C Pointers. 6th April 2017 Giulio Picierro

Transcription

1 C Pointers 6th April 07 Giulio Picierro

2 Functions Return type Function name Arguments list Function body int sum(int a, int b) { return a + b; } Return statement (return keyword + value)

3 Functions Function argument pass By value void my_function(int a) { a++; } int main(void) { int b = 0; my_function(b); printf("b=%d\n", b); } A copy of variable is made. Modifying a variable inside a function, DOES NOT modify variable outside.

4 Functions Function argument pass By reference No copy is made: a reference is passed Modifying a variable inside called function, MODIFY variable outside C doesn t support this, but you can pass pointers!

5 Data type that represents a memory address Typically pointers to variables, i.e. address of variables. Declaration: or: T* var_name; T *var_name; How to read it: Pointer to T

6 Data type that represents a memory address You can retrieve address of a variable using the & operator Memory 48 int num = 5; int *num_addr = # num num_addr

7 Dereference You can retrieve pointed value using the * operator Memory int a = *num_addr; a Here, a is a new variable which holds a copy of num. This means that address of a is different from address of num num_addr num

8 Dereference You can change pointed value using the * operator Memory *num_addr = 3; 5 3 num_addr num

9 Functions Function argument pass By address : Address of variable passed by value void my_function(int *a) { (*a)++; } int main(void) { int b = 0; my_function(&b); printf("b=%d\n", b); } Increase value pointed by a (dereference) Passing memory address of b.. Passing by reference doesn t exists in C

10 NULL A pointer with address value of 0 is considered NULL pointer Special value => Point to nothing => Invalid pointer Memory num int *num_addr = NULL; num_addr

11 Dereference Dereference a NULL pointer is an error! Memory int *num_addr = NULL; 48 int a = *num_addr; // the program will crash num 60 num_addr

12 Arithmetic Different from integer arithmetic int num = 5; Memory num

13 Arithmetic Different from integer arithmetic int num = 5; num++; num now is 6 Memory num

14 Arithmetic Different from integer arithmetic int num = 5; num++; Pointer arithmetic int *num_addr = # 48 5 Memory 6 num num_addr

15 Arithmetic Different from integer arithmetic int num = 5; num++; Pointer arithmetic int *num_addr = # num_addr++; Memory num 60 Pointer is increased by 4!!! num_addr

16 Arithmetic Given a pointer T* Increase a pointer by one equals to increase by sizeof(t) num_addr Memory

17 Arithmetic Given a pointer T* Increase a pointer by one equals to increase by sizeof(t) char *a = ; short *b = ; double *c = ; a++; // a now is 3 b++; // b now is 4 c++; // c now is 0 num_addr Memory

18 Pointers and Arrays Same values: array &(array[0]) array[i] *(array + i) Different types: array is an int[7] array+i is an int* array array[0] array[] array[] array[3] array[4] array[5] array[6]

19 Pointers and Arrays array int array[] = {,,, 3, 5, 8, 3}; 6 array[0] int* ptr = array; ptr array[] array[] 8 3 array[3] 3 5 array[4] 36 8 array[5] 3 array[6]

20 Pointers and Arrays array int array[] = {,,, 3, 5, 8, 3}; 6 array[0] int* ptr = array; ptr++; ptr array[] array[] 8 3 array[3] 3 5 array[4] 36 8 array[5] 3 array[6] NOTE: Pointers have their arithmetic, increase a pointer T* by one, means increase address by sizeof(t)

21 Pointers and Arrays array int array[] = {,,, 3, 5, 8, 3}; 6 array[0] int* ptr = array; ptr++; ptr 0 4 array[] array[] ptr = array + 6; 8 3 array[3] 3 5 array[4] 36 8 array[5] 3 array[6] NOTE: Pointers have their arithmetic, increase a pointer T* by one, means increase address by sizeof(t)

22 Pointers and Arrays array int array[] = {,,, 3, 5, 8, 3}; 6 array[0] int* ptr = array; ptr++; ptr 0 4 array[] array[] ptr = array + 6; 8 3 array[3] *ptr = ; 3 5 array[4] 36 8 array[5] array[6] NOTE: Pointers have their arithmetic, increase a pointer T* by one, means increase address by sizeof(t)

23 Pointers and Arrays array int array[] = {,,, 3, 5, 8, 3}; 6 array[0] int* ptr = array; ptr++; ptr array[] array[] ptr = array + 6; 8 3 array[3] *ptr = ; 3 5 array[4] ptr = &array[]; 36 8 array[5] array[6] NOTE: Pointers have their arithmetic, increase a pointer T* by one, means increase address by sizeof(t)

24 Pointers and Arrays array int array[] = {,,, 3, 5, 8, 3}; 6 array[0] int* ptr = array; ptr++; ptr array[] array[] ptr = array + 6; 8 3 array[3] *ptr = ; 3 5 array[4] ptr = &array[]; ptr[0] = 7; 36 8 array[5] array[6] NOTE: Pointers have their arithmetic, increase a pointer T* by one, means increase address by sizeof(t)

25 Arrays and functions Arrays are always passed by base address 6 0 array array[0] array[] void function(int v[]) { v[3] = 7; } array[] array[3] array[4] 36 8 array[5] 3 array[6]

26 Arrays and functions Arrays are always passed by base address 6 0 array array[0] array[] void function(int v[]) { v[3] = 7; } array[] array[3] array[4] function(array); 36 8 array[5] You re not copying the array, but the base address!!! Change an element of the array inside the function equals to change the element of the original array!!! 3 array[6]

27 Arrays and functions Arrays are always passed by base address 6 0 array array[0] array[] int* function(void) { int array[] = {,, 3, 4, 5}; } return array; array[] array[3] array[4] array[5] Returning a pointer to a local variable IS AN ERROR! Since local variables lives on the stack, the memory of those variables could be overwritten by subsequent function calls! 3 array[6]

28 Pointers and Const const int* p; *p = 5; // compiler error p = NULL; // ok int* const p; *p = 5; // ok p = NULL; // compiler error () () Read from right to left: ) Pointer to int const ) Const pointer to int 3) Const pointer to int const const int* const p; *p = 5; // compiler error p = NULL; // compiler error (3)