C: Advanced Topics

C: Advanced Topics Summer 2014 COMP 2130 Intro Computer Systems Computing Science Thompson Rivers University

Course Objectives • The better knowledge of computer systems, the better programing. Data Representation

Course Contents • Introduction to computer systems: B&O 1 • Introduction to C programming: K&R 1 – 4 • Data representations: B&O 2.1 – 2.4 • C: advanced topics: K&R 5.1 – 5.10, 6 – 7 • Introduction to IA32 (Intel Architecture 32): B&O 3.1 – 3.8, 3.13 • Compiling, linking, loading, and executing: B&O 7 (except 7.12) • Dynamic memory management – Heap: B&O 9.9.1 – 9.9.2, 9.9.4 – 9.9.5, 9.11 • Code optimization: B&O 5.1 – 5.6, 5.13 • Memory hierarchy, locality, caching: B&O 5.12, 6.1 – 6.3, 6.4.1 – 6.4.2, 6.5, 6.6.2 – 6.6.3, 6.7 • Virtual memory (if time permits): B&O 9.4 – 9.5 C: Advanced Topics

Unit Learning Objectives • Use pointers and references. • Use pointers for dynamic memory management. • Use open(), read(), write(), close(), and FILE* functions to manipulate files. • Use user-defined data structures. • More coming C: Advanced Topics

Unit Contents • Pointers and Arrays • Input and Output • Structures • Self Referential structures C: Advanced Topics

1. Pointers and Arrays C: Advanced Topics

Pointers and Addresses • A pointer is a variable that contains the address of a variable. • Pointers and arrays are very closely related. int x = 1, y = 2, z[10]; int *ip; /* ip is a pointer to int */ ip = &x; /* ip now points to x */ y = *ip; /* y is now 1 */ *ip = 0; /* x is now 0 */ ip = &z[1]; /* ip now points to z[1] */ • Declaration of a pointer variable: * • Reference operator: & • Indirection operator (oac dereference operator): * C: Advanced Topics

int x = 1; int y = 2; int z[10]; int *ip; ip = &x; y = *ip; *ip = 0; ip = &z[1]; C: Advanced Topics

int x = 1; int y = 2; int z[10]; int *ip; ip = &x; y = *ip; *ip = 0; ip = &z[1]; *ip = 5; C: Advanced Topics

Pointers and Function Arguments • How to write a function that swaps the values stored in two variables? swap(a, b); ... void swap(int x, int y) { int temp; temp = x; x = y; y = temp; } • Is the above function correct? Why? At assembly time, this function call will be expanded to create three variables in the stack area. The stack pointer will be increased as the result. The return statement is hidden. At assembly time, the return statement will be expanded to delete three variables in the stack area. The stack pointer will be decreased as the result. C: Advanced Topics

swap(a, b); ... void swap(int x, int y) { int temp; temp = x; x = y; y = temp; } At this moment C: Advanced Topics

How to write a function that swaps the values stored in two variables? swap(&a, &b); ... void swap(int *px, int* py) { int temp; temp = ???; ???; ??? = temp; } At this moment C: Advanced Topics

swap(&a, &b); ... void swap(int *px, int* py) { int temp; temp = *px; *px = *py; *py = temp; } C: Advanced Topics

int *test; int number = 20; test = &number; printf(“%d, %d, %p, %p\n”, number, *test, test, &test); // the content of the var pointed by test // the content of test *test = 30; printf(“%d, %d, %p, %p\n”, number, *test, test, &test); printf(“Enter an integer: ”); scanf(“%d”, &number); // the address of number printf(“%d, %d, %d, %p, %p, %p\n”, number, *test, *(&number), test, &test, &number); // the content pointed by test // the content pointed // by the address of number // the content of test // the address of test // the address of number C Programming

Can you write a function to read multiple integers into an array? int data[10]; getint(data, 10); // read 10 integers into data[] ... ??? getint(???, ???) { ??? // using scanf() } C: Advanced Topics

