XPG4 provides a set of C functions that allow processes to communicate through one or more message queues in an operating system's kernel. A process can create, read from, or write to a message queue. Each message is identified with a "type" number, a length value, and data (if the length is greater than 0).
A message can be read from a queue based on its type rather than on its order of arrival. Multiple processes can share the same queue. For example, a server process can handle messages from a number of client processes and associate a particular message type with a particular client process. Or the message type can be used to assign a priority in which a message should be dequeued and handled.
A common client/server implementation on the same system uses two message queues for communication between client and server. An inbound message queue allows group write access and limits read access to the server. An outbound message queue allows universal read access and limits write access to the server. This implementation allows users to place invalid messages on the inbound queue or remove messages belonging to another process from the outbound queue. To solve this problem, you can use two new z/OS® message queue types, ipc_SndTypePID and ipc_RcvTypePID to enforce source and destination process identification.
Create the inbound queue to the server with ipc_SndTypePID and the outbound queue from the server with ipc_RcvTypePID. This arrangement guarantees that the server knows the process ID of the client, and that the client is the only process that can receive the server's returned message. The server can also issue msgrcv() with TYPE=0 to see if any messages belong to process IDs that have gone away. Security checks on clients are not needed, since clients are unable to receive messages intended for another process.
The ipc_PLO constants provide possible message queue performance improvements based on workload. For information on the ipc_PLO constants, see the msgget() function in the z/OS XL C/C++ Runtime Library Reference.