Pointers, Pointers, Pointers
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1 Pointers, Pointers, Pointers CSE 333 Summer 2018 Instructor: Hal Perkins Teaching Assistants: Renshu Gu William Kim Soumya Vasisht
2 Administrivia v Exercise 2 out today and due Monday morning v Exercise grading We will do our best to keep up (some delays while we learn how to work with Gradescope) Things to watch for: Input sanity check No funcponal abstracpon (single blob of code) FormaRng funnies (e.g. tabs instead of spaces) 2
3 Administrivia v Homework 0 due Monday, 6/25 LogisPcs and infrastructure for projects clint and valgrind will be useful for exercises, too Should have set up an ssh key and cloned GitLab repo by now Do this ASAP so we have Pme to fix things if necessary v Homework 1 out today, due in 2 weeks (Thu 7/5) Linked list and hash table implementapons in C Get starter code using git pull in your course repo See CSE 333 Git Tutorial for Pps Might have merge conflict if your local copy has unpushed changes If git drops you into vi(m), :q to quit or :wq if you want to save changes 3
4 Administrivia v DocumentaPon: man pages, books Reference websites: cplusplus.org, man7.org, gcc.gnu.org, etc. v Folklore: Google-ing, stackoverflow, that rando in lab v Tradeoffs? RelaPve strengths & weaknesses? Discuss 4
5 Lecture Outline v Pointers & Pointer Arithme>c v Pointers as Parameters v Pointers and Arrays v FuncPon Pointers 5
6 Box-and-Arrow Diagrams int x = 1; int arr[3] = {2, 3, 4; int* p = &arr[1]; boxarrow.c printf("&x: %p; x: %d\n", &x, x); printf("&arr[0]: %p; arr[0]: %d\n", &arr[0], arr[0]); printf("&arr[2]: %p; arr[2]: %d\n", &arr[2], arr[2]); printf("&p: %p; p: %p; *p: %d\n", &p, p, *p); return 0; address name value 6
7 Box-and-Arrow Diagrams int x = 1; int arr[3] = {2, 3, 4; int* p = &arr[1]; boxarrow.c printf("&x: %p; x: %d\n", &x, x); printf("&arr[0]: %p; arr[0]: %d\n", &arr[0], arr[0]); printf("&arr[2]: %p; arr[2]: %d\n", &arr[2], arr[2]); printf("&p: %p; p: %p; *p: %d\n", &p, p, *p); return 0; address name value &arr[2] arr[2] value &arr[1] arr[1] value &arr[0] arr[0] value &p p value &x x value stack frame for main() 7
8 Box-and-Arrow Diagrams int x = 1; int arr[3] = {2, 3, 4; int* p = &arr[1]; boxarrow.c printf("&x: %p; x: %d\n", &x, x); printf("&arr[0]: %p; arr[0]: %d\n", &arr[0], arr[0]); printf("&arr[2]: %p; arr[2]: %d\n", &arr[2], arr[2]); printf("&p: %p; p: %p; *p: %d\n", &p, p, *p); return 0; address name value &arr[2] arr[2] 4 &arr[1] arr[1] 3 &arr[0] arr[0] 2 &p p &arr[1] &x x 1 8
9 Box-and-Arrow Diagrams int x = 1; int arr[3] = {2, 3, 4; int* p = &arr[1]; boxarrow.c printf("&x: %p; x: %d\n", &x, x); printf("&arr[0]: %p; arr[0]: %d\n", &arr[0], arr[0]); printf("&arr[2]: %p; arr[2]: %d\n", &arr[2], arr[2]); printf("&p: %p; p: %p; *p: %d\n", &p, p, *p); return 0; address name value 0x7fff 78 arr[2] 4 0x7fff 74 arr[1] 3 0x7fff 70 arr[0] 2 0x7fff 68 p 0x7fff 74 0x7fff 64 x 1 9
10 Pointer Arithme>c v Pointers are typed Tells the compiler the size of the data you are poinpng to ExcepPon: void* is a generic pointer (i.e. a placeholder) v Pointer arithmepc is scaled by sizeof(*p) Works nicely for arrays Does not work on void*, since void doesn t have a size! v Valid pointer arithmepc: Add/subtract an integer to a pointer Subtract two pointers (within stack frame or malloc block) Compare pointers (<, <=, ==,!=, >, >=), including NULL 10
