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AIX Version 4.3 Base Operating System and Extensions Technical Reference, Volume 1
msgget Subroutine
Purpose
Gets a message queue identifier.
Library
Standard C Library (libc.a)
Syntax
#include <sys/msg.h>
int msgget (Key, MessageFlag)
key_t Key;
int MessageFlag;
Description
The msgget subroutine returns the
message queue identifier associated with the specified Key parameter.
A message queue identifier, associated message
queue, and data structure are created for the value of the Key parameter if
one of the following conditions is true:
- The Key parameter is equal to the
IPC_PRIVATE value.
- The Key parameter does not already have a message
queue identifier associated with it, and the IPC_CREAT value is set.
Upon creation, the data structure associated with
the new message queue identifier is initialized as follows:
- The msg_perm.cuid,
msg_perm.uid, msg_perm.cgid, and msg_perm.gid
fields are set equal to the effective user ID and effective group ID, respectively,
of the calling process.
- The low-order 9 bits of the msg_perm.mode
field are set equal to the low-order 9 bits of the MessageFlag parameter.
- The msg_qnum, msg_lspid,
msg_lrpid, msg_stime, and msg_rtime fields are
set equal to 0.
- The msg_ctime field is set equal to the
current time.
- The msg_qbytes field is set equal to the
system limit.
The msgget subroutine performs the following
actions:
- The msgget subroutine either finds or creates
(depending on the value of the MessageFlag parameter) a queue with the Key parameter.
- The msgget subroutine returns the ID of the queue
header to its caller.
The following limits apply to the message queue:
- Maximum message size is
4 Mega
bytes.
- 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.
Parameters
Key |
Specifies either the value IPC_PRIVATE or an Interprocess
Communication (IPC) key constructed by the ftok subroutine (or by a similar algorithm). |
MessageFlag |
Constructed by logically ORing one or more of the following values:
- IPC_CREAT
- Creates the data structure if it does not already exist.
- IPC_EXCL
- Causes the msgget subroutine to fail if the IPC_CREAT value
is also set and the data structure already exists.
- S_IRUSR
- Permits the process that owns the data structure to read it.
- S_IWUSR
- Permits the process that owns the data structure to modify it.
- S_IRGRP
- Permits the group associated with the data structure to read it.
- S_IWGRP
- Permits the group associated with the data structure to modify it.
- S_IROTH
- Permits others to read the data structure.
- S_IWOTH
- Permits others to modify the data structure.
Values that begin with S_I are defined in the sys/mode.h file and are
a subset of the access permissions that apply to files.
|
Return Values
Upon successful completion, the msgget
subroutine returns a message queue identifier. Otherwise, a value of -1 is returned
and the errno global variable is set to indicate the error.
Error Codes
The msgget subroutine is unsuccessful if
any of the following conditions is true:
EACCES |
A message queue identifier exists for the Key parameter, but
operation permission as specified by the low-order 9 bits of the MessageFlag
parameter is not granted. |
ENOENT |
A message queue identifier does not exist for the Key parameter and
the IPC_CREAT value is not set. |
ENOSPC |
A message queue identifier is to be created, but the system-imposed limit
on the maximum number of allowed message queue identifiers system-wide would be
exceeded. |
EEXIST |
A message queue identifier exists for the Key parameter, and both
IPC_CREAT and IPC_EXCL values are set. |
Implementation Specifics
This subroutine is part of Base Operating
System (BOS) Runtime.
Related Information
The ftok
subroutine, msgctl subroutine, msgrcv subroutine, msgsnd subroutine, msgxrcv subroutine.
The mode.h file.
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