Using messages

Before a message can be sent or received, a uniquely identified message queue and data structure must be created. The unique identifier is called the message queue identifier (msqid); it is used to identify or refer to the associated message queue and data structure. This identifier is accessible by any process in the system, subject to normal access restrictions.

The message queue is used to store (header) information about each message being sent or received. This information, which is for internal use by the system, includes the following for each message:

• pointer to the next message on queue

• message type

• message text size

• message text address

There is one associated data structure for the uniquely identified message queue. This data structure contains the following information related to the message queue:

• operation permissions data (operation permission structure)

• pointer to first message on the queue

• pointer to last message on the queue

• current number of bytes on the queue

• number of messages on the queue

• maximum number of bytes on the queue

• process identification (PID) of last message sender

• PID of last message receiver

• last message send time

• last message receive time

• last change time

The definition for the associated message-queue data structure msqid_ds includes the following members:

struct msqid_ds
{
       struct ipc_perm  msg_perm;          /* operation permission struct */
       struct msg       *msg_first;        /* ptr to first message on q */
       struct msg       *msg_last;         /* ptr to last message on q */
       ulong            msg_cbytes;        /* current # bytes on q */
       ulong            msg_qnum;          /* # of messages on q */
       ulong            msg_qbytes;        /* max # of bytes on q */
       pid_t            msg_lspid;         /* pid of last msgsnd */
       pid_t            msg_lrpid;         /* pid of last msgrcv */
       time_t           msg_stime;         /* last msgsnd time */
       time_t           msg_rtime;         /* last msgrcv time */
       time_t           msg_ctime;         /* last change time */
};

The C programming language data structure definition for the message-queue data structure msqid_ds is located in the sys/msg.h header file.

Note that the msg_perm member of this structure uses ipc_perm as a template. The figure below breaks out the operation permissions data structure. In UnixWare, the definition of the ipc_perm data structure is as follows:

struct ipc_perm
{
	uid_t           uid;	/* owner's user id */
	gid_t           gid;	/* owner's group id */
	uid_t           cuid;	/* creator's user id */
	gid_t           cgid;	/* creator's group id */
	mode_t          mode;	/* access modes */
	ulong           seq;	/* slot usage sequence number */
	key_t           key;	/* key */
	long            pad[4];	/* reserve area */
};

ipc_perm data structure

The C programming language data structure definition for the interprocess communication permissions data structure ipc_perm is located in the sys/ipc.h header file and is common to all IPC facilities.

The msgget system call is used to perform one of two tasks:

• to get a new message queue identifier and create an associated message queue and data structure for it

• to return an existing message queue identifier that already has an associated message queue and data structure

Both tasks require a key argument passed to the msgget system call. For the first task, if the key is not already in use for an existing message queue identifier, a new identifier is returned with an associated message queue and data structure created for the key.

There is also a provision for specifying a key of value zero, known as the private key (IPC_PRIVATE). When specified, a new identifier is always returned with an associated message queue and data structure created for it unless a system-tunable parameter would be exceeded. The ipcs command will show the key field for the msqid as all zeros.

For the second task, if a message queue identifier exists for the key specified, the value of the existing identifier is returned. If you do not want to have an existing message queue identifier returned, a control command (IPC_EXCL) can be specified (set) in the msgflg argument passed to the system call (see ``Using msgget'' for how to use this system call).

When performing the first task, the process that calls msgget becomes the owner/creator, and the associated data structure is initialized accordingly. Remember, ownership can be changed but the creating process always remains the creator. The message queue creator also determines the initial operation permissions for it.

Once a uniquely identified message queue and data structure are created, msgop (message operations) and msgctl (message control) can be used.

Message operations, as mentioned before, consist of sending and receiving messages. The msgsnd and msgrcv system calls are provided for each of these operations (see ``Operations for messages'' for details of the msgsnd and msgrcv system calls.

The msgctl system call permits you to control the message facility in the following ways:

• by retrieving the data structure associated with a message queue identifier (IPC_STAT)

• by changing operation permissions for a message queue (IPC_SET)

• by changing the size (msg_qbytes) of the message queue for a particular message queue identifier (IPC_SET)

• by removing a particular message queue identifier from the UNIX operating system along with its associated message queue and data structure (IPC_RMID)