msgget(2)msgget(2)NAMEmsgget - Return (and possibly create) the ID for a message queue
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 pro‐
grams 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 stan‐
dards 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 cre‐
ates a message queue ID and associates it with the specified
If the specified key does exist, and IPC_EXCL is not set, the
msgget() function returns the message queue ID associated with
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 notifi‐
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() func‐
tion. 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
After creating a new message queue ID, the msgget() function initial‐
izes 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 mem‐
ber 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 fol‐
lowing conditions: A message queue identifier exists for the key param‐
eter, 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 mes‐
sage 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)