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msgget(2) [redhat 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 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 exis- tence 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: 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. RETURN VALUE
If successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with errno indicating the error. ERRORS
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 for removal. 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. 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 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. BUGS
Use of IPC_PRIVATE does not actually prohibit other processes from getting access to the allocated message queue. 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 TO
SVr4, SVID. SVr4 does not document the EIDRM error code. SEE ALSO
ftok(3), ipc(5), msgctl(2), msgsnd(2), msgrcv(2) Linux 0.99.13 1993-11-01 MSGGET(2)

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MSGGET(2)						      BSD System Calls Manual							 MSGGET(2)

NAME
msgget -- get message queue identifier LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/msg.h> int msgget(key_t key, int msgflg); DESCRIPTION
The msgget() system call returns the message queue identifier associated with key. A message queue identifier is a unique integer greater than zero. A message queue is created if either key is equal to IPC_PRIVATE, or key does not have a message queue identifier associated with it and the IPC_CREAT bit is set in msgflg. If both the IPC_CREAT bit and the IPC_EXCL bit are set in msgflg, and key has a message queue identifier associated with it already, the operation will fail. If a new message queue is created, the data structure associated with it (the msqid_ds structure, see msgctl(2)) is initialized as follows: o msg_perm.cuid and msg_perm.uid are set to the effective uid of the calling process. o msg_perm.gid and msg_perm.cgid are set to the effective gid of the calling process. o msg_perm.mode is set to the lower 9 bits of msgflg. o msg_qnum, msg_lspid, msg_lrpid, msg_rtime, and msg_stime are set to 0. o msg_qbytes is set to the system wide maximum value for the number of bytes in a queue (MSGMNB). o msg_ctime is set to the current time. RETURN VALUES
Upon successful completion a positive message queue identifier is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error. ERRORS
[EACCES] A message queue is already associated with key and the caller has no permission to access it. [EEXIST] Both IPC_CREAT and IPC_EXCL are set in msgflg, and a message queue is already associated with key. [ENOSPC] A new message queue could not be created because the system limit for the number of message queues has been reached. [ENOENT] IPC_CREAT is not set in msgflg and no message queue associated with key was found. SEE ALSO
msgctl(2), msgrcv(2), msgsnd(2), ftok(3) STANDARDS
The msgget system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5''). HISTORY
Message queues appeared in the first release of AT&T System V UNIX. BSD
May 13, 2004 BSD
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