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msgget(2) [linux man page]

MSGGET(2)						     Linux Programmer's Manual							 MSGGET(2)

NAME
msgget - get a message queue identifier SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgget(key_t key, int msgflg); DESCRIPTION
The msgget() system call returns the message queue identifier associated with the value of the key argument. A new message queue is cre- ated 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. 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. RETURN VALUE
If successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with errno indicating the error. ERRORS
On failure, errno is set to one of the following values: EACCES 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. EEXIST A message queue exists for key and msgflg specified both IPC_CREAT and IPC_EXCL. ENOENT No message queue exists for key and msgflg did not specify IPC_CREAT. ENOMEM A message queue has to be created but the system does not have 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. CONFORMING TO
SVr4, POSIX.1-2001. NOTES
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: MSGMNI System wide maximum number of message queues: policy dependent (on Linux, this limit can be read and modified via /proc/sys/ker- nel/msgmni). Linux Notes Until version 2.3.20 Linux would return EIDRM for a msgget() on a message queue scheduled for deletion. BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function. SEE ALSO
msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mq_overview(7), svipc(7) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2004-05-27 MSGGET(2)

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msgget(2)							System Calls Manual							 msgget(2)

Name
       msgget - get message queue

Syntax
       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/msg.h>

       int msgget (key, msgflg)
       key_t key;
       int msgflg;

Description
       The system call returns the message queue identifier associated with key.

       A message queue identifier and associated message queue and data structure are created for key if one of the following is true:

       o    The key is equal to IPC_PRIVATE

       o    The  key  does not already have a message queue identifier associated with it, and (msgflg & IPC_CREAT) is true.  For further informa-
	    tion, see

       Upon creation, the data structure associated with the new message queue identifier is initialized as follows:

       o    The msg_perm.cuid, msg_perm.uid, msg_perm.cgid and msg_perm.gid members are set equal to the effective user ID and effective group ID,
	    respectively, of the calling process.

       o    The low-order nine bits of msg_perm.mode are set equal to the low-order nine bits of msgflg.

       o    The msg_qnum, msg_lspid, msg_lrpid, msg_stime, and mgs_rtime members are set equal to 0.

       o    The msg_ctime is set equal to the current time.

       o    The msg_qbytes is set equal to the system limit.

Return Values
       Upon  successful  completion,  a  non-negative  integer,  which	is  a  message queue identifier, is returned.  Otherwise, a value of -1 is
       returned, and errno is set to indicate the error.

Diagnostics
       The system call fails under the following conditions:

       [EACCES]       A message queue identifier exists for key but operations permission, as specified by the	low-order  nine  bits  of  msgflg,
		      would not be granted.  For further information, see

       [ENOENT]       A message queue identifier does not exist for key and the logical operation (msgflg & IPC_CREAT) is false.

       [ENOSPC]       A  message  queue  identifier  is to be created, but the system-imposed limit on the maximum number of allowed message queue
		      identifiers system wide would be exceeded.

       [EEXIST]       A message queue identifier exists for key but the logical operation ((msgflg & IPC_CREAT ) & (msgflg & IPC_EXCL )) is true.

See Also
       msgctl(2), msgop(2), ftok(3)

																	 msgget(2)
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