Control - Procedures and Environments Control Procedure definition - PowerPoint PPT Presentation

Control - Procedures and Environments

Control  Procedure definition and activation:  A procedure is a mechanism in a programming language for abstracting a group of actions or computations.  The group of actions is called the body of the procedure.  A procedure is represented by a specification including A name,  The types and names of parameters.  The type of the return value.   We shall not make a significant distinction between a function and a procedure, although differences exist. Dr. Sherif G. Aly 2

Control  Procedure definition and activation: Formal  Example: Parameters //C++ code void intSwap (int &x, int &y) //Specification { int temp = x; //Body x = y; //Body y = temp; //Body } Dr. Sherif G. Aly 3

Control  Procedure definition and activation:  A procedure is called or activated by stating its name, together with arguments to the call corresponding to the parameters.  Example: Actual Parameters intSwap(a, b); Dr. Sherif G. Aly 4

Control  Procedure definition and activation:  A call to the procedure transfers control to the beginning of the body of the called procedure (The Callee).  In some languages, control can be returned to the caller even before reaching the end of the callee ’ s body by using a return statement. Dr. Sherif G. Aly 5

Control  Procedure definition and activation:  Example: //C++ code void intSwap (int &x, int &y) { If x is equal to y, the function will exit here if (x == y) return; int temp = x; x = y; y = temp; } Dr. Sherif G. Aly 6

Control  Procedure definition and activation:  In some languages such as FORTRAN, to call a procedure, one must also include the keyword CALL.  CALL INTSWAP (A, B)  In FORTRAN, procedures are called subroutines. Dr. Sherif G. Aly 7

Control  Procedure definition and activation:  In some languages, procedure and function declarations are written in a form similar to constant declarations:  Example: ML (* ML code *) fun swap (x, y) = let val t = !x in x:= !y y:= t end; Dr. Sherif G. Aly 8

Control  Procedure definition and activation:  In such ML case, we can say that a procedure declaration creates a constant procedure value.  It associates a symbolic name (the name of the procedure) with the value.  A procedure communicates with the rest of the program through its: Parameters  Non-local references (references to variables outside the procedure body)  Dr. Sherif G. Aly 9

Control  Procedure Semantics:  When a block is encountered during execution, it causes the allocation of local variables and other objects corresponding to the declarations of the block.  The memory allocated for the local objects of the block is called the activation record (stack frame). Dr. Sherif G. Aly 10

Control  Procedure Semantics:  When a block is encountered during execution, it causes the allocation of local variables and other objects corresponding to the declarations of the block.  The memory allocated for the local objects of the block is called the activation record (stack frame). Dr. Sherif G. Aly 11

Control Global Environment x x  Procedure Semantics: Activation Record of A y int x; void B(void){ int i; i Activation Record of B Using Lexical i = x/2; Scoping, which } x is this? void A(void){ int x, y; B(); } main(){ A(); return 0; } Dr. Sherif G. Aly 12

Control Global Environment x x  Procedure Semantics: Activation Record of A y int x; void B(void){ int i; i Activation Record of B Using Lexical i = x/2; Scoping, which } x is this? void A(void){ int x, y; B(); The global environment is called the defining } environment of B The activation record of A is called the calling main(){ environment of B A(); return 0; } Dr. Sherif G. Aly 13

Control Global Environment x x  Procedure Semantics: Activation Record of A y int x; void B(void){ int i; i Activation Record of B Using Lexical i = x/2; Scoping, which } x is this? void A(void){ int x, y; B(); The global environment is called the defining } environment of B The activation record of A is called the calling main(){ environment of B A(); return 0; } The x is that of the defining environment : The global x Dr. Sherif G. Aly 14

Control  Procedure Semantics:  A procedure may have multiple calling environments (could be called from more than one place).  However, a procedure will have one defining environment. Dr. Sherif G. Aly 15

Control  Parameter Passing Mechanisms:  Pass by value.  Pass by reference.  Pass by value result.  Pass by name and delayed evaluation. Dr. Sherif G. Aly 16

