Interprocess communication

• Pipes do not allow unrelated processes to communicate.

• Named pipes allow unrelated processes to communicate, but they cannot provide private channels for pairs of communicating processes; that is, any process with appropriate permission may read from or write to a named pipe.

• Sending signals, via the kill system call, allows arbitrary processes to communicate, but the message consists only of the signal number.

SCO OpenServer also provides an InterProcess Communication (IPC) package that supports three, more versatile types of interprocess communication. For example,

• Messages allow processes to send formatted data streams to arbitrary processes.

• Semaphores allow processes to synchronize execution.

• Shared memory allows processes to share parts of their virtual address space.

When implemented as a unit, these three mechanisms share common properties such as

• each mechanism contains a ``get'' system call to create a new entry or retrieve an existing one

• each mechanism contains a ``control'' system call to query the status of an entry, to set status information, or to remove the entry from the system

• each mechanism contains an ``operations'' system call to perform various operations on an entry

This section describes the system calls for each of these three forms of IPC.

This information is for programmers who write multiprocess applications. These programmers should have a general understanding of what semaphores are and how they are used.

Information from other sources would also be helpful. See ipcs(ADM) and ipcrm(ADM), as well as the following manual pages:

intro(S)	aclipc(S)
msgget(S)	msgctl(S)	msgop(S)
semget(S)	semctl(S)	semop(S)
shmget(S)	shmctl(S)	shmop(S)

Included in this section are several example programs that show the use of these IPC system calls. Since there are many ways to accomplish the same task or requirement, keep in mind that the example programs were written for clarity and not for program efficiency. Usually, system calls are embedded within a larger user-written program that makes use of a particular function provided by the calls.