MSGGETSection: Linux Programmer's Manual (2)
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NAMEmsgget - get a message queue identifier
# include <sys/types.h> # include <sys/ipc.h> # include <sys/msg.h>
DESCRIPTIONThe 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 is nonzero). 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 of the message queue. These permission bits have the same format and semantics as the access permissions parameter in open(2) or creat(2) system calls. (The execute permissions are not used.)
Furthermore, while creating, the system call initializes the system message queue data structure msqid_ds as follows:
RETURN VALUEIf successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with errno indicating the error.
ERRORSFor a failing return, errno will be set to one among the following values:
NOTESIPC_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:
BUGSUse of IPC_PRIVATE does not actually prohibit other processes from getting 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.
CONFORMING TOSVr4, SVID. SVr4 does not document the EIDRM error code.
SEE ALSOftok(3), ipc(5), msgctl(2), msgsnd(2), msgrcv(2)
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Time: 15:08:28 GMT, July 17, 2018