Operating Systems 2INC0 C course Pointer Advanced. Dr. Ir. Ion Barosan

Transcription

1 Operating Systems 2INC0 C course Pointer Advanced Dr. Ir. Ion Barosan (i.barosan@tue.nl)

2 Containt Pointers Definition and Initilization Ponter Operators Pointer Arithmetic and Array Calling Functions by Reference Pointers to Pointers Returning A Pointer Pointers and const void Pointer Function Pointers Structure and Pointer to Structures Pointer to File / Faculteit Wiskunde en Informatica PAGE 1

3 Pointers Definition and Initilization Pointer variables Contain memory addresses as their values Normal variables contain a specific value (direct reference) int a = 10; Pointers contain address of a variable that has a specific value (indirect reference) Indirection referencing a pointer value int a = 10; int *ptr = &a ; ptr a 10 / Faculteit Wiskunde en Informatica PAGE 2

4 Pointers Definition and Initilization Pointer declarations * used with pointer variables int *myptr; Declares a pointer to an int (pointer of type int *) Multiple pointers require using a * before each variable declaration int *myptr1, *myptr2; Can declare pointers to any data type Initialize pointers to 0, NULL, or an address 0 or NULL points to nothing (NULL preferred) / Faculteit Wiskunde en Informatica PAGE 3

5 Pointer Operators & (address operator) Returns address of operand int y = 5; int *yptr; yptr = &y; // yptr gets address of y // yptr points to y yptr y 5 Address 0x7A120 yptr ox927c0 Address y ox927c0 5 Address of y is value of yptr / Faculteit Wiskunde en Informatica PAGE 4

6 Ponter Operators * (indirection/dereferencing operator) Returns a synonym/alias of what its operand points to *yptr returns y (because yptr points to y) * can be used for assignment Returns alias to an object *yptr = 7; // changes y to 7 Dereferenced pointer (operand of *) must be an lvalue (no constants) * and & are inverses They cancel each other out / Faculteit Wiskunde en Informatica PAGE 5

7 1 /* 2 Using the & and * operators */ 3 #include <stdio.h> 4 5 int main() 6 { 7 int a; /* a is an integer */ 8 int *aptr; /* aptr is a pointer to an integer */ 9 10 a = 7; 11 aptr = &a; /* aptr set to address of a */ printf( "The address of a is %p" 14 "\nthe value of aptr is %p", &a, aptr ); printf( "\n\nthe value of a is %d" 17 "\nthe value of *aptr is %d", a, *aptr ); printf( "\n\nshowing that * and & are inverses of " 20 "each other.\n&*aptr = %p" 21 "\n*&aptr = %p\n", &*aptr, *&aptr ); return 0; 24 } The address of a is 0012FF88 The value of aptr is 0012FF88 The value of a is 7 The value of *aptr is 7 Proving that * and & are complements of each other. &*aptr = 0012FF88 / Faculteit Wiskunde en Informatica *&aptr = 0012FF PAGE 6

8 More Pointers Example / Faculteit Wiskunde en Informatica PAGE 7

9 Pointers to Pointers b 002DF728 a 002DF71C 002DF728 x 7 002DF71C / Faculteit Wiskunde en Informatica PAGE 8

10 Pointer Arithmetic and Array float a[4]; float *ptr; ptr = &(a[2]); *ptr = 3.14; ptr++; *ptr = 9.0; ptr = ptr - 3; *ptr = 6.0; ptr += 2; Type of a is float * a[2] *(a + 2) ptr = &(a[2]) ptr = &(*(a + 2)) ptr = a + 2 a is a memory address constant ptr is a pointer variable *ptr = 7.0; / Faculteit Wiskunde en Informatica PAGE 9

11 Calling Functions by Reference Call by reference with pointer arguments Pass address of argument using & operator Allows you to change actual location in memory Arrays are not passed with & because the array name is already a pointer * operator Used as alias/nickname for variable inside of function void double( int *number ) { *number = 2 * ( *number ); } // *number used as nickname for the variable passed / Faculteit Wiskunde en Informatica PAGE 10

12 Calling Functions by Reference Modify behaviour in argument passing void f(int j) { } j = 5; void g() { } int i = 3; f(i); void f(int *ptr) { } *ptr = 5; void g() { int i = 3; f(&i); } i = 3 i = 5 / Faculteit Wiskunde en Informatica PAGE 11

13 Calling Functions by Reference / Faculteit Wiskunde en Informatica PAGE 12

14 Calling Functions by Reference Pointers are also used in C to enable a function to modify a variable held by the caller: / Faculteit Wiskunde en Informatica PAGE 13

