock_test(int fd, int type, off_t offset, int whence, off_t len)

{