User Manual. (C) 2010, Matt Pedersen, Matthew Sowders, Brian Kauke, Jason Hurt, Sean Kau

Transcription

1 User Manual (C) 2010, Matt Pedersen, Matthew Sowders, Brian Kauke, Jason Hurt, Sean Kau 1

2 1 Introduction to ProcessJ 2 Primitive Processes 2.1 Assignment ProcessJ, like occam is a process oriented language, and therefore, everything is built from processes. One of the simplest processes we can have is that of an assignment. The syntax for an assignment is: assignment variable = expression Note to occam programmers: In ProcessJ we do not use := as the assginment operator, but simply the = operator like in Java. In general, ProcessJ attempts to stay as close to Java syntax as possible. Also, note that where occam allows multiple assignments like x,y := y,x, ProcessJ does not. The expression on the right hand side of the assignment must be of a type that is assignment compatible with the variable on the left hand side. We will return to the meaning of assignment compatible, but in general we use the coercion rules of Java: If a value can be assigned to the variable without loss of precission, the assignment is legal. The variable on the left hand side can be a number of different things: a local variable or parameter as in x = 5. a record member variable as in myrecord.x = 5. an array index as in myarray[100][233] = 5. or even a mixture of the three above mentioned possibilities: myrecord.myarray[12][34][56].x = 6; which represents a local variable or parameter named myrecord (or record type), which has a record field named myarray of three dimensional of array type of a record type which has a record field named x of integer type. 2

3 2.2 Communication Without communication, a ProcessJ program would not be all that interesting; it would be roughly comparable to a good old-fashioned Pascal program. Communication is extremely important in ProcessJ. Since two concurrent processes do not have any shared data, data must be passed from one to another through communication. In ProcessJ (like in occam) communication takes place over unidirectional channels carrying values of a specific data type or protocol. Channels can be shared at both ends (multiple readers/multiple writers), shared at one end (single reader/multiple writer or multiple reader/single writer) or not shared at all (single reader/single writer). We will return to channels, and channel communication, as well a shared channels and extended rendez-vous plus a discussion about channels versus channel ends later Input Inputting from a channel, or as we prefer to call it in ProcessJ, reading from a channel (techincally this should read reading from a channel end ) is easy: mychan.read() This will block until a value can be read of the reading end of the channel stored in the variable mychan. A channel read is an expression, so it can be used as a right hand side of an assignment: or in an expression: x = mychan.read() a mychan.read() (under the assumption that mychan is a channel that carries numbers that can be added to the value of a and 42. The general syntax (at least for now) for a read is: channel read expression channel.read() Note to occam programmers: In occam a channel input must appear with a variable or in a case read; this is not the case in ProcessJ; a channel read is an expression in ProcessJ. 3

4 2.2.2 Output Where a channel read in an input event, a channel write is an output event. An example is: yourchan.write(42 + x) which writes the value of 42+x to the writing end of yourchan. (It might seem a little silly saying writes to the writing end as you cannot write to the reading end of a channel; this is of course true, but as we will see soon, the read and the write that we have shown so far are both really using that fact to choose the right end. We ought to have written yourchan.write.write(42+x), that is, write to the writing end of yourchan. Indeed you may write that (and you sometimes need to refer to channel ends; namely when passing them as parameters), but you do not need to add the.write or.read when the context is obviuos. The syntax for a channel write is (for now at least): channel write statement channel.write(expression) 2.3 Skip and Stop skip is a primitive process well known to occam programmers. To a Java programmer the equivalent is ;, that is, do nothing. In ProcessJ you may use the skip process to perform nothing, or alternatively you could use the semicolon. A skip process does nothing and terminates immediately. This is opposite to the stop process, which, like skip does nothing, but never terminates. In other words, it causes the code following a stop (in sequence) never to be executed. 3 Combining Processes In ProcessJ other processes can be built from other (simpler) processes; much line in occam. Java programmers will be familiar with if conditionals, case selections and while, for and do loops, but new to the parallel and the alternation constructs, and the occam programmer will be missing the SEQ construct. In general we can categorize these constructed processes as being either sequential or parallel. 4

5 3.1 Sequences In object oriented and imperative languages we do not have any parallel constructs; statements are executed one after another, where as a language like occam, which is highly concurrent, one frequently distinguishes between a sequential execution of a number of processes and a parallel execution of a number of processes. In ProcessJ two processes are executed sequentially simply by separating them by a semicolon (;) - exactly like in Java or C. Note to occam programmers: You will be familiar with the SEQ keyword, which creates a sequential sequence of processes. The SEQ has the same semantic meaning as the semicolon in ProcessJ. Example: in ProcessJ is equivalent to x = 7; y = 9; SEQ x := 7 y := 9 in occam. 3.2 Conditionals Conditionals in ProcessJ are exactly like in Java. Either an if-then or an if-then-else. more to come 3.3 Selection 3.4 Loops do and while 5

