Lab 15 Record Locking NCHU System Network Lab

Lab 15 Record Locking NCHU System & Network Lab

Record Locking (1/4) • What happens when two process attempt to edit the same file at the same time ? • Record locking is the ability of a process to prevent other processes from modifying or reading a region of a file while the first process is working on it.

Record Locking (2/4) • Here are two types of locks : – Read lock (shared lock) • Any number of processes can have a shared read lock on a given region of a file. • All of them can read from the file but can’t write to it. – Write lock (exclusive lock) • Only one process can have an exclusive lock on a given region of a file. • The other process are prevented from getting locks or reading and writing.

Record locking (3/4) • Advisory V. S. Mandatory locking – Advisory lock (cooperative lock) • All access functions in a library handle record locking in a consistent way. • But advisory locking doesn’t prevent some other process that has write permission for file from writing. A rude process with write permission of the file database Advisory lock access library

Record locking (4/4) – Mandatory locking cause the kernel to check every read(), write(), open() to prevent the calling process from violating the lock rule. Locking check fget() write() file kernel readn( ) fput()

fcntl() (1/3) • fcntl() function can change the properties of a file that is already open. – The fcntl() is used for five different purposes : cmd description F_DUPFD Duplicate an existing descriptor F_GETFD, F_SETFD Get/set file descriptor flags F_GETFL, F_SETFL Get/set file status flags (O_RDWR … etc) F_GETOWN, F_SETOWN Get/set asynchronous I/O ownership F_GETLK, F_SETLK, Get/set/lock file lock, it discussed later. F_SETLKW #include <fcntl. h> int fcntl (int filedes, int cmd, struct flock *lockptr…);

fcntl() (2/3) • For record locking , the third argument of fcntl() is a pointer to a flock structure : struct flock { short l_type; off_t l_start; short l_whence; off_t l_len; pid_t l_pid; } /* F_RDLCK, F_WRLCK, F_UNLCK */ /* offset in bytes, relative to l_whence */ /* SEEK_SET, SEEK_CUR, SEEK_END */ /* length, in bytes ; 0 means lock to EOF */ /* returned with F_GETLK */

fcntl() (3/3) • Three commands of fcntl( record lock ) : – F_GETLK • To check the lock described by flockptr : – The information pointed by flockptr is overwritten by an existing lock. – If no lock exists, flockptr. l_type = F_UNLCK – F_SETLK • Set the lock described by flockptr. • If we try to obtain an illegal record lock, fcntl() will return errono = EAGAIN. – F_SETLKW • A blocking version of F_SETLK. It will wait until the desired region released.

Deadlock • Deadlock – Deadlock occurs when two processes are each waiting for a resource that the other has locked. – When a deadlock is detected , the kernel must choose one to receive the error return. • It depends on different implementations of systems. Process A Lock request Process B

Inheritance and Release of Locks • Inheritance and release of locks : – Release of locks : • When a process terminates, all its locks are released. • When a descriptor is closed, any locks on it are automatically released. – Inheritance • Locks are never inherited by the child process across a fork(). • Locks are inherited by a new program across an exec().

Ex : Free. BSD file lock structure Figure : Free. BSD data structures for record locking

Lab 1 • Write a program that : – User can define a lock with type, start, and length. – This program will try to get a lock or it will be blocked by an existing lock. • If it is blocked , it should show the lock information. – By running this program in two or more windows, you can see how programs interact while waiting locks.

Lab 1 (cont. ) Input : R/W, start, length Show the lock and wait. . Is it locked? No Lock the region Wait for user unlock yes

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#include <stdio. h> #include <stdlib. h> #include <errno. h> #include <fcntl. h> #include <unistd. h> int main() { struct flock fl = { F_WRLCK, SEEK_SET, 0, 0, 0 }; int fd; fd = open(“test", O_RDWR) fcntl (fd, F_SETLKW, &fl) ; // set the lock ‘fl’ getchar(); printf(“file is locked…”); … exit(1); }

Lab 2 • Use the program you have done before : – You try to make a deadlock situation to see that how does Linux solve this problem.

Reference • Advanced Programming in the UNIX Environment 2 nd Author : Richard Stevens, Stephen A. Rago, Publisher : Addison-Wesley • Beginning Linux Programming Author : Richard Stones, Neil Matthew Publisher : Wrox • http: //linux. vbird. org/ • http: //www. jollen. org/blog/ jollen’s Blog