msgget - get a System V message queue identifier
int msgget(key_t key, int msgflg);
() system call returns the System V message queue
identifier associated with the value of the key
argument. It may be
used either to obtain the identifier of a previously created message queue
is zero and key
does not have the value
), or to create a new set.
A new message queue is created if key
has the value IPC_PRIVATE
, no message queue with the given key
exists, and IPC_CREAT
is specified in msgflg
specifies both IPC_CREAT
message queue already exists for key
, then msgget
() fails with
set to EEXIST
. (This is analogous to the effect of the
combination O_CREAT | O_EXCL
Upon creation, the least significant bits of the argument msgflg
the permissions of the message queue. These permission bits have the same
format and semantics as the permissions specified for the mode
. (The execute permissions are not used.)
If a new message queue is created, then its associated data structure
) is initialized as follows:
- msg_perm.cuid and msg_perm.uid are set to the
effective user ID of the calling process.
- msg_perm.cgid and 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 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
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 integer), otherwise -1 with errno
indicating the error.
On failure, 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 CAP_IPC_OWNER capability in the user namespace that governs its
- IPC_CREAT and IPC_EXCL were specified in
msgflg, but a message queue already exists for key.
- No message queue exists for key and msgflg
did not specify IPC_CREAT.
- 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
POSIX.1-2001, POSIX.1-2008, SVr4.
The inclusion of <sys/types.h>
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.
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
The following is a system limit on message queue resources affecting a
- System-wide limit on the number of message queues. Before
Linux 3.19, the default value for this limit was calculated using a
formula based on available system memory. Since Linux 3.19, the default
value is 32,000. On Linux, this limit can be read and modified via
Until version 2.3.20, Linux would return EIDRM
for a msgget
() on a
message queue scheduled for deletion.
The name choice IPC_PRIVATE
was perhaps unfortunate, IPC_NEW
more clearly show its function.
This page is part of release 4.16 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at