Procedures & Concurrency in Ada Thanks to: Fatemeh Salehi, Maryam Foroughi Fatemeh Farzian, Maryam Khademi Fatemeh Farzian, Maryam Khademi

Procedures & Procedures & ConcurrencyConcurrency

in Adain Ada

Procedures & Procedures & ConcurrencyConcurrency

in Adain AdaThanks to:Thanks to:

Fatemeh Salehi, Maryam ForoughiFatemeh Salehi, Maryam Foroughi Fatemeh Farzian, Maryam KhademiFatemeh Farzian, Maryam Khademi

Control StructuresControl StructuresControl StructuresControl Structures

Rich set of Control Structures

• A Conditional• An iterator (definite and indefinite)• A case-statement• Subprograms• A goto-statement• Facilities of handling exceptions• Facilities for concurrent programming

All-Purpose Iterator• Quite general, quite baroque• Syntax:

loop <sequence of statements> end loop

• exit-statement:• aborts the loop and resumes control after the end

loop• Any number of exits, at any depth• Exit from blocks (not from subprograms)• Has some of the characteristics of nonlocal goto

• Multiple mid-decision loops

Loop exampleI := A’First;loop

if I > A’Last then exit; end if;if A(I) = k then exit; end if;I := I + 1;

end loop;

exit when (abbreviation)

I := A’First;loop

exit when I > A’Last;exit when A(I) = k;I := I + 1;

end loop; Saves us from typing Loop termination conditions are clearly marked X Exit can be buried deeply in the body of the loop

therefore be hard to spot

While loop

I := A’First;

When I <= A’Last

Loop

exit when A(I) = k;

I := I + 1;

end loop;

For loopfor I in A’Rangeloop

exit when A(I) = K;end loop

• The for-phrase automatically declares the control variable I, thereby making it local to the loop

• Outside of the loop it will not be possible to determine where K was found

Using a controlled variable to save K’s

location

for J in A’Rangeloop

if A(I) = K thenI := J;exit;

end if;end loop;

K was actually found or not?

DeclareFound: Boolean := False;

beginfor J in A’Rangeloop

if A(J) = K thenI := J;Found := True;exit;

end if;end loop;if Found then

--Handle found caseelse

--Handle not found caseend if;

end;

Ada’s Exception Mechanism

• Ada is intended for embedded computer applications

• Allows us to define exceptional situations, signal their occurrence, and respond to their occurrence

Exception Handling• Example

– User attempts to push an item when the stack is full

– Push function raises Stack_Error, causing the exception mechanism to be called• Assume exception mechanism pops a few

elements off the top of the stack and throws them away since they’re older. Then it adds the new data.

– Execution of Push and the body of Producer is aborted.

Definition of an Exception Handler

Use Stack1;procedure Producer (...);begin ... Push (Data); ...Exception when Stack_Error =>

declare Scratch: Integer; begin for I in 1..3 loop if not Empty then Pop (Scratch); end if; end loop; Push (Data); end;end Producer;

Propagation of exceptions

• If the exception is defined in the local environment, we go to its handler, otherwise we look for a handler in the caller’s environment.

• Exceptions are bound dynamically:– Violate Structure and Regularity

Principles.

Parameters• In

– Transmit information into a procedure – Analogous to constant parameters in

original Pascal– Read-only– Elementary types pass by value– Composite types up to compiler– Remaining types pass by reference

Parameters• Out

– Transmit results out of a subprogram– Ada 83: write-only– Ada 95: may be read after value set

within subprogram– Elementary types pass by value– Composite types up to compiler

Parameters• In Out

– Used for parameters that are to be used as both a source and a destination by the subprogram

– Elementary types pass by value-result

– Composite types pass by reference or value-result, compiler’s choice

Ada’s orthogonal solution

How does the compiler decide?

• Composite parameter: s words• Each component of the parameter: 1 word• Accessing components: n times• Cost of passing the parameter by copying:

C = 2s + n

• Cost of passing the parameter by reference:R = 2n + 1

Reference or Copy

Position-Independent & Default Parameters

Procedure Draw_Axes (X_Origin, Y_Origin: Coord; X_Scale ,Y_Scale: Float;

X_Spacing, Y_Spacing: Natural; X_Logarithmic, Y_Logarithmic: Boolean; X_Labels, Y_Labels: Boolean;

Full_Grid: Boolean);

Draw_Axes (500, 500, 1.0, 0.5, 10, 10, False, True, True, True, False )


Problem of programs with many parameters:

hard memorization error-proneSolution (also suggested by OS command

Languages):Position-Independent Parameters:Parameters can be listed in any order.A name is associated with each

parameter.


