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()) |
