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msgget() - Unix, Linux System Call
msgget - get a message queue identifier
int msgget(key_t key, int msgflg);
msgget() system call returns the message queue identifier associated
with the value of the
A new message queue is created if
key has the value
IPC_PRIVATE, no message queue with the given key
key exists, and
IPC_CREAT is specified in
msgflg specifies both
IPC_EXCL and a message queue already exists for
msgget() fails with
errno set to
EEXIST. (This is analogous to the effect of the combination
O_CREAT | O_EXCL for
Upon creation, the least significant bits of the argument
msgflg define the permissions of the message queue.
These permission bits have the same format and semantics
as the permissions specified for the
mode argument of
(The execute permissions are not used.)
If a new message queue is created,
then its associated data structure
is initialised as follows:
If the message queue already exists the permissions are
verified, and a check is made to see if it is marked for
msg_perm.uid are set to the effective user ID of the calling process.
msg_perm.gid are set to the effective group ID of the calling process.
The least significant 9 bits of
msg_perm.mode are set to the least significant 9 bits of
msg_qnum, msg_lspid, msg_lrpid, msg_stime and
msg_rtime are set to 0.
msg_ctime is set to the current time.
msg_qbytes is set to the system limit
If successful, the return value will be the message queue identifier (a
nonnegative integer), otherwise -1
errno indicating the error.
errno is set to one of the following values:
A message queue exists for
key, but the calling process does not have permission to access the queue,
and does not have the
A message queue exists for
msgflg specified both
No message queue exists for
msgflg did not specify
A message queue has to be created but the system does not have enough
memory for the new data structure.
A message queue has to be created but the system limit for the maximum
number of message queues
(MSGMNI) would be exceeded.
IPC_PRIVATE isnt a flag field but a
If this special value is used for
key, the system call ignores everything but the least significant 9 bits of
msgflg and creates a new message queue (on success).
The following is a system limit on message queue resources affecting a
System wide maximum number of message queues: policy
(on Linux, this limit can be read and modified via
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW
would more clearly show its function.
Until version 2.3.20 Linux would return EIDRM for a
msgget() on a message queue scheduled for deletion.