Unix message queues allow processes to communicate asynchronously through kernel-managed buffers. A message queue is identified by a message queue identifier and contains a linked list of messages. Each message contains a message type, byte count, and pointer to the actual message data. Processes use message queue APIs like msgget(), msgsnd(), msgrcv(), and msgctl() to open, send, receive, and manage messages on queues.

1. UNIX
MESSAGE
QUEUES
Reference:
Rohit Sinha Unix System Programming
2BV07CS081 Using C++
http://www.rohitsinha.com -- Terrence Chan

2. MESSAGE QUEUES
• Linked list of messages
• Message queue table in kernel address
space
• Identified by message queue identifier

3. KERNEL DATA STRUCTURE
•A name which
Struct msqid_dsis anA MessageKey
integer ID Queue
{•A creator user ID and group ID
•Assigned owner user ID and group ID
struct ipc_perm msg_perm;
•Read-write access permission
msgqnum_t msg_qnum;
of the queue for owners,
msglen_t msg_qbytes;
Message Record others
group members and
pid_t msg_lspid;
•Last read time and process ID
pid_t msg_lrpid;
•Last write time and process ID
time_t msg_stime
•Pointer to a linked list of message
time_t msg_rtime
records.
time_t msg_ctime
Message Table :
:
}

4. Kernel Handling Message
Queue
Sender
Process
•Message type
•No of bytes of
message Receiver
Message Table •Pointer to Process
actual
message data
Actual
Message

5. SYSTEM-IMPOSED LIMITS
<sys/msg.h>
SYSTEM LIMIT Meaning
MSGMNI The maximum number of message
queue that may exists at any given
time in a system
MSGMAX The maximum number of bytes of
data allowed for a message
MSGMNB The maximum number of bytes of
all messages allowed in a queue
MSGTQL The maximum number of messages
in all queues allowed in a system

6. APIs FOR MESSAGE
MANIPULATION
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/message.h>
MESSAGE API USES
msgget Open and create if needed, a
message queue for access
msgsnd Sends a message to message
queue
msgrcv Receives a message from message
queue
msgctl Manipulate the control data of a
message queue

7. MSGGET
• Prototype
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/message.h>
int msgget (key_t key, int flag);
Returns -1 when fails
• Opens a message queue with ID as key
• If the key value is a manifested constant IPC_PRIVATE the API
allocates a special message queue to be used exclusively by the
calling process.
– Flag can take two values:
• 0: API aborts if there is no message queue whose key ID matches the given
key value otherwise return a descriptor for that queue.
• IPC_CREAT: Creates a new queue (if none exists) with given key ID and
read-write access permission.
– Ex: int msgfdesc = msgget(15, IPC_CREAT|0644);
» IPC_EXCL ensures the API succeeds only if it creates a new msg queue.

8. MSGSND
• Prototype is a pointer to an object that contains the actual message
msgPtr: it
text and #include <sys/types.h> be sent
and message type to
#include <sys/ipc.h>
#include <sys/message.h>
int msgsnd (int msgfd, const void* msgPtr, int len, int flag);
struct msgbuf
Returns -1 when fails 0 on success
{
• Send a message pointed by msgPtr to a message
long mtype
queue designated message type
//
by the msgfd descriptor
char mtext[MSGMAX]
• The len value is the sizehold bytes of the message
// buffer to in the message text
}
• Two values of flag:
– 0: Process is blocked if needed until the process call is
completed.
– IPC_NOWAIT: Function abort if the process is to be
blocked ----- (EAGAIN)

9. MSGRCV
• Prototype
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/message.h>
If there is message in queuemsgfd, const void* msgPtr, criteria but mtype, int flag); the
int msgrcv (int that satisfies the selection int len, int is larger than len,
Success: Returns number of bytes written
function returns a failure status
• Receive a message of type mtype from a message queue
If MSG_NOERROR is set in the flag value, a message-1 queue is selectable
Fail: Returns in the
designated by the msgfd descriptor in an object pointed by the
msgPtr
irrespective of length.
• The len value is the max size in bytes of the message that can be
received
• Two values of flag:
– 0: Process is blocked if no message in queue matched the selection
criteria specified by mtype
– IPC_NOWAIT: The call is non blocking (returns -1 with errno set to
ENOMSG)

10. mtype Values
(type of the message to be received)
mtype values Meaning
0 Receive the oldest message of any
type in the queue
Positive integer Receive the oldest message of
specified message type
Negative integer Receive a message whose message
type is less than or equal to the
absolute value of mytpe. If more than
1 receive the oldest and the smallest.

11. MSGCTL
• Prototype
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/message.h>
int msgctl (int msgfd, int cmd, struct msqid_ds* mbufPtr);
Fail: returns -1
• Used to query the control data of a message queue
designated by the msgfd:argument 0
Success returns
• To change the information within the control data of
the queue
• To remove a message queue

12. cmd Values
cmd value Meaning
IPC_STAT Copy control data of the queue to the
object pointed by the mbufPtr
IPC_SET Change the control data of the queue
by those specified in the object
pointed by mbufPtr
IPC_RMID Remove the queue from the system
Note: The calling process for IPC_SET and IPC_RMID must be
superuser, creator or the assigned owner of the queue.

13. CLIENT/SERVER EXAMPLE

14. THANK YOU
• Queries ?
– Just Google it 