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AIX Version 4.3 Base Operating System and Extensions Technical Reference, Volume 1
msgctl Subroutine
Purpose
Provides message control operations.
Library
Standard C Library (libc.a)
Syntax
#include <sys/msg.h>
int msgctl (MessageQueueID,Command,Buffer)
int MessageQueueID, Command;
struct msqid_ds *Buffer;
Description
The msgctl subroutine provides a variety
of message control operations as specified by the Command parameter and stored
in the structure pointed to by the Buffer parameter. The msqid_ds
structure is defined in the sys/msg.h file.
The following limits apply to the message queue:
- Maximum message size is 65,535
bytes for releases prior to AIX 4.1.5 and is 4 Megabytes for release 4.1.5 and later releases.
- Maximum number of messages per queue is 8192.
- Maximum number of message queue IDs
is 4096 for AIX
releases before 4.3.2 and 131072 for AIX 4.3.2 and following.
- Maximum number of bytes in a queue is
4 65,535 for releases prior to AIX 4.1.5 and is 4 Megabytes for release 4.1.5
and later releases.
Parameters
MessageQueueID |
Specifies the message queue identifier. |
Command |
The following values for the Command parameter are available:
- IPC_STAT
- Stores the current value of the above fields of the data structure
associated with the MessageQueueID parameter into the msqid_ds
structure pointed to by the Buffer parameter.
The current process must have read permission in order to perform this operation.
- IPC_SET
- Sets the value of the following fields of the data structure associated
with the MessageQueueID parameter to the corresponding values found in the
structure pointed to by the Buffer parameter:
msg_perm.uid
msg_perm.gid
msg_perm.mode/*Only the low-order
nine bits*/
msg_qbytes
The effective user ID of the current process must have root user authority or its
process ID must equal the value of the msg_perm.uid or
msg_perm.cuid field in the data structure associated with the
MessageQueueID parameter in order to perform this operation. To raise the
value of the msg_qbytes field, the effective user ID of the current
process must have root user authority.
- IPC_RMID
- Removes the message queue identifier specified by the
MessageQueueID parameter from the system and destroys the message queue and
data structure associated with it. The effective user ID of the current process must
have root user authority or be equal to the value of the msg_perm.uid or
msg_perm.cuid field in the data structure associated with the
MessageQueueID parameter to perform this operation.
|
Buffer |
Points to a msqid_ds structure. |
Return Values
Upon successful completion, the msgctl
subroutine returns a value of 0. Otherwise, a value of -1 is returned and the
errno global variable is set to indicate the error.
Error Codes
The msgctl subroutine is unsuccessful if
any of the following conditions is true:
EINVAL |
The Command or MessageQueueID parameter is not valid. |
EACCES |
The Command parameter is equal to the IPC_STAT value, and
the calling process was denied read permission. |
EPERM |
The Command parameter is equal to the IPC_RMID value and the
effective user ID of the calling process does not have root user authority. Or, the
Command parameter is equal to the IPC_SET value, and the effective user
ID of the calling process is not equal to the value of the msg_perm.uid
field or the msg_perm.cuid field in the data structure associated with
the MessageQueueID parameter. |
EPERM |
The Command parameter is equal to the IPC_SET value, an
attempt was made to increase the value of the msg_qbytes field, and the
effective user ID of the calling process does not have root user authority. |
EFAULT |
The Buffer parameter points outside of the process address
space. |
Implementation Specifics
This subroutine is part of Base Operating
System (BOS) Runtime.
Related Information
The msgget subroutine, msgrcv subroutine, msgsnd subroutine, msgxrcv subroutine.
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