11 Prac>ce Ques>on int arr[3] = {2, 3, 4; int* p = &arr[1]; int** dp = &p; // pointer to a pointer boxarrow2.c *(*dp) += 1; p += 1; *(*dp) += 1; return 0; address name value At this point in the code, what values are stored in arr[]? 0x7fff 78 arr[2] 4 0x7fff 74 arr[1] 3 0x7fff 70 arr[0] 2 0x7fff 68 p 0x7fff 74 0x7fff 60 dp 0x7fff 68 11
12 Prac>ce Solu>on L03: Pointers int arr[3] = {2, 3, 4; int* p = &arr[1]; int** dp = &p; // pointer to a pointer Note: arrow points to next instrucpon to be executed. boxarrow2.c *(*dp) += 1; p += 1; *(*dp) += 1; return 0; address name value 0x7fff 78 arr[2] 4 0x7fff 74 arr[1] 3 0x7fff 70 arr[0] 2 4 0x7fff 68 p 0x7fff 74 0x7fff 60 dp 0x7fff 68 12
13 Prac>ce Solu>on L03: Pointers int arr[3] = {2, 3, 4; int* p = &arr[1]; int** dp = &p; // pointer to a pointer Note: arrow points to next instrucpon to be executed. boxarrow2.c *(*dp) += 1; p += 1; *(*dp) += 1; return 0; address name value 0x7fff 78 arr[2] 4 0x7fff 74 arr[1] 4 0x7fff 70 arr[0] 2 0x7fff 68 p 0x7fff 74 0x7fff 60 dp 0x7fff 68 13
14 Prac>ce Solu>on L03: Pointers int arr[3] = {2, 3, 4; int* p = &arr[1]; int** dp = &p; // pointer to a pointer Note: arrow points to next instrucpon to be executed. boxarrow2.c *(*dp) += 1; p += 1; *(*dp) += 1; return 0; address name value 0x7fff 78 arr[2] 4 0x7fff 74 arr[1] 4 0x7fff 70 arr[0] 2 0x7fff 68 p 0x7fff 78 0x7fff 60 dp 0x7fff 68 14
15 Prac>ce Solu>on L03: Pointers int arr[3] = {2, 3, 4; int* p = &arr[1]; int** dp = &p; // pointer to a pointer Note: arrow points to next instrucpon to be executed. boxarrow2.c *(*dp) += 1; p += 1; *(*dp) += 1; return 0; address name value 0x7fff 78 arr[2] 4 0x7fff 74 arr[1] 4 0x7fff 70 arr[0] 2 5 0x7fff 68 p 0x7fff 78 0x7fff 60 dp 0x7fff 68 15
16 Endianness v Memory is byte-addressed, so endianness determines what ordering that mulp-byte data gets read and stored in memory Big-endian: Least significant byte has highest address Lijle-endian: Least significant byte has lowest address v Example: 4-byte data 0xa1b2c3d4 at address 0x100 Big-Endian Lijle-Endian 0x100 0x101 0x102 0x103 a1 b2 c3 d4 0x100 0x101 0x102 0x103 d4 c3 b2 a1 16
17 Pointer Arithme>c Example int arr[3] = {1, 2, 3; int* int_ptr = &arr[0]; char* char_ptr = (char*) int_ptr; Note: Arrow points to next instrucpon. Stack (assume x86-64) int_ptr += 1; int_ptr += 2; // uh oh char_ptr += 1; char_ptr += 2; return 0; pointerarithmepc.c arr[2] arr[1] arr[0] char_ptr int_ptr 17
18 Pointer Arithme>c Example int arr[3] = {1, 2, 3; int* int_ptr = &arr[0]; char* char_ptr = (char*) int_ptr; Note: Arrow points to next instrucpon. Stack (assume x86-64) int_ptr += 1; int_ptr += 2; // uh oh char_ptr += 1; char_ptr += 2; return 0; pointerarithmepc.c arr[2] arr[1] arr[0] char_ptr int_ptr 18
19 Pointer Arithme>c Example int arr[3] = {1, 2, 3; int* int_ptr = &arr[0]; char* char_ptr = (char*) int_ptr; Note: Arrow points to next instrucpon. Stack (assume x86-64) int_ptr += 1; int_ptr += 2; // uh oh char_ptr += 1; char_ptr += 2; return 0; pointerarithmepc.c arr[2] arr[1] arr[0] char_ptr int_ptr 19
