CS452 - Real-Time Programming - Spring 2008

Lecture 15 - Send-Receive-Reply

Questions & Comment

1. Due dates now on the web.

Producer/Consumer

Two ways to do this

1. Producer Sends, consumer Receives
   • Send means "Here is some data"
   • Receive means "I'm ready to receive data"
   • Reply means "I got the data"

   After Reply buffers holding data can be overwritten at will

2. Producer Receives and Replys
   • Send means "I'm ready for data"
   • Receive means "I'm ready to transmit data"
   • Reply means "Here's the data"

   After Reply buffers can be overwritten at will.

What is the difference?

1. If relationship is one-to-one
   • no difference
2. If relationship is many-to-one
   • the one doing many MUST Receive.
3. If relationship is many-to-many
   • both MUST Receive
   • Placing a Send among Receives is a bad idea. Why?
   • How do you solve the problem: a worker Sends to both.

int Send( Tid tid, char *arg, int arg-length, char *reply-buffer, int reply-buffer-size )

These are pretty self explanatory, except

1. The return value is the number of characters actually returned
   • including the \0 if the contents of the reply buffer is a string
2. If something goes wrong, the return value is negative, coded to indicate what went wrong

   What can go wrong

   1. Illegal tid
   2. tid not an existing task

   It's up to Send to check that the reply-buffer was big enough by looking at its return value

   It's not an error if the task to which we Send never Receives

   • Should it be?
3. Parsing argument and reply-buffer is potentially costly and error-prone
   • A type system might be nice
   • But then you would feel compelled to implement run-time type checking
4. This form of message passing requires user and kernel code to cooperate to avoid malignancies
   • which is just barely okay when one implementer does both
   • but is otherwise monumentally insecure

int Receive( Tid *tid, char *arg-buffer, int arg-buffer-length )

These are pretty self explanatory, except

1. How is the task id copied form kernel to receiver?
   • That is, where does the pointer point to?
2. What if the buffer wasn't big enough?
3. If several tasks have Sended, which one gets Received first?

int Reply( Tid tid, char *reply, int reply-length )

These are pretty self explanatory

Implementation

Code should be constant time

Send before Receive

• Kernel needs to find the Sender
• Need a FIFO Send queue in the Receiver's TD

Receive before Send

• Tid of Receiver in the arguments
• Test state to know which case

How is the message copying done?

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