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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 							 msgget(2)

NAME
msgget - get message queue SYNOPSIS
#include <sys/msg.h> int msgget(key_t key, int msgflg); DESCRIPTION
The msgget() argument returns the message queue identifier associated with key. A message queue identifier and associated message queue and data structure (see Intro(2)) are created for key if one of the following are true: o key is IPC_PRIVATE. o key does not already have a message queue identifier associated with it, and (msgflg&IPC_CREAT) is true. On creation, the data structure associated with the new message queue identifier is initialized as follows: o msg_perm.cuid, msg_perm.uid, msg_perm.cgid, and msg_perm.gid are set to the effective user ID and effective group ID, respec- tively, of the calling process. o The low-order 9 bits of msg_perm.mode are set to the low-order 9 bits of msgflg. o msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime are set to 0. o msg_ctime is set to the current time. o msg_qbytes is set to the system limit. See NOTES. RETURN VALUES
Upon successful completion, a non-negative integer representing a message queue identifier is returned. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The msgget() function will fail if: EACCES A message queue identifier exists for key, but operation permission (see Intro(2)) as specified by the low-order 9 bits of msgflg would not be granted. EEXIST A message queue identifier exists for key but (msgflg&IPC_CREAT) and (msgflg&IPC_EXCL) are both true. ENOENT A message queue identifier does not exist for key and (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 identi- fiers system wide would be exceeded. See NOTES. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ SEE ALSO
rctladm(1M), Intro(2), msgctl(2), msgrcv(2), msgsnd(2), setrctl(2), ftok(3C), attributes(5), standards(5) NOTES
The system-defined limit used to initialize msg_qbytes is the minimum enforced value of the calling process's process.max-msg-qbytes resource control. The system-imposed limit on the number of message queue identifiers is maintained on a per-project basis using the project.max-msg-ids resource control. The zone.max-msg-ids resource control restricts the total amount of message queue identifiers that can be allocated by a zone. See rctladm(1M) and setrctl(2) for information about using resource controls. SunOS 5.11 14 Aug 2006 msgget(2)

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