20 Pointer Arithme>c Example int arr[3] = {1, 2, 3; int* int_ptr = &arr[0]; char* char_ptr = (char*) int_ptr; Note: Arrow points to next instrucpon. Stack (assume x86-64) int_ptr += 1; int_ptr += 2; // uh oh char_ptr += 1; char_ptr += 2; return 0; pointerarithmepc.c arr[2] arr[1] arr[0] char_ptr int_ptr 20
21 Pointer Arithme>c Example int arr[3] = {1, 2, 3; int* int_ptr = &arr[0]; char* char_ptr = (char*) int_ptr; Note: Arrow points to next instrucpon. Stack (assume x86-64) int_ptr += 1; int_ptr += 2; // uh oh char_ptr += 1; char_ptr += 2; return 0; pointerarithmepc.c int_ptr: 0x0x7ffffffde010 *int_ptr: 1 arr[2] arr[1] arr[0] char_ptr int_ptr 21
22 Pointer Arithme>c Example int arr[3] = {1, 2, 3; int* int_ptr = &arr[0]; char* char_ptr = (char*) int_ptr; Note: Arrow points to next instrucpon. Stack (assume x86-64) int_ptr += 1; int_ptr += 2; // uh oh char_ptr += 1; char_ptr += 2; return 0; pointerarithmepc.c int_ptr: 0x0x7ffffffde014 *int_ptr: 2 arr[2] arr[1] arr[0] char_ptr int_ptr 22
23 Pointer Arithme>c Example int arr[3] = {1, 2, 3; int* int_ptr = &arr[0]; char* char_ptr = (char*) int_ptr; Note: Arrow points to next instrucpon. Stack (assume x86-64) int_ptr += 1; int_ptr += 2; // uh oh char_ptr += 1; char_ptr += 2; return 0; pointerarithmepc.c int_ptr: 0x0x7ffffffde01C *int_ptr:??? arr[2] arr[1] arr[0] char_ptr int_ptr 23
24 Pointer Arithme>c Example int arr[3] = {1, 2, 3; int* int_ptr = &arr[0]; char* char_ptr = (char*) int_ptr; Note: Arrow points to next instrucpon. Stack (assume x86-64) int_ptr += 1; int_ptr += 2; // uh oh char_ptr += 1; char_ptr += 2; return 0; pointerarithmepc.c char_ptr: 0x0x7ffffffde010 *char_ptr: 1 arr[2] arr[1] arr[0] char_ptr int_ptr 24
25 Pointer Arithme>c Example int arr[3] = {1, 2, 3; int* int_ptr = &arr[0]; char* char_ptr = (char*) int_ptr; Note: Arrow points to next instrucpon. Stack (assume x86-64) int_ptr += 1; int_ptr += 2; // uh oh char_ptr += 1; char_ptr += 2; return 0; pointerarithmepc.c char_ptr: 0x0x7ffffffde011 *char_ptr: 0 arr[2] arr[1] arr[0] char_ptr int_ptr 25
26 Pointer Arithme>c Example int arr[3] = {1, 2, 3; int* int_ptr = &arr[0]; char* char_ptr = (char*) int_ptr; Note: Arrow points to next instrucpon. Stack (assume x86-64) int_ptr += 1; int_ptr += 2; // uh oh char_ptr += 1; char_ptr += 2; return 0; pointerarithmepc.c char_ptr: 0x0x7ffffffde013 *char_ptr: 0 arr[2] arr[1] arr[0] char_ptr int_ptr 26
27 Lecture Outline v Pointers & Pointer ArithmePc v Pointers as Parameters v Pointers and Arrays v FuncPon Pointers 27
28 C is Call-By-Value v C (and Java) pass arguments by value Callee receives a local copy of the argument Register or Stack If the callee modifies a parameter, the caller s copy isn t modified void swap(int a, int b) { int tmp = a; a = b; b = tmp; int a = 42, b = -7; swap(a, b);... 28
29 Broken Swap Note: Arrow points to next instrucpon. main OS kernel [protected] Stack brokenswap.c void swap(int a, int b) { int tmp = a; a = b; b = tmp; int a = 42, b = -7; swap(a, b);... Heap Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 29
30 Broken Swap OS kernel [protected] Stack main a 42 b -7 brokenswap.c void swap(int a, int b) { int tmp = a; a = b; b = tmp; int a = 42, b = -7; swap(a, b);... Heap Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 30
