NAME

msgctl — XSI message control operations

SYNOPSIS

[XSI] #include <sys/msg.h>

int msgctl(int msqid, int cmd, struct msqid_ds *buf);

DESCRIPTION

The msgctl() function operates on XSI message queues (see XBD 3.206 Message Queue). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in 2.8 Realtime.

The msgctl() function shall provide message control operations as specified by cmd. The following values for cmd, and the message control operations they specify, are:

IPC_STAT

Place the current value of each member of the msqid_ds data structure associated with msqid into the structure pointed to by buf. The contents of this structure are defined in <sys/msg.h>.

IPC_SET

Set the value of the following members of the msqid_ds data structure associated with msqid to the corresponding value found in the structure pointed to by buf:

msg_perm.uid
msg_perm.gid
msg_perm.mode
msg_qbytes

Also, the msg_ctime timestamp shall be set to the current time, as described in 2.7.1 IPC General Description.

IPC_SET can only be executed by a process with appropriate privileges or that has an effective user ID equal to the value of msg_perm.cuid or msg_perm.uid in the msqid_ds data structure associated with msqid. Only a process with appropriate privileges can raise the value of msg_qbytes.

IPC_RMID

Remove the message queue identifier specified by msqid from the system and destroy the message queue and msqid_ds data structure associated with it. IPC_RMD can only be executed by a process with appropriate privileges or one that has an effective user ID equal to the value of msg_perm.cuid or msg_perm.uid in the msqid_ds data structure associated with msqid.

RETURN VALUE

Upon successful completion, msgctl() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.

ERRORS

The msgctl() function shall fail if:

[EACCES]

The argument cmd is IPC_STAT and the calling process does not have read permission; see 2.7 XSI Interprocess Communication.

[EINVAL]

The value of msqid is not a valid message queue identifier; or the value of cmd is not a valid command.

[EPERM]

The argument cmd is IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to that of a process with appropriate privileges and it is not equal to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid.

[EPERM]

The argument cmd is IPC_SET, an attempt is being made to increase to the value of msg_qbytes, and the effective user ID of the calling process does not have appropriate privileges.

EXAMPLES

None.

APPLICATION USAGE

The POSIX Realtime Extension defines alternative interfaces for interprocess communication (IPC). Application developers who need to use IPC should design their applications so that modules using the IPC routines described in 2.7 XSI Interprocess Communication can be easily modified to use the alternative interfaces.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

2.7 XSI Interprocess Communication, 2.8 Realtime, mq_close, mq_getattr, mq_notify, mq_open, mq_receive, mq_send, mq_setattr, mq_unlink, msgget, msgrcv, msgsnd

XBD 3.206 Message Queue, <sys/msg.h>

CHANGE HISTORY

First released in Issue 2. Derived from Issue 2 of the SVID.

Issue 5

The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to a new APPLICATION USAGE section.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0395 [345] is applied.