Example of Position-Independent:Draw_Axes (X_Origin => 500,

Y_Origin => 500, X_Spacing =>10, Y_Spacing => 10, Full_Grid => False,

X_Scale => 1.0, Y_Scale=>0.5 , X_Labels => True, Y_Labels=> True, X_Logarithmic => False, Y_Logarithmic=>

True);


Position-Independent:Readable.Much less prone to mistakes than the

position-dependent version.Illustration of the Labeling Principle.Like advantages of Pascal’s labeled

case-statement over earlier unlabeled case-statements.


Most users will not want a full grid or logarithmic axes.

All users must specify those options. Violation of the Localized Cost Principle

(users should not have to pay for what they do not use).

Solution:Default Parameters


Example of Default Parameters:Procedure Draw_Axes (X_Origin, Y_Origin:

Coord := 0; X_Scale ,Y_Scale: Real :=1.0; X_Spacing, Y_Spacing: Natural :=1; X_Label, Y_Label: Boolean:= True; X_Logarithmic, Y_Logarithmic: Boolean:= False; Full_Grid: Boolean:= False);

Draw_Axes (500, 500, Y_Scale =>0.5, Y_Logarithmic => True, X_Spacing => 10, Y_Spacing => 10);


Position-dependent and Position-Independent parameters can be mixed in a single call.

Draw_Axes (500, 500, Y_Scale =>0.5, Y_Logarithmic => True, X_Spacing => 10, Y_Spacing => 10);

Position-Independent & Default Parameters Complicate Operator

Identification

In Ada:A less obvious cost results from

feature interaction; in this case, the interaction of overloading with Position-Independent and Default Parameters.

Position-Independent & Default Parameters Complicate Operator

IdentificationProcedure P (x: Integer ; Y:Boolean := False);Procedure P (x: Integer ; Y:Integer := 0);Procedure P bears 2 meaning at onceP is

overloaded P(9,True) P(5,8)What is the meaning of the call P(3)?P(3) is ambiguous Ada dose not allow the two

procedure declaration shown above.

Position-Independent & Default Parameters Complicate Operator Identification

A set of declarations is illegal if it introduces the potential for ambiguous calls.

Type primary is (Red, Blue, Green);Type Stop_Light is (Red, Yellow, Green);Procedure Switch (Color: Primary; x: Float; Y: Float);Procedure Switch (Light: Stop_Light; Y: Float;

x:Float)

Switch (Red, x => 0.0, Y => 0.0)

Call is ambiguous declarations is illegal

Position-Independent & Default Parameters Complicate Operator Identification

Switch (Red, x => 0.0, Y => 0.0)

2 over-loading:I. Overloaded enumerationII. Overloaded procedure Human reader and the compiler can

have difficulty with a program that makes extensive use of overloading and position- independent and default parameters.

Ada Permits Concurrent Execution

Ada provides a tasking facility that allows a program to do more than one thing at a time.

Example:Small, stand-alone word-processing

system that allows users to print one file while they are editing another.

Ada Permits Concurrent ExecutionDefining disjoint tasks:

Procedure word_Processor istask Edit; end Edit;task body Edit isbegin------ edit the file selected ------end;task Print; end Print;task body Print isbegin------ print the file selected ------end;Begin--- initiate tasks and wait for their

completion ---end word_Processor;


A task is declared very much like a package, with a separate specification and body.

We do 3 things at once:We begin executing the bodies of

Word_ProcessorEditPrint


What does Word_Processor do?In this case not very much.Body of procedure is empty we

immediately encounter the end and try to return.

Ada prevents a procedure from returning as long as it has active local tasks.


Edit Print

Word_Processor


Why Ada require all local tasks to finish before a procedure can exit?

Suppose Word_Processor had some local variables; those are visible to Edit and Print.

When Word_Processor exits:- its activation record is deleted any reference

in Edit and Print to the local variables of Word_Processor will be meaningless

dangling references


Alternative:

Delay Word_Processor’s return until it can be done safely.

This is an example of the Security Principle.

1: Tasks Synchronize by 1: Tasks Synchronize by RendezvousRendezvous

1: Tasks Synchronize by 1: Tasks Synchronize by RendezvousRendezvous

2: Control Access to Data 2: Control Access to Data StructuresStructures

Will See:Will See:

task DB_System istask Summary; end Summary;task body Summary is

{…};task Retrieval;

entry Seek (K: Key);entry Fetch (R: out Recd);

end Retrieval;task body Retrieval is

{seek record and return it};begin

...await completion of local tasks end DB_System;

task body Retrieval is{

loop accept Seek (K: Key) do

RK := K; end Seek;

... Seek record RK and put in Recd_Value

accept Fetch(R:outRecd) do

R := Recd_Value; end Fetch;

end loop;};

task body Summary is{

. Seek(id);

.

. fetch(New_Rec);

.

.