31 Broken Swap OS kernel [protected] Stack main a 42 b -7 brokenswap.c void swap(int a, int b) { int tmp = a; a = b; b = tmp; int a = 42, b = -7; swap(a, b);... swap a 42 b -7 tmp?? Heap Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 31
32 Broken Swap OS kernel [protected] Stack main a 42 b -7 brokenswap.c void swap(int a, int b) { int tmp = a; a = b; b = tmp; int a = 42, b = -7; swap(a, b);... swap a 42 b -7 tmp 42 Heap Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 32
33 Broken Swap OS kernel [protected] Stack main a 42 b -7 brokenswap.c void swap(int a, int b) { int tmp = a; a = b; b = tmp; int a = 42, b = -7; swap(a, b);... swap a -7 b -7 tmp 42 Heap Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 33
34 Broken Swap OS kernel [protected] Stack main a 42 b -7 brokenswap.c void swap(int a, int b) { int tmp = a; a = b; b = tmp; int a = 42, b = -7; swap(a, b);... swap a -7 b 42 tmp 42 Heap Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 34
35 Broken Swap OS kernel [protected] Stack main a 42 b -7 brokenswap.c void swap(int a, int b) { int tmp = a; a = b; b = tmp; int a = 42, b = -7; swap(a, b);... Heap Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 35
36 Faking Call-By-Reference in C v Can use pointers to approximate call-by-reference Callee spll receives a copy of the pointer (i.e. call-by-value), but it can modify something in the caller s scope by dereferencing the pointer parameter void swap(int* a, a, int* b) b) { { int tmp = *a; *a a = = b; *b; *b b = = tmp; int a = 42, b = -7; swap(a, swap(&a, &b);... 36
37 Fixed Swap Note: Arrow points to next instrucpon. OS kernel [protected] Stack main a 42 b -7 swap.c void swap(int* a, int* b) { int tmp = *a; *a = *b; *b = tmp; int a = 42, b = -7; swap(&a, &b);... Heap Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 37
38 Fixed Swap OS kernel [protected] Stack main a 42 b -7 swap.c void swap(int* a, int* b) { int tmp = *a; *a = *b; *b = tmp; int a = 42, b = -7; swap(&a, &b);... swap a Heap b tmp?? Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 38
39 Fixed Swap OS kernel [protected] Stack main a 42 b -7 swap.c void swap(int* a, int* b) { int tmp = *a; *a = *b; *b = tmp; int a = 42, b = -7; swap(&a, &b);... swap a Heap b tmp 42 Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 39
40 Fixed Swap OS kernel [protected] Stack main a -7 b -7 swap.c void swap(int* a, int* b) { int tmp = *a; *a = *b; *b = tmp; int a = 42, b = -7; swap(&a, &b);... swap a Heap b tmp 42 Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 40
41 Fixed Swap OS kernel [protected] Stack main a -7 b 42 swap.c void swap(int* a, int* b) { int tmp = *a; *a = *b; *b = tmp; int a = 42, b = -7; swap(&a, &b);... swap a Heap b tmp 42 Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 41
42 Fixed Swap OS kernel [protected] Stack main a -7 b 42 swap.c void swap(int* a, int* b) { int tmp = *a; *a = *b; *b = tmp; int a = 42, b = -7; swap(&a, &b);... Heap Read/Write Segment.data,.bss Read-Only Segment.text,.rodata 42
43 Lecture Outline v Pointers & Pointer ArithmePc v Pointers as Parameters v Pointers and Arrays v FuncPon Pointers 43
