Advanced Programming in the UNIX® Environment

ptg10805159

Section 15.7 Message Queues 561

15.7 Message Queues

Amessage queue is a linked list of messages stored within the kernel and identified by

amessage queue identifier.We’ll call the message queue just aqueueand its identifier a

queue ID.

The Single UNIX Specification message-passing option includes an alternative IPC message

queue interface derived from the POSIX real-time extensions.We donot discuss it in this text.

Anew queue is created or an existing queue opened bymsgget.New messages

areadded to the end of a queue bymsgsnd.Every message has a positive long integer

type field, a non-negative length, and the actual data bytes (corresponding to the

length), all of which arespecified tomsgsndwhen the message is added to a queue.

Messages arefetched from a queue bymsgrcv.Wedon’t have to fetch the messages in

afirst-in, first-out order.Instead, we can fetch messages based on their type field.

Each queue has the followingmsqid_dsstructureassociated with it:

struct msqid_ds {

struct ipc_perm msg_perm; /* see Section 15.6.2 */

msgqnum_t msg_qnum; /* # of messages on queue */

msglen_t msg_qbytes; /* max # of bytes on queue */

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 */

..

.

};

This structuredefines the current status of the queue. The members shown arethe ones

defined by the Single UNIX Specification. Implementations include additional fields

not covered by the standard.

Typical values

FreeBSD Linux Mac OS X Solaris

8.0 3.2.0 10.6.8 10

Description

size in bytes of largest message we can send 16,384 8,192 16,384 derived

maximum size in bytes of a particular queue (i.e., 2,048 16,384 2,048 65,536

the sum of all the sizes of messages on the queue)

maximum number of messages queues, systemwide 40 derived 40 128

maximum number of messages, systemwide 40 derived 40 8,192

Figure 15.26System limits that affect message queues

Figure15.26 lists the system limits that affect message queues. We show ‘‘derived’’

wherealimit is derived from other limits. On Linux, for example, the maximum

number of messages is based on the maximum number of queues and the maximum

amount of data allowed on the queues. The maximum number of queues, in turn, is

based on the amount of RAM installed in the system. Note that the queue maximum

byte size limit further limits the maximum size of a message to be placed on a queue.