- Inter Process Communication
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- System V & Posix
- Pipes
- Named Pipes
- Shared Memory
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System V & Posix
Following table lists the differences between System V IPC and POSIX IPC.
| SYSTEM V | POSIX |
|---|---|
| AT & T introduced (1983) three new forms of IPC facilities namely message queues, shared memory, and semaphores. | Portable Operating System Interface standards specified by IEEE to define application programming interface (API). POSIX covers all the three forms of IPC |
| SYSTEM V IPC covers all the IPC mechanisms viz., pipes, named pipes, message queues, signals, semaphores, and shared memory. It also covers socket and Unix Domain sockets. | Almost all the basic concepts are the same as System V. It only differs with the interface |
| Shared Memory Interface Calls shmget(), shmat(), shmdt(), shmctl() | Shared Memory Interface Calls shm_open(), mmap(), shm_unlink() |
| Message Queue Interface Calls msgget(), msgsnd(), msgrcv(), msgctl() | Message Queue Interface Calls mq_open(), mq_send(), mq_receive(), mq_unlink() |
| Semaphore Interface Calls semget(), semop(), semctl() | Semaphore Interface Calls Named Semaphores sem_open(), sem_close(), sem_unlink(), sem_post(), sem_wait(), sem_trywait(), sem_timedwait(), sem_getvalue() Unnamed or Memory based semaphores sem_init(), sem_post(), sem_wait(), sem_getvalue(),sem_destroy() |
| Uses keys and identifiers to identify the IPC objects. | Uses names and file descriptors to identify IPC objects |
| NA | POSIX Message Queues can be monitored using select(), poll() and epoll APIs |
| Offers msgctl() call | Provides functions (mq_getattr() and mq_setattr()) either to access or set attributes 11. IPC - System V & POSIX |
| NA | Multi-thread safe. Covers thread synchronization functions such as mutex locks, conditional variables, read-write locks, etc. |
| NA | Offers few notification features for message queues (such as mq_notify()) |
| Requires system calls such as shmctl(), commands (ipcs, ipcrm) to perform status/control operations. | Shared memory objects can be examined and manipulated using system calls such as fstat(), fchmod() |
| The size of a System V shared memory segment is fixed at the time of creation (via shmget()) | We can use ftruncate() to adjust the size of the underlying object, and then re-create the mapping using munmap() and mmap() (or the Linux-specific mremap()) |
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