Control  Parameter Passing Mechanisms:  Pass by value: The value of the parameter is evaluated.  The parameter is passed as a constant.  It ’ s value cannot be modified, or if modified does not affect the actual  parameter. This is by far the most common mechanism for parameter passing.  In Java, all primitive data types are passed by value.  Dr. Sherif G. Aly 17

Control  Parameter Passing Mechanisms:  Pass by value (C): void increment (int x){ A copy of y is made and x++; passed to x. } Changing x DOES NOT increment (y); change y. Dr. Sherif G. Aly 18

Control  Parameter Passing Mechanisms:  Pass by reference: The location of the parameter is passed.  The formal parameter becomes an alias to the actual parameter.  Any change in the formal parameter affects the actual parameter.  In FORTRAN, passing by reference is the only allowed parameter  passing mechanism. In C++ and Pascal, passing by reference is specified using an extra  syntax. Dr. Sherif G. Aly 19

Control  Parameter Passing Mechanisms:  Pass by reference (C): void increment (int &x){ x becomes an alias of y. x++; Changing x affects y. } The & before x specifies passing by reference. increment (y); Dr. Sherif G. Aly 20

Control  Parameter Passing Mechanisms:  Pass by reference:  If FORTRAN only allows passing by reference.  How is a call such as inc(2) achieved?  A temporary variable is located, initialized with 2, and passed.  This mimics passing by value in a passing by reference mechanism. Dr. Sherif G. Aly 21

Control  Parameter Passing Mechanisms:  Pass by Value-Result: This mechanism achieves a similar result to passing by  reference. No actual alias is established.  A copy of the actual parameter is made, used in the  procedure, and then copied back to the actual parameter. Also known as copy-in, copy-out.  Also known as copy-restore.  Dr. Sherif G. Aly 22

Control  Parameter Passing Mechanisms:  Pass by Name and Delayed Evaluation:  The actual parameter passed to the procedure is NOT evaluated until it is actually used in the called procedure. Dr. Sherif G. Aly 23

Control  Parameter Passing Mechanisms:  Pass by Name and Delayed Evaluation:  Example: Void inc(int x) { … x++; a[i] only evaluated here! } If i somehow changed in procedure inc before this point, BIG problem!! We will not be incrementing inc(a[i]); the actually intended a[i] Dr. Sherif G. Aly 24

Control  Parameter Passing Specification:  Parameter passing specification is different than parameter passing mechanisms.  Parameter passing mechanisms are tied closely to the internal mechanics of the code used to implement them. Dr. Sherif G. Aly 25

Control  Parameter Passing Specification:  In Ada for example, we can specify parameters as In:  Cannot be legally assigned a new value inside the procedure, or otherwise  have its value changed! More like a constant.  Out:  Can only be assigned to.  Its value can never be used.  In Out :  Both   The meaning of such words is exactly what we can expect.  Any programs violating the rules above is considered erroneous! Dr. Sherif G. Aly 26

Control  Type Checking of Parameters:  In strongly typed languages:  Procedure calls must be checked so that the actual parameters agree in type and number with the formal parameters.  Procedures may not have a variable number of parameters.  Rules must be stated for type compatibility. Dr. Sherif G. Aly 27

Control  Procedure Environments, Activations, and Allocation:  In block structured language with recursion, such as C and Algol like languages, we need a stack to store various scope information.  In stack based runtime environments: An environment pointer is needed to point to the current  activation. A control link is needed to point to the previous activation  record of the block from which control passed to the current block and to which control will return. Dr. Sherif G. Aly 28

Control  Dynamic Memory Management:  In an imperative language such as C, the automatic allocation and deallocation of storage occurs only for activation records on the stack.  Space is allocated for an activation record on the stack when a procedure is called, and deallocated when the procedure exits.  Explicit dynamic allocation and use of pointers is also available under manual programmer control using a heap of memory separate from the stack. Dr. Sherif G. Aly 29