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. It may be used either to obtain the identifier of a previously created message queue (when msgflg is zero and key does not have the value IPC_PRIVATE), or to create a new set.
A new message queue is created if key has the value IPC_PRIVATE 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 permissions specified for the mode argument of open(2). (The execute permissions are not used.)
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 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.
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:
- msg_qbytes is set to the system limit MSGMNB.
- 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 namespace.
- 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.
POSIX.1-2001, POSIX.1-2008, SVr4.
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:
- A message queue has to be created but the system limit for the maximum number of message queues (MSGMNI) would be exceeded.
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 would more clearly show its function.
- 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 /proc/sys/kernel/msgmni.