Основы программирования в Linux

  printf("Process %d — failed to unlock region\n", getpid);

 } else {

  printf("Process %d — unlocked region\n", getpid);

 }

 region_to_lock.l_type = F_WRLCK;

 region_to_lock.l_whence = SEEK_SET;

 region_to_lock.lstart = 16;

 region_to_lock.l_len = 5;

 printf("Process %d, trying F_WRLCK, region %d to %d\n", getpid, (int)region_to_lock.l_start,

  (int)(region_to_lock.l_start + region_to_lock.l_len));

 res = fcntl(file_desc, F_SETLK, &region_to_lock);

 if (res == -1) {

  printf("Process %d — failed to lock region\n", getpid);

 } else {

  printf("Process %d — obtained lock on region\n", getpid);

 }

 region_to_lock.l_type = F_RDLCK;

 region_to_lock.l_whence = SEEK_SET;

 region_to_lock.l_start = 40;

 region_to_lock.l_len = 10;

 printf("Process %d, trying F_RDLCK, region %d to %d\n", getpid, (int)region_to_lock.l_start,

  (int)(region_to_lock.l_start + region_to_lock.l_len));

 res = fcntl(filedesc, F_SETLK, &region_to_lock);

 if (res == -1) {

  printf("Process %d — failed to lock region\n", getpid);

 } else {

  printf("Process %d — obtained lock on region\n", getpid);

 }

 region_to_lock.l_type = F_WRLCK;

 region_to_lock.l_whence = SEEK_SET;

 region_to_lock.l_start = 16;

 region_to_lock. l_len = 5;

 printf("Process %d, trying F_WRLCK with wait, region %d to %d\n", getpid, (int)region_to_lock.l_start,

  (int)(region_to_lock.l_start + region_to_lock.l_len));

 res = fcntl(file_desc, F_SETLKW, &region_to_lock);

 if (res == -1) {

  printf("Process %d — failed to lock region\n", getpid);

 } else {

  printf("Process %d — obtained lock, on region\n", getpid);

 }

printf ("Process %d ending\n", getpid);

 close(file_desc);

 exit(EXIT_SUCCESS);

}

Если вы сначала запустите программу lock3 в фоновом режиме, далее сразу запускайте новую программу:

$ ./lock3 &

$ process 227 locking file

$ ./lock5

Вы получите следующий вывод:

Process 227 locking file

Process 228, trying F_RDLCK, region 10 to 15

Process 228 — obtained lock on region

Process 228, trying F_UNLCK, region 10 to 15

Process 228 — unlocked region

Process 228, trying F_UNLCK, region 0 to 50

Process 228 — unlocked region

Process 228, trying F_WRLCK, region 16 to 21

Process 228 — failed to lock on region

Process 228, trying F_RDLCK, region 4 0 to 50

Process 228 - failed to lock on region

Process 228, trying F_WRLCK with wait, region 16 to 21

Process 227 closing file

Process 228 — obtained lock on region

Process 228 ending

Как это работает

Сначала программа пытается заблокировать участок с 10-го по 15-й байты с помощью разделяемой блокировки. Эта область уже заблокирована блокировкой того же типа, но одновременные разделяемые блокировки допустимы, и установка блокировки завершается успешно.