ics212-14-pointers

Transcript

1. ICS 212 Program Structure William McDaniel Albritton M.S.
2. Memory Allocation  Declaring and initializing pointers  Address operator and dereferencing operator  Pointer Arithmetic
4. Declare and Initialize a Pointer
5. Pointers  Pointers are variables that contain a memory address of another variable as its value  What are they used for?  Generic access to several similar variables  Call by reference function parameters  Dynamically allocated data structures  Creating copies of arrays and structures
6. Declare and Initialize a Variable  Every local variable has a name, a value, and an address in memory on the runtime stack name value address number1 0xffbffa4c int number1 = 5; 5
7. Declare and Initialize a Pointer  A pointer local variable also has a name, a value, and an address in memory on the runtime stack name value address number1 0xffbffa4c pointer1 0xffbffa48  NULL means that the pointer does not point to anything int *pointer1 = NULL; 5 NULL
8. Declare and Initialize a Pointer  When we declare a pointer, the asterisk (*) indicates that we are creating a pointer, which is a variable that stores an address to another variable int *pointer1 = NULL;
9. Declare and Initialize a Pointer  When we declare a pointer, the pointer’s type should match the variable’s type which it will point to  So the type for number1 and pointer1 is int int number1 = 5; int *pointer1 = NULL;
11. Address Operator and Dereferencing Operator
12. Assign an Address to a Pointer  The address operator (&) returns the address of number1 (0xffbffa4c), which is stored in pointer1 name value address number1 0xffbffa4c pointer1 0xffbffa48  Usually the “pointing” is represented with an arrow pointer1 = &number1; //0xffbffa4c 5 0xffbffa4c
13. Dereferencing a Pointer  The dereferencing operator (*) returns the value stored at the address that is stored in pointer1 name value address number1 0xffbffa4c pointer1 0xffbffa48  So the output is: 5 printf(“%i”, *pointer1); //5 5 0xffbffa4c
14. Dereferencing a Pointer  The dereferencing operator (*) returns the value which pointer1 points to, so number2 has value 5 name value address number1 0xffbffa4c pointer1 0xffbffa48 number2 0xffbffa44 number2 = *pointer1; //5 5 0xffbffa4c 5
15. Address Stored in the Pointer  We don’t use the dereferencing operator (*) if we want to access the address stored in the pointer name value address number1 0xffbffa4c pointer1 0xffbffa48  So the output is: 0xffbffa4c printf(“0x%p”, pointer1); //0xffbffa4c 5 0xffbffa4c
16. Pointer’s Address on Runtime Stack  We do use the address operator (&) if we want to access the pointer’s address on the runtime stack name value address number1 0xffbffa4c pointer1 0xffbffa48  So the output is: 0xffbffa48 printf(“0x%p”, &pointer1);//0xffbffa48 5 0xffbffa4c
17. Pointer’s Address on Runtime Stack  Similarly, we use the address operator (&) to access the local variable’s address on the runtime stack name value address number1 0xffbffa4c pointer1 0xffbffa48  So the output is: 0xffbffa4c printf(“0x%p”, &number1); //0xffbffa4c 5 0xffbffa4c
18. Local Variable’s Address  Same example as the last slide, using number2  Note that %p is used to display addresses name value address number1 0xffbffa4c pointer1 0xffbffa48 number2 0xffbffa44 printf(“0x%p”, &number2); //0xffbffa44 5 0xffbffa4c 5
19. What Not to Do with Pointers!  Never, ever dereference (*) a NULL pointer  This will crash your program with this error message: Segmentation fault (core dumped) int *pointer1 = NULL; printf(“%i\n”, *pointer1);
20. Segmentation Fault and Core Dump  A segmentation fault happens when your program tries to access memory at an address that it is not supposed to have access to  A core dump is a file that stores the complete state of a process (running program) that has crashed  The term comes from magnetic-core memory, which was used as RAM in computers from 1955 to 1975