Pointers and Arrays • Strong relation between and pointers float *px; // px is ready to store an address. float x[10]; // x represents 10 float type variables, // x[0], x[1], ..., x[9], that are // alocated in cosecutive memory area. // x has the address of x[0]. float y; x[0] = 2; x[1] = 3; x[2] = 4; x[3] = 5; x[4] = 6; x[5] = 7; px = x; // the same data type? y = *px; printf(“%f, %f, %f\n”, x[0], *px, y); // ??? px = &x[2]; y = *(px+2); printf(“%f, %f, %f\n”, x[0], *px, y); // ??? C: Advanced Topics

int number[10]; printf(“%p\n”, &(number[0])); printf(“%p\n”, &(number[5])); printf(“%p\n”, number); // related to reference printf(“%p\n”, number + 5); // the address of number[5] // not 5 * 4 newval(number); ... void newval(int num[]) { num[0] = 5; ... }, or void newval(int* num) { *num = 5; *(num+1) = 10; num[2] = 20; ... } • int num[] and int *num are equivalent. C Programming

int number[10]; int *p, *q; *p = 10; // Is it wrong? p = number; q = &number[0]; number[0] = 2; number[1] = 9; number[2] = 5; printf(“%p, %p\n”, p, q); printf(“%d, %d, %d, %d\n”, *(p+1), p[1], *(q+1), q[1]); • Pointer variables and array variables can be used interchangeably. • Why do we need to use pointer variables? C Programming

Do we really need to use pointers? • Dynamic memory management #include <stdlib.h> void *malloc(int size); // allocate size bytes // and return the addr void free(void *); // free the memory space ... int *p, n; scanf(“%d”, &n); // space for n-many integer variables. p = (int*)malloc(sizeof(int) * n); p[0] = 10; *(p+1) = 20; *(p+2) = 30; p++; *p = 4; C Programming

Character Pointers #include <string.h.> gets(), puts() strcpy(), strlen(), strcmp(), strcat(), ... toupper(), ... C: Advanced Topics

char name[256], tmp[256]; name[0] = ‘C’; name[1] = ‘O’; name[2] = ‘M’; name[3] = ‘P’; name[4] = ‘\0’; // it is very important. name[5] = ‘ ’; name[6] = ‘2’; name[7] = ‘1’; name[8] = ‘3’; name[9] = ‘0’; name[10] = ‘\0’; // it is very important. printf(“course number = %s\n”, name); printf(“%p\n”, name); printf(“course number = %s\n”, &(name[5])); scanf(“%s”, name); // not &name sprintf(tmp, “course name is %s.”, name); C Programming

3. Input and Output • Standard input from keyboard $ prog < infile input redirection $ otherprog | prog pipe • <stdio.h> • int getchar() • int putchar(int c) C: Advanced Topics

Formatted input • int scanf (char *format, arg1, arg2, ...) // from stdin • int sscanf (char *string, char *format, arg1, arg2, ...); // from string • The arguments must be references. C: Advanced Topics

File Access #include <stdio.h> FILE *in, *out; // FILE is defined in <stdio.h> in = fopen(“in_filename”, “r”); // mode: r, w, a, r+, w+, a+ if (in == NULL) ... out = fopen(“out_filename”, “w”); fclose(in); fprintf(out, “format ...”, variables...); fscanf(...); fgets(...); int fseek(FILE*, long, SEEK_SET or SEEK_CURRENT or SEEK_END); // move file position pointer int fwrite(void*, int memb_size, int no_memb, FILE*); int fread(void*, int memb_size, int no_memb, FILE*); C: Advanced Topics

int fputc(int, FILE*); int fputcs(char*, FILE*); int fgetc(FILE*); int fscanf(FILE*, char* format, ...); int fprintf(FILE*, char* format, ...); Examples: • A file copy program, using fopen(), fseek(), fwrite(), fread(), fclose(). • Files containing student records struct student { ... }; struct student record; FILE *fp = fopen(“test”, “w+”); // read and write; file truncated; fwrite(&record, sizeof(struct student), 1, fp); fread(&record, sizeof(struct student), 1, fp); C: Advanced Topics

How to obtain the current position:: • long ftell(FILE*); C: Advanced Topics

Error Handling – Stderr and Exit fprintf(stderr, char*, ...); exit(int); // non zero means error C: Advanced Topics

math.h • Some MATH related functions • # include <math.h> • double sqrt(double); • double pow(double, double); • double fabs(double); • ... • Link with –lm-lm means libm.a, that contains math utilities, is used • $ gcc program3-5.c –lm C Programming

Pointer Arrays: Pointers to Pointers void f(int *x[13]); // 13 int* variables void f(int (*x)[13]); // pointer to an array of 13 ints // equivalent to int x[][13] • Command-line arguments int main(int argc, char *argv[]); • argc the number of arguments • argv[0] the program name, e.g., a.out • argv[1] the first argument from the user • E.g., $ ./a.out test this comp • argc: 4 • argv[0]: “./a.out” • argv[1]: “test C: Advanced Topics