44 Pointers and Arrays v A pointer can point to an array element You can use array indexing notapon on pointers ptr[i] is *(ptr+i) with pointer arithmepc reference the data i elements forward from ptr An array name s value is the beginning address of the array Like a pointer to the first element of array, but can t change int a[] = {10, 20, 30, 40, 50; int* p1 = &a[3]; // refers to a's 4th element int* p2 = &a[0]; // refers to a's 1st element int* p3 = a; // refers to a's 1st element *p1 = 100; *p2 = 200; p1[1] = 300; p2[1] = 400; p3[2] = 500; // final: 200, 400, 500, 100,
45 Array Parameters v Array parameters are actually passed as pointers to the first array element The [] syntax for parameter types is just for convenience OK to use whichever best helps the reader This code: void f(int a[]); int main(... ) { int a[5];... f(a); return 0; void f(int a[]) { Equivalent to: void f(int* a); int main(... ) { int a[5];... f(&a[0]); return 0; void f(int* a) { 45
46 Lecture Outline v Pointers & Pointer ArithmePc v Pointers as Parameters v Pointers and Arrays v Func>on Pointers 46
47 Func>on Pointers v Based on what you know about assembly, what is a funcpon name, really? Can use pointers that store addresses of funcpons! v Generic format: returntype (* name)(type1,, typen) Looks like a funcpon prototype with extra * in front of name Why are parentheses around (* name) needed? v Using the funcpon: (*name)(arg1,, argn) Calls the pointed-to funcpon with the given arguments and return the return value 47
48 Func>on Pointer Example v map() performs operapon on each element of an array #define LEN 4 int negate(int num) {return -num; int square(int num) {return num*num; funcptr parameter // perform operation pointed to on each array element void map(int a[], int len, int (* op)(int n)) { for (int i = 0; i < len; i++) { a[i] = (*op)(a[i]); // dereference function pointer funcptr dereference int arr[len] = {-1, 0, 1, 2; funcptr definipon int (* op)(int n); // function pointer called 'op' op = square; // function name returns addr (like array) map(arr, LEN, op); funcptr assignment... map.c 48
49 Extra Exercise #1 v Use a box-and-arrow diagram for the following program and explain what it prints out: #include <stdio.h> int foo(int* bar, int** baz) { *bar = 5; *(bar+1) = 6; *baz = bar + 2; return *((*baz)+1); int arr[4] = {1, 2, 3, 4; int* ptr; arr[0] = foo(&arr[0], &ptr); printf("%d %d %d %d %d\n", arr[0], arr[1], arr[2], arr[3], *ptr); return 0; 49
50 Extra Exercise #2 v Write a program that determines and prints out whether the computer it is running on is lijle-endian or big-endian. Hint: pointerarithmetic.c from today s lecture or show_bytes.c from
51 Extra Exercise #3 v Write a funcpon that: Arguments: [1] an array of ints and [2] an array length Malloc s an int* array of the same element length IniPalizes each element of the newly-allocated array to point to the corresponding element of the passed-in array Returns a pointer to the newly-allocated array 51
52 Extra Exercise #4 v Write a funcpon that: Accepts a funcpon pointer and an integer as arguments Invokes the pointed-to funcpon with the integer as its argument 52
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