msgget

Returns a message queue identifier.

Syntax

   #include <sys/types.h>

   #include <sys/ipc.h>

   #include <sys/msg.h>



   int msgget (key_t key, int msgflg);

Parameters

key

Either a user-defined key value to rendezvous with the message queue, or IPC_PRIVATE. If IPC_PRIVATE is specified, a new message queue is created, but other processes cannot rendezvous by key. Refer to the description of ftok() for details about obtaining user-defined key values

msgflg

Valid flags for this function are:

IPC_CREAT: If a message queue is not already associated with key, a new message queue identifier is allocated and a message queue and data structure are associated with it. If a message queue identifier is already associated with key, and IPC_EXCL is not specified, msgget() returns the message queue identifier associated with key.
IPC_EXCL: If specified with IPC_CREAT, msgget() returns an error if a message queue identifier is already associated with key.
MODE: The lower nine bits of msgflg contain access permission bits (similar to the nine-bit mask found in file entries). They define access permissions for the owner, the group, and other users on the system.

Return Values

>0: Success. A message queue identifier is returned.
-1: An error occurred, and errno is set to indicate the error condition.

Description

The msgget() function returns a message queue identifier associated with the value passed in key. A new message queue identifier is allocated and a message queue and data structure are associated with it if:

The value passed in key is equal to IPC_PRIVATE.
The value passed in key does not already have a message queue identifier associated with it, and msgflg specifies IPC_CREAT.

The data structure associated with the new message queue identifier is initialized to the following values:

msg_perm.cuid: Effective user ID of the calling process (creator user ID)
msg_perm.uid: Effective user ID of the calling process (owner user ID)
msg_perm.cgid: Effective group ID of the calling process (creator group ID)
msg_perm.gid: Effective group ID of the calling process (owner group ID)
msg_perm.mode: Low-order 9 bits are set equal to the low-order 9 bits of msgflg
msg_qnum: Zero
msg_lspid: Zero
msg_lrpid: Zero
msg_stime: Zero
msg_rtime: Zero
msg_ctime: Current time
msg_qbytes: System limit

Implementation Considerations

The maximum size of a message is 65536 bytes.

An MPE/iX system manager can use the MPE/iX SVIPC utility to interactively configure:

The maximum message size
The maximum number of bytes on a message queue
The total number of message queues allowed system wide

Refer to the section "Managing SVID IPC Services" for more information.

Errors

If an error occurs, errno is set to one of the following values.

`EACCES`	CAUSE	A message queue identifier exists for `key` and the calling process does not have permission (specified by the low-order 9 bits of `msgflg`).
ACTION	Ensure that the calling process has appropriate permissions to access the existing message queue associated with `key`, or specify a unique `key` value to create a new message queue.
`EEXIST`	CAUSE	A message queue identifier exists for `key` and `msgflg` specifies both `IPC_CREATE` and `IPC_EXCL`.
ACTION	To access the existing message queue associated with `key`, remove the `IPC_EXCL` option. Otherwise, a unique `key` value must be specified.
`ENOENT`	CAUSE	A message queue identifier does not exist for `key` and `msgflg` does not specify `IPC_CREATE`.
ACTION	Specify `IPC_CREATE` to indicate a message queue should be created if one does not already exist for the specified `key` value.
`ENOSPC`	CAUSE	The number of message queue identifiers would exceed the system-defined limit.
ACTION	A new message queue cannot be created unless a previously allocated message queue is removed.
`ESYSERR`	CAUSE	An operating system error occurred that does not map directly to any of the above errors.
ACTION	Examine the MPE/iX process error stack for the type of system error.

msgget

Technical documentation

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