Procedural Programming

Transcription

1 Procedural Programming Lecture 4 - Summer Semester 2016 Parameters & Pointers Faculty of Engineering Institute of Computer Engineering What we know so far... Data type serves to organize data (in the memory), its possible values, allowed changes and use of storage. (Function) procedure is a self-contained block of instructions (a subpart of the program) making use of data that have specific types. Variables and (function) procedures must be declared to be known or visible within their scope and defined to allocate memory to them for storing values during their lifetime. (Function) procedures have parameters, which correspond to their local variables. Formal parameters of a procedure declare these local variables, while actual parameters initiate them by values given on a procedure call. 2 1

2 Passing parameters requires knowing which values to use? When calling a procedure, it needs to know which values to use for replacing a formal by an actual parameter. There are different kinds of method how to replace a formal by an actual parameter, for example we will discuss Call-by-value Call-by-reference Procedure: a block of instructions / program statements Procedure call: starts execution of a procedure Caller: the one who starts execution of procedure Callee: is the called procedure PROCEDURE Example(x:integer, y:integer) y = y + 1; x = x * 2; DECLARE a,b:integer; ENDPROCEDURE Example(...):void; START a = 7; b = 4; CALL Example with a and a CALL Example with b and b END 3 Call-by-value (1/2) In call-by-value, a local copy of the passed variables is made and passed into the (function) procedure. The caller's copy cannot be modified. If the (function) procedure is able to assign values to its parameters, only its local copy is assigned that is, anything passed into a call is unchanged in the caller's scope when the (function) procedure returns. Data structures such as arrays are copied too, which can be undesired especially if a huge amount of data must be coped with. 4 2

3 Call-by-value (2/2) PROCEDURE Example(x:integer, y:integer) x = x * 2; y = y + 1; ENDPROCEDURE Variable Value of variable a b x y DECLARE a,b:integer; Example(...):void; START a = 7; b = 4; CALL Example with a and a CALL Example with b and b END END 5 Call-by-reference (1/2) In Call-by-reference, a (function) procedure receives an implicit reference to a variable (its memory address) used as argument, rather than a copy of its value. This typically means that the function can modify the variable used as argument (the caller's copy). Changes remain after end of procedure. Call-by-reference can therefore be used to provide an additional channel of communication between the caller and the callee, which may introduce subtle bugs. 6 3

4 Call by reference (2/2) PROCEDURE Example(x:AdrOf(integer), y:adrof(integer)) (ContentOfAdrIn)x = (ContentOfAdrIn)x * 2; (ContentOfAdrIn)y = (ContentOfAdrIn)y + 1; ENDPROCEDURE Variable Value of variable DECLARE a,b:integer; Example(...):void; START a = 7; b = 4; CALL Example with adr a and adr a CALL Example with adr b and adr b END a b x adr a adr a adr b adr b y adr a adr a adr b adr b 7 Pointers A pointer is a variable that stores a reference to a variable. Its value is the address or location of a variable. In C, a memory address is called a pointer. DECLARE b:integer; C lets you access memory locations directly. a:(integer)pointer; b = 4; A pointer is a data type, a = (AdrOf)b; which is usually 2, 4, or 8 bytes, depending upon machine architecture. Variable Value of variable Comments a adr b a is a pointer variable to an integer value at address of b b 4 b is an integer variable 8 4

5 Benefits of using of pointers Pointers are very flexible in their use. Sometimes pointers are the only way to express a computation. Pointers usually lead to more efficient code. Pointers allow for changing g values passed by a (function) procedure call. Dynamic memory management is based on using pointers. 9 Declaration of pointers (1/4) Just like variables, pointers must be declared before using them. The keyword at the beginning (int, char and so on) declares the type of variable that the pointer will point to. An asterisk (*) is placed before the variable name. Read pointer declarations from right to left. type *s; a pointer to an object of type type int *p; a pointer to an integer double *q; a pointer to a double char **r; a pointer to a pointer to a char 10 5