Command-line arguments int main(intargc, char *argv[]); • argc the number of arguments • argv[0] the program name, e.g., a.out • argv[1] the first argument from the user • E.g., $ ./a.out test this comp • argc: 4 • argv[0]: “./a.out” • argv[1]: “test C: Advanced Topics

Command Line Arguments • It is possible to pass some values from the command line in the C programs when they are executed. • These values are called command line arguments • These are important as they can control the ‘C’ program from outside instead of hard coding those values inside the code. • The command line arguments are handled using main() function arguments • argc refers to the number of arguments passed, • argv[] is a pointer array which points to each argument passed to the program. Introduction

Example #include <stdio.h> int main( intargc, char *argv[] ) { if( argc == 2 ) { printf("The argument supplied is %s\n", argv[1]); } else if( argc > 2 ) { printf("Too many arguments supplied.\n"); } else { printf("One argument expected.\n"); } } $./a.out testing The argument supplied is testing Introduction

Structures • It is a way to have a single name referring to a group of a related values. • Structures provide a way of storing many different values in variables of potentially different types under the same name. • its design makes things more compact. • Structs are generally useful whenever a lot of data needs to be grouped together • Example: a contact group may have {name, address, phone number, and mobile number} Introduction

Example struct database { intid_number; int age; float salary; }; int main() { struct database employee; /* There is now an employee variable that has modifiable variables inside it.*/ employee.age = 22; employee.id_number = 1; employee.salary = 12000.21; printf(“Employee ID = %d, Age = %d and Salary = %7.2f”, employee.id, employee.age, employee.salary); } Introduction

2. Structures • User-defined data structure structstudent_rcd { // class without methods in Java intstudent_number; char name[128]; ... }; ... structstudent_rcd record[10], *rp; structstudent_rcd test; // how to declare a struct variable test.student_number = 10; // how to access a member print_rcd(test); read_rcd(&test); record[0].student_number = 5; C: Advanced Topics

void print_rcd(struct student_rcd rcd) { printf(“Number: %d\n”, rcd.student_number); printf(“Name: %s\n”, rcd.name); // name is an array. // not &(rcd.name) } void read_rcd(struct student_rcd *rcd) { printf(“Enter number: “); scanf(“%d”, &(rcd->student_number)); // reference rqd printf(“Enter name: “); scanf(“%s”, rcd->name); // name is an array. // not &(rcd->name) } C: Advanced Topics

Self-Referential Structures struct tnode { /* the tree node: */ char *word; /* points to the text */ int count; /* number of occurrences */ struct tnode *left; /* left child */ struct tnode *right; /* right child */ struct tnode *parent; }; struct tnode root; root.left = (struct tnode *)malloc(...); C: Advanced Topics

Typedef typedefint Length; // Now Length is a data type. typedef char *String; // Now String is a data type. typedefstructtnode { /* the tree node: */ char *word; /* points to the text */ int count; /* number of occurrences */ structtnode *left, *right; /* children */ structtnode *parent; } Treenode; // Now Treenode is a data type. ... Lengthlen, maxlen; Length *lengths[]; String p, lineptr[MAXLINES]; Treenode tnode1; C: Advanced Topics

Union • A union is a special data type available in C that enables you to store different data types in the same memory location like structures • The main difference is that a union may have many members, but only one member can contain a value at any given time. • Unions provide an efficient way of using the same memory location for multi-purpose. #include <stdio.h> union Data { inti; float f; char str[20]; }; int main( ) { union Data data; data.i = 10; printf( "data.i : %d\n", data.i); return 0; } Introduction

Dynamic Memory Allocation Name Number next Name Number next Name Number next struct Student { char name[128]; int number; Student *next; }; typedef struct Student Student; Student *head = null; // important Student *new, *tmp; new = create_student(); // create a record, read data from the user, // store them into the record add_student(head, new); // add the new record at the end of the list tmp = find_student(head, 968374); // find the record whose number is .. printf(“%d: %s\n”, tmp->number, tmp->name); // print the record delete_student(head, 968374); Head null C: Advanced Topics

Allocation functions • Malloc() • Calloc() • Free() Introduction

C: Advanced Topics