Description
II. Background – communicating by message passing
Processes can communicate with each other by sending and receiving messages. Sender
process creates a message and sends it to a specific receiver process. The receiver receives
the message and then waits until the next message comes from any senders. In order to
implement message passing, the Xinu process table is used; a flag (process has message) is
used in addition to storage in the table to store the message itself. For message passing,
Xinu provides a send/receive set of system calls as follows:
● send – send a message to a process
● receive – receive a message from some other process
● recvclr – check if a message has been received from another process and return it if
it exists, but do not wait if one does not exist. Recvclr removes (clears) the message
from the process table if one exists.
● recvtime – same as receive, yet will only wait for a limited specified time for the
message to be received.
III. Extending send/receive system calls
In this lab assignment, you will extend Xinu message passing and will implement your own
system calls for handling two capabilities. For the first, your system call will allow sending
multiple messages to a single process. In the second, a sender process will be able to send a
message to multiple receivers.
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A. System calls for sending/receiving multiple messages
A sender process may need to send multiple messages to a single process. For instance,
different types of messages need to be sent at the same time. Or a message should be
segmented in a fixed size and then sent to a receiver process. The messages should be
received in the order in which they were sent. You may use a circular buffer for queuing (or
dequeuing) the messages as you did in Lab 1. You will be implementing the following
system calls:
syscall sendMsg (
pid32 pid,
umsg32 msg
);
Sending message (msg) to process (pid). In case a message
or a number of messages are already waiting on pid to
receive them, the new msg will be queued. It will return
OK on success or SYSERR on error.
umsg32 receiveMsg (void); Receiving a message from any sender. It causes the calling
process to wait for a message to be sent (by any process).
When a message is sent, it is received and returned.
uint32 sendMsgs (
pid32 pid,
umsg32* msgs,
uint32 msg_count
);
Sending a group (msg_count) of messages (msgs) to
process (pid). It will return the number of msgs
successfully sent or SYSERR on error.
syscall receiveMsgs (
umsg32* msgs,
uint32 msg_count
);
Receiving a group (msg_count) of messages (msgs). It
causes the calling process to wait for msg_count messages
to be sent. When all messages are in the queue, they are
then all together immediately received.
B. System call for sending to multiple recipients
Sometimes a sender process is required to send a message to multiple processes. For
instance, after a process changes data, it should update the change in other processes which
use the data. You will be implementing the following system calls;
uint32 sendnMsg (
uint32 pid_count,
pid32* pids,
umsg32 msg
);
Sending the message (msg) to the number (pid_count) of
processes (pids). It will return the number of pids
successfully sent to or SYSERR on error.
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C. Requirements and notes
1. You need to add your new system calls in Xinu by updating include/prototypes.h.
Insert the following:
extern syscall sendMsg (pid32, umsg32);
extern unit32 sendMsgs (pid32, umsg32*, uint32);
extern unit32 sendnMsg (uint32, pid32*, umsg32,);
extern umsg32 receiveMsg (void);
extern syscall receiveMsgs (umsg32*, unit32);
2. Do not change the existing messaging system calls (send, receive, recvclr, and
recvtime), because they are used by other system processes. Also do not change the
process table entry in any way that causes existing system calls to no longer work. If
changes are required, make sure to only add onto the existing structure; never remove
any existing variables.
3. The maximum number of messages is 10. Also make the number of recipient
processes no more than 3 for each of the senders and receivers (3 senders and 3
receivers).
4. Processes should print message(s) sent or received while the program is running.
Senders should show recipients’ pid and msgs, and how many messages successfully
sent. In receivers’ side, received messages should be displayed. Make sure you
indicate in the print message whether a message is being sent or has been received.
5. You are able to add more functions and internal calls if necessary, as long as you
never change existing functions. Only files in the include/ and system/ directories
should be altered. Do not add any new source files, as this would require altering the
build process.
IV. Submission
You should submit your main() program along with your syscall implementations.
Also be sure to include any Xinu headers/sources that you modified, with comments
on what you have added – everything should be packaged into a .zip file. For grading,
the TAs will use their own main() function to compile and test the functionality of
your system calls, in addition, of course, to inspecting your code itself.