6 Declaration of pointers (2/4) const int *p; p is a pointer to an integer constant. i.e., pointer can change, thing it points to cannot. Variable Value of variable Comments p adr k (or) adr m p is a pointer variable to addresses of const int k 4 k is a constant integer variable of value 4 m 9 m is a constant integer variable of value 9 11 Declaration of pointers (3/4) int * const q; q is a constant pointer to an integer variable. i.e., pointer cannot change, thing it points to can! Variable Value of variable Comments q adr v q is a constant pointer variable to address v v 4 (or) 9 v is an integer variable 12 6

7 Declaration of pointers (4/4) const int * const r; r is a constant pointer to an integer constant Variable Value of variable Comments r adr z adr z q is a constant pointer variable to address z z z is a constant t integer variable ab of value Addresses pointed to a reference The ampersand (&) operator gives the address of a variable. integer a = 123; a is a variable of type integer, whose value is 123 integer *x; x is declared as a pointer variable to an integer x = &a; x is assigned with the address of variable a, x references or points to variable a Variable Value of variable Comments a a is a variable of type integer x - adr a x is a pointer variable, which refers or points to variable a 14 7

8 Dereference The asterisk (*) operator allows to access the value of a variable, which is referenced by pointer. This is known as to dereference a pointer. integer a = 123; integer variable a is assigned with the value 123 integer *y; pointer variable y is declared for referencing variables of type integer y =&a; the value of pointer variable y is assigned with the address of variable a integer b = *y; Dereferencing pointer variable y is used to assign the value of y s pointee (which is variable a) to variable b *y = 666; Dereferencing pointer variable y is used to assigne the value 666 to its pointee a. Variable Value of variable y adr a adr a adr a a b Dereference (2/2) The "dereference" operation follows a pointer's reference to get the value of its pointee. The only restriction is that the pointer must have a pointee for the dereference to access. Almost all bugs in pointer code involve violating that one restriction. 16 8

9 The NULL Pointer is a special pointer value. encodes the idea of "points to nothing. in a graphical representation it is usually drawn as a diagonal line between the corners of the pointer variable's box. Example: int *numptr = NULL; The C language uses the symbol NULL for this purpose. In C, NULL is equal to the integer constant 0. Deferencing a NULL pointer leads to an exception or unpredictable behavior of the program. 17 Pointer operations Dereference: asterisk (*) operator enables access to value of pointed variable (pointee). Increment/Decrement: ++/-- operator on pointer changes adress to subsequent or preceding variable in memory Address Assignment: to pointers should be done with care. References to invalid addresses should be avoided to prevent unpredictable program behavior. Compare: with other pointers or NULL. 18 9

10 Dangling Pointers when points to released storage (1/2) void main(void){ integer x = 88; integer *y = NULL; ExampleFunction(y); void ExampleFunction(int *ptrtox){ integer localx = 42;... ptrtox = &localx; } x = *y;... Variable Value of variable } x ??? y NULL NULL NULL adr localx adr localx ptrtox adr y adr y adr localx localx Dangling Pointers when points to released storage (2/2) ExampleFunction(...) redirects pointer to an address of a local variable: ptrtox = &localx; After leaving ExampleFunction(...) the pointer is used again: x = *y; Problem: local variable localx does not exist anymore after leaving ExampleFunction(...). Its storage has been released. This is named 'dangling pointer. If storage is reused again is left to chance. The program has an error, but it still can work (sometimes). Double check your damn pointers! 20 10

11 Pointers and Memory Data program ti.uni due.de Prof. Dr. Ing. Axel Hunger Procedural Programming Overview Data Types Data Types Simple / Scalar Access / Pointer Structured / / Primitive / Dynamic Composite coming next ordinal real array record/struct set/union file integer, bool, char text/string Enumerations 22 11

12 END of 4th Lecture 12