MPATE-GE 2618: C Programming for Music Technology. Unit 5.1

Transcription

1 MPATE-GE 2618: C Programming for Music Technology Unit 5.1

2 Review: automatic vs. static variables Variables declared and passed to functions are automatic variables. As soon as you leave the function, they disappear. Static variables, on the other hand, stay around for the life of the entire program no matter where they are. void func(); void main(void) { int i; for (i = 0; i < 3; i++) { func(); void func() { static int i = 0; i++; 2

3 Review: the sizeof operator e sizeof operator returns the number of bytes for a given type or variable. unsigned long number; int size; size = sizeof(unsigned long); size = sizeof(number); // equiv. to line above 3

4 Allocating memory with malloc e functions malloc and calloc are used to allocate memory of a specified size. e size is always in bytes. malloc takes the total size of the chunk of memory you want as its single parameter. calloc takes the size of a particular type and the number of those elements. 4

5 Deallocating dynamic memory Use the free function to delete the memory you ve allocated. Example: allocate a float: float *val = (float *)malloc(sizeof(float)); if (vall == NULL) { printf("out of memory\n"); // exit or return free(val); Example: allocating and freeing memory for an array of integers: int *array = (int *)malloc(15*sizeof(int)); if (array == NULL) { printf("out of memory\n"); // exit or return free(array); 5

6 Allocating multi-dimensional arrays #include <stdlib.h> int **array; array = (int **)malloc(nrows * sizeof(int *)); if(array == NULL) { fprintf(stderr, "out of memory\n"); exit(1); for(i = 0; i < nrows; i++) { array[i] = (int *)malloc(ncolumns * sizeof(int)); if(array[i] == NULL) { fprintf(stderr, "out of memory\n"); exit(1); 6

7 Deallocating multi-dimensional arrays Each individual array needs to be deallocated. for(i = 0; i < nrows; i++) { free(array[i]); free(array); 7

8 Memory leaks: e inputlib library revisted Do you recall the getstring() function? Let s take a look at the actual code for it... 8

9 Some examples Allocate a single double precision float: double *val; val = (double *) malloc(sizeof(double)); Allocate an array of MAX_SIZE integers int *array; array = (int *) malloc(max_size * sizeof(int)); Allocate a two-dimensional array of characters char **array; array = (char **) malloc(nrows * sizeof(char *)); for(i = 0; i < nrows; i++) { array[i] = (char *) malloc(ncolumns * sizeof(char)); Allocate a three-dimensional array of unsigned integers unsigned int ***array; int x = 5, y = 3, z = 12; int i, j; array = (unsigned int ***) malloc(x * sizeof(unsigned int **)); for (i = 0; i < x; i++) { array[i] = (unsigned int **)malloc(y * sizeof(unsigned int *)); for (i = 0; i < x; i++) { for (j = 0; j < y; j++) { array[i][j] = (unsigned int *)malloc(z * sizeof(unsigned int)); 9

10 Debugging memory problems and making programs more efficient Why is writing to wrong place in memory bad (even if it doesn t crash)? If it s an invalid address the program will crash On a system with protected memory spaces (UNIX, MacOS X, Windows 98/NT/XP/Vista/ 7), accessing/writing to memory that your program doesn t own will cause the system to terminate execution of the program. Early systems (Windows 3.1, earlier MacOS) were not aware of what you are doing and so the result could be damage to the code or variables of another application. ings might seem OK initially but it might affect the behavior of the program down the line. Memory debugger - programming tool for finding memory leaks and buffer overflows. Profiler - tool that investigates a program's behavior using information gathered as the program executes (i.e. it is a form of dynamic program analysis, as opposed to static code analysis). e usual goal of performance analysis is to determine which sections of a program to optimize (increase speed or decrease memory).

11 Structures revisited When we wanted to group a collection of data of the same type together, we used an array. Sometimes it s useful to collect data of different types. at s where structs come in handy. Say we have have a student type: typedef struct _student { char *firstname; char *lastname; int age; double gpa; student; student s1; student *s1ptr; s1ptr = &s1; s1.age = 19; s1ptr->gpa = 3.8;

12 Passing a struct to a function Why is the struct passed as a pointer? In main(): student s; setfirstname("mary", &s); e function header: setfirstname(char *name, student *s) {...

13 Function to set a string in the struct void setfirstname(const char *name, student *s)! {!!! if (s!= NULL) {! if (s->firstname!= NULL)! free(s->firstname);! s->firstname = (char *)malloc(sizeof(char)*strlen(name)+1);! strcpy(s->firstname, name);!

14 A program to enter a student record Example: let s write a program that allows you to enter in data for a student. Add a function that prints the student s info. Add a function to change the student s first name. See file student1.c How could we extend this program so that we can store more than one student record? See file student2.c