6 3.5 Parallel Something new to a Java programmer is the ability to execute two processes in parallel (at least without having to created and explicityly start threads to do so). In occam a parallel counterpart to the SEQ, namely PAR exists. In ProcessJ, in order to execute a number of processes in parallel, they are combined in a block prefixed the keyword par. For example, to concurrently assign 7 to x and 9 to y we can write: x = 7; y = 9; Note, the indentation is not necessary (unlike in occam, where indentation matters), but good indentation improves readability and maintainability of the code. In general we apply the same indentation guidelines as Java and/or C. The par construct is a very poweful construct; consider for example the following example: editor (term_in.read, term_out.write); keyboard (term_in.write); screen (term_out.read); This code executes three procedures (editor, keyboard, and screen) concurrently, passing channel ends to them, so they can communicate across the supplied channels. The entire par construct does not terminate until each of the three sub-processes terminate. par blocks may be nested: p1(); p2(); p3{; 6

7 The above code is equivalent to: p1(); p2(); p3(); However, it is not equivalent to p1(); { p2(); p3(); which is a concurrent execution of the process p1 and a sequential block that first executes p2 and then executes p Parallel disjointness With great power comes great responsibility, a wise man once said. This is true about the par construct; luckily we have a compiler that can perform the checking for us. When executing process in parallel, sharing variables and channels can potentially be dangerous. Consider this example: x := 42; x := 67; What is the value of x after the par block has finished executing? if executed on a true parallel (or multicore) architecture, there is no way to predict, and if executed on a single core interleaving arcitecture it entirely depends on the scheduling. This kind of nondeterministic behavious is not very attractive; therefore, in ProcessJ (like in occam) a particular variable cannot be assigned to in more than one process in a parallel constructs, as well as a channel cannot be written to in more than one process or read from. See Appendix?? for more information about parallel usage rules. 7

8 3.5.2 Parallel for loops occam: replicated parallel... par for (int i=0; i<4; i++) myproc(i); is equivalent to myproc(0); myproc(1); myproc(2); myproc(3); something about usage rules here too! 3.6 Alternation An alternation is a nwe construct for Java programmers. An alternation combines a number of processes guarded by inputs. The alternation performs the process associated with one of the set of ready guards. A simple example is: while (true) alt { x = top_in.read(): out.write(x); x = bottom_in.read(): out.write(x); which implements a simple multiplexer. It reads input from the top in or the bottom in channels and places the input in x before writing the out again onto the out channel. If input is ready on the top in channel and nothing is ready on the bottom in channel, the top in channel is read and vice versa. If both of the channels are read to be read (i.e., both input guards are ready), one is chosen at random. An input guard may be prefixed by a boolean expression like for example in this example: 8

9 while (true) alt { top_enabled & x = top_in.read(): out.write(x); x = bottom_in.read(): out.write(x); When an input guard is prefixed by a boolean expression, only when the boolean expression evaluates to true will the input guard be considered. If the boolean expression is false, then the entire guard is not considered ready. (If top ready is set to false, then the multiplexer will only move input from the bottom in channel onto the out channel. A special type of guard called skip guard can be used. Recall, the skip process does nothing and terminates, so using a skip guard (possibly prefixed by a boolean expression) acts like a guard that is always ready (if prefixed by a boolean expression, a skip guard is always ready when the boolean expression evaluates to true). Also note, the boolean prefix expression may not contain any channel reads or any procedure calls. Note to compiler programmers: the boolean expression may not contain any procedure calls or any channel reads! should we have replicated alternation? something like this: alt for (i=0; i<10; i++) x = mychanarray[i] : out.write(x); To allow something like this there may not be any channel reads in the for expressions, and the entire index set must be calculatable at runtime... maybe more.. this is why occam does it the way they do! 9

10 4 Basic Data Types 4.1 Primitive Data Types 4.2 Arrays 5 Variables and Values 6 Structured Data Types 6.1 Record Data Types 6.2 Protocol Data Types 7 Channels 8 Sharing 9 Timers 10 Barriers 11 Expressions 12 Procedures 13 Mobility 14 Packages 15 Libraries 16 Native Code 17 Choice of Backend 10