msgget
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 veri-
fied, 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 val-
ues:
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 depen-
dent.
BUGS
Use of IPC_PRIVATE does not actually prohibit other processes from get-
ting 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 cre-
ated, 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)
