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The message type of IPC allows processes (executing programs) to communicate through the exchange of data stored in buffers. This data is transmitted between processes in discrete portions called messages. Processes using this type of IPC can send and receive messages.
Before a process can send or receive a message, it must have the UNIX operating system generate the necessary software mechanisms to handle these operations. A process does this using the msgget system call. In doing this, the process becomes the owner/creator of a message queue and specifies the initial operation permissions for all processes, including itself. Subsequently, the owner/creator can relinquish ownership or change the operation permissions using the msgctl system call. However, the creator remains the creator as long as the facility exists. Other processes with permission can use msgctl to perform various other control functions.
Processes which have permission and are attempting to send or receive a message can suspend execution if they are unsuccessful at performing their operation. That is, a process which is attempting to send a message can wait until it becomes possible to post the message to the specified message queue; the receiving process isn't involved (except indirectly, for example, if the consumer isn't consuming, the queue space will eventually be exhausted) and vice versa. A process which specifies that execution is to be suspended is performing a ``blocking message operation.'' A process which does not allow its execution to be suspended is performing a ``nonblocking message operation.''
A process performing a blocking message operation can be suspended until one of three conditions occurs:
System calls make these message capabilities available to processes. The calling process passes arguments to a system call, and the system call either successfully or unsuccessfully performs its function. If the system call is successful, it performs its function and returns applicable information. Otherwise, an error code (-1) is returned to the process, and an external error number variable, errno, is set accordingly.