}

Guards and Protected Types Control Concurrent Access to Data Structures

Our word processor• Edit sends documents to Print for

printing.

task Print is entry Send (D: Document) entry Terminateend Print;

task body Print isbegin loop select accept Send (D: Document) do … print the document end Send; or accept Terminate do exit end Terminate; end select; end loop;End Print;

Task Edit …. Print.Send(D);

Making it more loosely coupled

• Put a buffer (Communication) in between

• We have a Send : used by Edit• And a Receive : used by Print

Communication Specification (1)

• Package communication isSize : constant integer :=100;Avail: integer rang 0..size:=0;Procedure Send (D: in Document);Procedure Receive (d: out Document);

Privatein_ptr, out_ptr: integer range o..size-1 :=0;buffer: array (0..size-1) of Document;

End Communication;

Communication Body (1)

• Package body Communication isProcedure Send( d: in document){

buffer (in_ptr) := d;In_ptr := (in_ptr+1) mod size;avail++;

}Procedure receive( d: out document){

d := buffer (out_ptr);out_ptr := (out_ptr+1) mod size;avail--;

}

• Avail is a critical section, we need to have mutual exclusion.

• Or the implementation will be incorrect.

• One way: have Communication as a task and Send and Receive as guarded entries

Communication Specification (2)• Task communication is

entry Send (D: in Document);entry Receive (D: out Document);entry Terminate;

PrivateSize : constant integer :=100;Avail: integer rang 0..size:=0;in_ptr, out_ptr: integer range o..size-1 :=0;buffer: array (0..size-1) of Document;

End communication;


task body Communication isBegin

loopselect

when Avail < size accept Send (D: in Document)

do…end Send;or when Avail > 0 accept Receive (D: out

Document)do…end Receive;or accept Terminate do exit end Terminate;

end select;end loop;

end Communication;

• It has an overhead: an additional task, actively waiting

• Ada 95 has an additional construct :– Protected type– Only one of its entries can be

executed at a time

Communication Specification (3)

• Protected type communication is entry Send (D: in Document);entry Receive (D: out Document);

PrivateSize : constant integer :=100;Avail: integer range 0..size:=0;in_ptr, out_ptr: integer range 0..size-1 :=0;buffer: array (0..size-1) of Document;

End communication;


Protected body Communication isBegin

entry Send (D: in Document) when Avail< size is

begin…

end Send;entry Receive (D: out Document)

when Avail>0 isbegin

…end Receive;

end Communication;

Syntactic Structures

• Similar to Pascaldeclare procedure <name> (<formals>) is <local declarations> <local declarations>begin begin <statements> <statements>exceptions exceptions <exception handlers> <exception handlers>end end

Procedures vs. Blocks

• Procedures– Have a name– Have formal parameters– Can be “called” from diff. contexts

• Blocks-are called where they are written

Which Principle?

Syntactic Consistency Principle

Things that look similar should be similar; things that are

different should look different

Semicolons• Pascal

– semicolons are separators – between statements

• Ada– semicolons are terminators– Believed to be less error-prone

Fully Bracketed syntax• Compound statement have a terminating

statement• Pascal examples:

for i:= ... do begin ... endif ... then begin ...end else begin ... endprocedure ... begin ... endfunction ... begin ... endcase ... of a: begin ... end; ... endwhile ... do begin ... endwith ... do begin ... endrecord ... end

Ada Brackets• loop ... end loop• if ... end if• case ... end case• record ... end record• function <name> (<formals>) is ... begin ... end

<name>;• procedure <name (<formals>) is ... begin ... end

<name>;• package <name> is ... end <name>;• package body <name> is ... end <name>;• accept <name> (<formals>) do ... end <name>;

Ada Summary• An engineering trade-off• DoD Still disallows subsets• Criticized for Size

– Suffers from “feature bloat” in some ways– Ada-83 BNF: 1600 tokens– Pascal: 500– Algol: 600– Ada 95 is even larger

Ada Summary• C. A. R. Hoare suggested Ada is so big that

few programmers will master it.– “Do not allow this language in its present state

to be used in applications were reliability is critical ... By careful pruning of the Ada language, it is still possible to select a very powerful subset that would be reliable and efficient in implementation with safe and economic use. ... If you want a language with no subsets, you must make it small.”

Ada Summary• Moderately Successful

– Widely used in DoD– Not widely used in universities or the

commercial market

Fourth-Generation Languages

• Both consolidation of 3rd generation characteristics and addition of new characteristics

• Major contribution: extensions to name structures to give full data abstraction language.

• Control structure improvements

Fourth-Generation Languages

• Dynamically scoped exception mechanism

• Data structure constructors corrected some problems with 3rd generation constructors (e.g., array parameters)– Name equivalency is the rule

• Syntactic structures– Fully bracketed notation

Bottom Line

“... culmination and fulfillment of the evolutionary process that began

with FORTRAN.”

Any Question?

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Procedures & Concurrency in Ada Thanks to: Fatemeh Salehi, Maryam Foroughi Fatemeh Farzian, Maryam Khademi Fatemeh Farzian, Maryam Khademi