21. Pointer Operator Review  Only use two operators for pointers  & operator: address operator  Returns the address of a variable pointer1 = &number1; //0xffbffa4c
22. Pointer Operator Review  * operator: dereferencing (indirection) operator  Returns the value of the object to which the address points to  See example code at: pointers.c number2 = *pointer1; //5
23. Binky Pointer Video  Check out a really cool 3 minute claymation video that also explains how pointers work!  See the link on the class webpage: http://cslibrary.stanford.edu/  From the Stanford CS Education Library
25. Pointers and Arithmetic
26. Valid Pointer Operations  You can:  Assign pointers of the same type to each other  Add or subtract a pointer & an integer  Assign or compare a pointer to zero (NULL) or to an address
27. Invalid Pointer Operations  You cannot:  Assign pointers of different types to each other without a cast  Add a float or double to a pointer  Add two pointers  Multiply, divide, do modulus, or shift a pointer
28. Adding with a Pointer’s Address  With pointers, you can alter the value that the pointer is pointing to by: 1. Dereferencing the pointer 2. Changing the value that the pointer points to
29. Assigning an Address to a Pointer  Runtime stack for each line of executable code address name value 0xffbffa47 letter1 0xffbffa40 pointer1 char letter1 = 'A'; char *pointer1 = NULL; pointer1 = &letter1; ‘A’ 0xffbffa47
30. Adding One with a Pointer  Runtime stack for each line of executable code address name value 0xffbffa47 letter1 0xffbffa40 pointer1 /*unary operators associate right to left, so dereference, then add*/ ++*pointer1; ‘B’ 0xffbffa47
31. Adding One Again with a Pointer  Dereference pointer, then add one address name value 0xffbffa47 letter1 0xffbffa40 pointer1 /*unary operators associate right to left, so have to use parentheses*/ (*pointer1)++; ‘C’ 0xffbffa47
32. You Can Re-assign a Pointer  Now the pointer points to letter2! address name value 0xffbffa47 letter1 0xffbffa40 pointer1 0xffbffa3f letter2 char letter2 = 'z'; pointer1 = &letter1; pointer1 = &letter2; ‘C’ 0xffbffa3f ‘z’
33. Dereferenced Pointer  Just like any other variable, you can do as you wish with a dereferenced pointer address name value 0xffbffa47 letter1 0xffbffa40 pointer1 0xffbffa3f letter2 *pointer1 = ((*pointer1 / 2) * 2) - 2; ‘C’ 0xffbffa3f ‘x’
34. Example Program  See example code at: pointers2.c
35. Adding to a Pointer’s Address  Can also add to (or subtract from) the address that is stored in the pointer variable
36. Assigning an Address to a Pointer  Runtime stack representation of above code address name value 0xffbffa40 x 0xffbffa38 y 0xffbffa30 z 0xffbffa2c pointer1 double x=3.3, y=6.6, z=9.9; double *pointer1=&y; 3.3 6.6 9.9 0xffbffa38
37. Dereferencing a Pointer  Change the value that the pointer points to address name value 0xffbffa40 x 0xffbffa38 y 0xffbffa30 z 0xffbffa2c pointer1 *pointer1 = *pointer1 + 1.1; 3.3 7.7 9.9 0xffbffa38
38. Adding to a Pointer’s Address  Increments by 8 bytes, because points in an double address name value 0xffbffa40 x 0xffbffa38 y 0xffbffa30 z 0xffbffa2c pointer1 pointer1 = pointer1 + 1; 3.3 7.7 9.9 0xffbffa40
39. Dereferencing a Pointer  Change the value of another variable with pointer2 address name value 0xffbffa40 x 0xffbffa38 y 0xffbffa30 z 0xffbffa2c pointer1 *pointer1 = *pointer1 + 1.1; 4.4 7.7 9.9 0xffbffa40
40. Subtracting from a Pointer’s Address  What variable will pointer1 point to? address name value 0xffbffa40 x 0xffbffa38 y 0xffbffa30 z 0xffbffa2c pointer1 pointer1 = pointer1 - 2; 4.4 7.7 9.9 0xffbffa30
41. Example Code  See example code at: pointers3.c
42. Memory Management  Declaring and initializing pointers  Address operator & dereferencing operator  Pointers and Arithmetic