msgget - get a message queue identifier
# include
<sys/types.h>
# include <sys/ipc.h>
# include <sys/msg.h>
int msgget ( key_t key, int msgflg )
The function returns the message queue identifier associated to the value of the key argument. A new message queue is created if key has value IPC_PRIVATE or key isn’t IPC_PRIVATE, no existing message queue is associated to key, and IPC_CREAT is asserted in msgflg (i.e. msgflg&IPC_CREAT isn’t zero). The presence in msgflg of the fields IPC_CREAT and IPC_EXCL plays the same role, with respect to the existence of the message queue, as the presence of O_CREAT and O_EXCL in the mode argument of the open(2) system call: i.e. the msgget function fails if msgflg asserts both IPC_CREAT and IPC_EXCL and a message queue already exists for key.
Upon creation, the lower 9 bits of the argument msgflg define the access permissions (for owner, group and others) to the message queue in the same format, and with the same meaning, as for the access permissions parameter in the open(2) or creat(2) system calls (though the execute permissions are not used by the system).
Furthermore, while creating, the system call initializes the system message queue data structure msqid_ds 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 lowest order 9 bits of msg_perm.mode are set to the lowest order 9 bit 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 access 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 positive integer), otherwise -1 with errno indicating the error.
For a failing return, errno will be set to one among the following values:
|
EACCES |
A message queue exists for key, but the calling process has no access permissions to the queue. | ||
|
EEXIST |
A message queue exists for key and msgflg was asserting both IPC_CREAT and IPC_EXCL. | ||
|
EIDRM |
The message queue is marked as to be removed. | ||
|
ENOENT |
No message queue exists for key and msgflg wasn’t asserting IPC_CREAT. | ||
|
ENOMEM |
A message queue has to be created but the system has not 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. |
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 lowest order 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. |
Use of IPC_PRIVATE don’t inhibits to other processes the access to the allocated message queue.
As for the files, there is currently no intrinsic way for a process to ensure exclusive access to a message queue. Asserting both IPC_CREAT and IPC_EXCL in msgflg only ensures (on success) that a new message queue will be created, it doesn’t imply exclusive access to the message queue.
ftok(3), ipc(5), msgctl(2), msgsnd(2), msgrcv(2).