Editing Message queue (section)

==Implementation in UNIX==
There are two common message queue implementations in [[Unix|UNIX]]. One is part of the SYS V API, the other one is part of [[POSIX]].

===SYS V===
UNIX SYS V implements message passing by keeping an array of linked lists as message queues. Each message queue is identified by its index in the array, and has a unique descriptor. A given index can have multiple possible descriptors. UNIX gives standard functions to access the message passing feature.<ref name="The Design of the UNIX Operating System">
{{cite book | title=The Design of the UNIX Operating System | author=Bach, M.J. | year=1986 | publisher=Prentice-Hall | isbn=9780132017992 | url-access=registration | url=https://archive.org/details/designofunixoper00bach }}
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;<code>msgget()</code>: This system call takes a key as an argument and returns a descriptor of the queue with the matching key if it exists. If it does not exist, and the <code>IPC_CREAT</code> flag is set, it makes a new message queue with the given key and returns its descriptor.
;<code>msgrcv()</code>: Used to receive a message from a given queue descriptor. The caller process must have read permissions for the queue. It is of two types.<ref name="Operating Systems Concepts">
{{cite book | title=Operating Systems Concepts |author=Abraham Silberschatz, Peter B. Galvin |year=1994 |publisher=Addison-Wesley |  isbn=9780201504804 }}
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:*Blocking receive puts the child to sleep if it cannot find a requested message type. It sleeps until another message is posted in the queue, and then wakes up to check again.
:*Non-blocking receive returns immediately to the caller, mentioning that it failed.
;<code>msgctl()</code>: Used to change message queue parameters like the owner. Most importantly, it is used to delete the message queue by passing the <code>IPC_RMID</code> flag. A message queue can be deleted only by its creator, owner, or the superuser.

===POSIX===
The POSIX.1-2001 message queue API is the later of the two UNIX message queue APIs. It is distinct from the SYS V API, but provides similar function. The unix man page <code>mq_overview(7)</code> provides an overview of POSIX message queues.