msgget - Return (and possibly create) the ID for a message
queue
#include <sys/msg.h>
int msgget(
key_t key,
int msgflg );
Application developers may want to specify #include statements
for <sys/types.h> and <sys/ipc.h> before the one for
<sys/msg.h> if programs are being developed for multiple
platforms. The additional #include statements are not
required on Tru64 UNIX systems or by ISO or X/Open standards,
but may be required on other vendors' systems that
conform to these standards.
Interfaces documented on this reference page conform to
industry standards as follows:
msgget(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information
about industry standards and associated tags.
Specifies the key that identifies the message queue. The
value for the key parameter can be IPC_PRIVATE or a random
number other than zero (0). To ensure that the msgget()
function returns a new message queue ID, specify the
IPC_PRIVATE keyword. Specifies flags that control the
msgget() function. Possible values are as follows: Specifies
creating a message queue ID.
If the specified key does not exist, the msgget()
function creates a message queue ID and associates
it with the specified key.
If the specified key does exist, and IPC_EXCL is
not set, the msgget() function returns the message
queue ID associated with key.
If the specified key does exist and IPC_EXCL is
set, the msgget() function fails and returns an
error. Specifies that you want to ensure that your
application is the only one using the specified key
parameter. If the specified key already exists, the
msgget() function fails and returns an error notification.
The msgget() function returns the message queue ID for the
message queue identified by the key parameter. If the
message queue ID does not exist, the msgget() function
attempts to create it.
If you specify IPC_PRIVATE for the key parameter, the
msgget() function returns the ID for a private (that is,
newly created) message queue.
The msgflg parameter supplies creation options for the
msgget() function. The IPC_CREAT flag specifies that you
want the kernel to create a message queue for your application.
The kernel returns the message queue ID to your
application as the return value of the successful call to
the msgget() function.
The msgget() function does not create a message queue ID
for your application under the following conditions: A
message queue ID that is associated with the specified key
already exists. In this case, the msgget() function
returns the message queue ID associated with the key
parameter (unless you have set the IPC_EXCL flag). You
have set the IPC_EXCL flag and another application is
using the specified key. In this case, the msgget() function
fails and returns an error to your application.
After creating a new message queue ID, the msgget() function
initializes the msqid_ds structure associated with
the ID as follows: The msg_perm.cuid and msg_perm.uid members
are set equal to the effective user ID of the calling
process. The msg_perm.cgid and msg_perm.gid members are
set equal to the effective group ID of the calling process.
The low-order nine bits of the msg_perm.mode member
are set equal to the low-order nine bits of msgflg. The
msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime
members are all set equal to zero. The msg_ctime member
is set equal to the current time. The msg_qbytes member
is set equal to the system limit.
Upon successful completion, the msgget() function returns
a message queue identifier. Otherwise, the function
returns a value of -1 and sets errno to indicate the
error.
The msgget() function sets errno to the specified values
for the following conditions: A message queue identifier
exists for the key parameter, but the calling process does
not have the appropriate privileges to perform the operation.
The permission is specified by the low-order nine
bits of the msgflg parameter. A message queue identifier
exists for the key parameter, and both IPC_CREAT and
IPC_EXCL are set. A message queue identifier does not
exist for the key parameter and the IPC_CREAT value is not
set. A message queue identifier can be created, but the
system-imposed limit on the maximum number of allowed message
queue identifiers has been exceeded.
Functions: msgctl(2), msgrcv(2), msgsnd(2), table(2),
ftok(3)
Data Structures: msqid_ds(4)
Standards: standards(5)
msgget(2)
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