15 Calling Functions by Reference Copy Example / Faculteit Wiskunde en Informatica PAGE 14

16 Pointers to Pointers A pointer can point to a pointer. One use of this is to pass a pointer so that a function can modify it: / Faculteit Wiskunde en Informatica PAGE 15

17 Returning a Pointer (Good) Pointers can also be used as return values: / Faculteit Wiskunde en Informatica PAGE 16

18 Returning a Pointer (Bad) NEVER return a pointer to an automatic local object: gcc compiler warning: function returns address of local variable / Faculteit Wiskunde en Informatica PAGE 17

19 Pointers and const / Faculteit Wiskunde en Informatica PAGE 18

20 Pointers and const This provides safety against inadvertent changes to a pointer and/or its target, and is certainly an underused feature in C. / Faculteit Wiskunde en Informatica PAGE 19

21 void Pointers In C, a pointer may be declared of type void: void* p; // target can be of ANY type; //no compile-time.type-checking occurs void pointers are not useful in many situations: the return value from malloc() is actually a void* they can be used to achieve generic programming, often with data structures, but also with a number of useful functions: void *memcpy(void *str1, const void *str2, size_t n) / Faculteit Wiskunde en Informatica PAGE 20

22 void Pointers - Example / Faculteit Wiskunde en Informatica PAGE 21

23 void Pointer A void pointer can be really useful if the programmer is not sure about the data type of data inputted by the end user. The programmer can use a void pointer to point to the location of the unknown data type / Faculteit Wiskunde en Informatica PAGE 22

24 void Pointer Pointer arithmetic can not be performed in a void pointer. ERROR / Faculteit Wiskunde en Informatica PAGE 23

25 Function Pointers Function Pointers are pointers - variables, which point to the address of a function. Running program gets a certain space in the mainmemory: Thus a function in the program code is nothing else than an address. It is only important how you, or better your compiler/processor, interpret the memory a pointer points to. / Faculteit Wiskunde en Informatica PAGE 24

26 Function Pointer int (*POINTER_NAME)(int a, int b); / Faculteit Wiskunde en Informatica PAGE 25

27 C structures : Overview A struct is a data structure composed for simpler data types. Like a class in Java/C++ but without methods or inheritance. struct point { int x; int y; }; void PrintPoint(point p) { printf( (%d,%d), p.x, p.y); } / Faculteit Wiskunde en Informatica PAGE 26

28 C structures: Pointers to them The C arrow operator (->) dereferences and extracts a structure field with a single operator. The following are equivalent: struct point *p; printf( x is %d\n, (*p).x); printf( x is %d\n, p->x); / Faculteit Wiskunde en Informatica PAGE 27

29 How big are structs? Recall C operator sizeof() which gives size in bytes (of type or variable) How big is sizeof(struct p)? struct p { char x; int y; }; 5 bytes? 8 bytes? Compiler may word align integer y / Faculteit Wiskunde en Informatica PAGE 28

30 Example Pointer To Structures / Faculteit Wiskunde en Informatica PAGE 29

31 Structure Declarations struct tag { member-list } variable-list; struct P { typedef struct { int a; int a; char b; char b; float c; float c; double y; double y; } Z[20], *ptrp; } Simple; Simple x; Simple y[20], *ptr; / Faculteit Wiskunde en Informatica PAGE 30

32 Example / Faculteit Wiskunde en Informatica PAGE 31

33 Structure Members struct COMPLEX { float f; int a[20]; long *lp; struct Simple s; struct Simple sa[10]; struct Simple *sp; }; Struct COMPLEX comp; ((comp.sa)[4]).c; comp.sa[4].c; // equivalent / Faculteit Wiskunde en Informatica PAGE 32

34 Structure Declarations struct REF1 { struct REF1{ int a; int a; struct REF1 b; struct REF1 *b; float c; float c; double y; double y; }; // ERROR }; The compiler knows the size of the *b pointer, even before the size of the structure has be determined The pointer will be pointing to a different structure of the same type / Faculteit Wiskunde en Informatica PAGE 33

35 Structure Declarations Structure that are mutually dependent: struct B; struct A { struct B{ char d; int a; double c; float f[10]; struct B b ; struct A *pb; }; }; The solution to this problem is the incomplete declaration struct B, which declares an identifier to be a structure tag. / Faculteit Wiskunde en Informatica PAGE 34

36 Pointer To Files / Faculteit Wiskunde en Informatica PAGE 35

37 Advice and Precaution Pros Efficiency Convenience Cons Error-prone Difficult to debug 36

38 Summary A pointer stores the address (memory location) of another entity Address-of operator (&) gets the address of an entity De-reference operator (*) makes a reference to the referee of a pointer Pointer and array Pointer arithmetic 37