MSGGET(2) Linux Programmer's Manual MSGGET(2)
msgget - get a System V message queue identifier
int msgget(key_t key, int msgflg);
The msgget() system call returns the System V message queue identifier associated with the
value of the key argument. A new message queue is created if key has the value IPC_PRI-
VATE or key isn't IPC_PRIVATE, no message queue with the given key key exists, and
IPC_CREAT is specified in msgflg.
If msgflg specifies both IPC_CREAT and IPC_EXCL and a message queue already exists for
key, then msgget() fails with errno set to EEXIST. (This is analogous to the effect of
the combination O_CREAT | O_EXCL for open(2).)
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 per-
missions specified for the mode argument of open(2). (The execute permissions are not
If a new message queue is created, then its associated data structure msqid_ds (see
msgctl(2)) is initialized as follows:
msg_perm.cuid and msg_perm.uid are set to the effective user ID of the calling
msg_perm.cgid and msg_perm.gid are set to the effective group ID of the calling
The least significant 9 bits of msg_perm.mode are set to the least significant 9
bits of msgflg.
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 MSGMNB.
If the message queue already exists the permissions are verified, and a check is made to
see if it is marked for destruction.
If successful, the return value will be the message queue identifier (a nonnegative inte-
ger), otherwise -1 with errno indicating the error.
On failure, errno is set to one of the following values:
EACCES A message queue exists for key, but the calling process does not have permission to
access the queue, and does not have the CAP_IPC_OWNER capability.
EEXIST A message queue exists for key and msgflg specified both IPC_CREAT and IPC_EXCL.
ENOENT No message queue exists for key and msgflg did not specify IPC_CREAT.
ENOMEM A message queue has to be created but the system does not have enough memory for
the new data structure.
ENOSPC A message queue has to be created but the system limit for the maximum number of
message queues (MSGMNI) would be exceeded.
The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux or by any version
of POSIX. However, some old implementations required the inclusion of these header files,
and the SVID also documented their inclusion. Applications intended to be portable to
such old systems may need to include these header files.
IPC_PRIVATE isn't a flag field but a key_t type. 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 msgget() call:
MSGMNI System wide maximum number of message queues: policy dependent (on Linux, this
limit can be read and modified via /proc/sys/kernel/msgmni).
Until version 2.3.20 Linux would return EIDRM for a msgget() on a message queue scheduled
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its
msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mq_overview(7), svipc(7)
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Linux 2012-05-31 MSGGET(2)