ics212-12-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; //WARNING: Don’t do this! 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; //WARNING: Don’t do this! 4.4 7.7 9.9 0xffbffa30
41. Example Code  See example code at: pointers3.c  WARNING: This is showing what NOT to do. It is okay to add integers to or subtract integers from pointers, if they point to an array. However, it is not okay to change pointer values this way, if the pointer points to a single variable. It might point to the wrong variable, or point to random data.
42. Memory Management  Declaring and initializing pointers  Address operator & dereferencing operator  Pointers and Arithmetic