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 associ
ated 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 fol
lows:
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.
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MSGGET(2) Linux Programmer's Manual MSGGET(2)
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 sys
tem has not enough memory for the new data
structure.
ENOSPC A message queue has to be created but the sys
tem 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 cre
ates 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 pro
cesses 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 cre
ated, it doesn't imply exclusive access to the message
queue.
CONFORMING TO
SVr4, SVID. SVr4 does not document the EIDRM error code.
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MSGGET(2) Linux Programmer's Manual MSGGET(2)
SEE ALSO
ftok(3), ipc(5), msgctl(2), msgsnd(2), msgrcv(2).
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