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
(2). (The execute permissions are not used.)
If a new message queue is created, then its associated data structure
(2)) 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 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 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
- 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
- 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.
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.