osi_kmsgctl — Perform message queue control operations

Function

The osi_kmsgctl service provides various message control operations as specified by command. These functions include reading and changing message variables within the MSQID_DS data structure and removing a message queue from the system.

This is a secondary interface to the msgctl service. It is provided for use by a PFS running in a colony address space. For information about the msgctl service, see msgctl (BPX1QCT, BPX4QCT) — Perform message queue control operations in z/OS UNIX System Services Programming: Assembler Callable Services Reference.

Requirements

Operation	Environment
Authorization:	Supervisor state or problem state; any PSW key
Dispatchable unit mode:	Task
Cross memory mode:	Any
AMODE:	31-bit
ASC mode:	Any
Interrupt status:	Enabled for interrupts
Locks:	Unlocked
Control parameters:	All parameters must be addressable by the caller and in the primary address space.

Format

CALL osi_kmsgctl,(Message_Queue_ID,
              Command,
              Buffer,
              Return_value,
              Return_code,
              Reason_code)

Parameters

Message_Queue_ID

Supplied parameter

Type:: Integer
Length:: Fullword

Specifies the message queue identifier.

Command

Supplied parameter

Type:: Integer
Character set:: N/A
Length:: Fullword

The name of a fullword field that indicates the message command that is to be executed. For the structure that contains these constants, see Mapping macros in z/OS UNIX System Services Programming: Assembler Callable Services Reference. The values for Command are:

Ipc_STAT: This command obtains status information about the message queue that is identified by the Message_Queue_ID parameter, if the current process has read permission. This information is stored in the area that is pointed to by argument Buffer and mapped by area MSQID_DS data structure. For the data structure, see MSQID_DS DSECT.
Ipc_SET: Set the value of the IPC_UID, IPC_GID, IPC_MODE and MSG_QBYTES for associated Message_queue_ID. The values that are to be set are taken from the MSQID_DS data structure that is pointed to by argument Buffer. Any value for IPC_UID and IPC_GID may be specified. Only mode bits that are defined by BPX1QGT under the Message_Flag argument may be specified in the IPC_MODE field. This Command can only be executed by a task that has an effective user ID equal to either that of a task with appropriate privileges, or the value of IPC_CUID or IPC_UID in the MSQID_DS data structure that is associated with Message_Queue_ID. This information is taken from the buffer that is pointed to by the Buffer parameter. For the data structure, see MSQID_DS DSECT.
Ipc_RMID: Remove the message identifier that is specified by Message_Queue_ID from the system, and destroy the message queue and MSQID_DS data structure that are associated with it. This Command can only be executed by a process that has an effective user ID equal to either that of a process with appropriate privileges, or the value of IPC_CUID or IPC_UID in the MSQID_DS data structure that is associated with Message_Queue_ID.

Buffer

Parameter supplied and returned

Type:: Address
Length:: Fullword

The name of the fullword that contains the address of the buffer into which or from which the message queue information is to be copied. This buffer is mapped by MSQID_DS.

Return_value

Returned parameter

Type:: Integer
Length:: Fullword

The name of a fullword in which the osi_kmsgctl service returns -1 or 0.

Return_code

Returned parameter

Type:: Integer
Length:: Fullword

The name of a fullword in which the osi_kmsgctl service stores the return code. The osi_kmsgctl service returns Return_code only if Return_value is -1. See z/OS UNIX System Services Messages and Codes for a complete list of possible return code values. The osi_kmsgctl service can return one of the following values in the Return_code parameter:

Return_code	Explanation
EACCES	The command specified was Ipc_STAT, and the calling process does not have read permission. The following reason code can accompany the return code: JRIpcDenied.
EINVAL	One of the following errors occurred: Message_Queue_ID is not a valid Message queue identifier The Command parameter is not a valid command. The mode bits were not valid (SET). The following reason codes can accompany the return code: JRIpcBadFlags, JRMsqQBytes, or JRIpcBadID.
EPERM	The command specified was Ipc_RMID or Ipc_SET, and the effective user ID of the caller is not that of a process with appropriate privileges, and is not the value of IPC_CUID or IPC_UID in the MSQID_DS data structure that is associated with Message_Queue_ID. The command specified was Ipc_SET, and an attempt is being made to increase MSG_QBYTES. The effective user ID of the caller does not have superuser privileges. The following reason codes can accompany the return code: JRIpcDenied or JRMsqQBytes.
EFAULT	The Buffer parameter specified an address that caused the syscall to program check. The following reason code can accompany the return code: JRBadAddress.

Reason_code

Returned parameter

Type:: Integer
Length:: Fullword

The name of a fullword in which the osi_kmsgctl service stores the reason code. The osi_kmsgctl service returns Reason_code only if Return_value is -1. Reason_code further qualifies the Return_code value. See z/OS UNIX System Services Messages and Codes for the reason codes.

Usage notes

Changing the access permissions only affects message queue syscall invocations that occur after msgctl has returned. msgsnd and msgrcv, which are waiting while the permission bits are changed by msgctl, are not affected.
Ipc_SET can change permissions, and might affect the ability of a thread to use the next message queue syscall.
Quiescing a message queue stops additional messages from being added, while allowing existing messages to be received. A message queue can be quiesced by clearing (Ipc_SET) write permission bits.
A message queue can also be quiesced by reducing MSG_QBYTES (Ipc_SET) to zero. (Note: it would take superuser authority to reraise the limit.) Requesters are told EAGAIN or wait.
When a message queue ID is removed (Ipc_RMID) from the system, all waiting threads regain control with RV=-1, RC=EIDRM, and RC=JRIpcRemoved.
If you do not want to change all the fields, first initialize (Ipc_STAT) the buffer, change the desired fields, and then make the change (Ipc_SET).
For Command Ipc_RMID, the remove is complete by the time control is returned to the caller.

Related services

osi_kmsgget — Create or find a message queue

Characteristics and restrictions

This service is invoked only from a colony address space.

The caller is restricted by ownership, read, and read/write permissions that are defined by OSI_kmsgget and OSI_kmsgctl Ipc_SET.