Dynamic Memory Allocation, Structure pointers

Dynamic Memory Allocation,Structure pointers Lecture 17b 20.3.2001. Sudeshna Sarkar, CSE, IIT Kharagpur

Basic Idea • Many a time we face situations where data is dynamic in nature. • Amount of data cannot be predicted beforehand. • Number of data item keeps changing during program execution. • Such situations can be handled more easily and effectively using dynamic memory management techniques. Sudeshna Sarkar, CSE, IIT Kharagpur

C language requires the number of elements in an array to be specified at compile time. • Often leads to wastage or memory space or program failure. • Dynamic Memory Allocation • Memory space required can be specified at the time of execution. • C supports allocating and freeing memory dynamically using library routines. Sudeshna Sarkar, CSE, IIT Kharagpur

Memory Allocation Process in C Local variables Stack Free memory Heap Global variables Permanent storage area Instructions Sudeshna Sarkar, CSE, IIT Kharagpur

The program instructions and the global variables are stored in a region known as permanent storage area. • The local variables are stored in another area called stack. • The memory space between these two areas is available for dynamic allocation during execution of the program. • This free region is called the heap. • The size of the heap keeps changing Sudeshna Sarkar, CSE, IIT Kharagpur

Memory Allocation Functions • malloc: Allocates requested number of bytes and returns a pointer to the first byte of the allocated space. • calloc: Allocates space for an array of elements, initializes them to zero and then returns a pointer to the memory. • free : Frees previously allocated space. • realloc: Modifies the size of previously allocated space. Sudeshna Sarkar, CSE, IIT Kharagpur

Dynamic Memory Allocation • used to dynamically create space for arrays, structures, etc. int main () { int *a ; int n; .... a = (int *) calloc (n, sizeof(int)); .... } a = malloc (n*sizeof(int)); Sudeshna Sarkar, CSE, IIT Kharagpur

Space that has been dynamically allocated with either calloc() or malloc() does not get returned to the function upon function exit. • The programmer must use free() explicitly to return the space. • ptr = malloc (...) ; • free (ptr) ; Sudeshna Sarkar, CSE, IIT Kharagpur

void read_array (int *a, int n) ; int sum_array (int *a, int n) ; void wrt_array (int *a, int n) ; int main () { int *a, n; printf (“Input n: “) ; scanf (“%d”, &n) ; a = calloc (n, sizeof (int)) ; read_array (a, n) ; wrt_array (a, n) ; printf (“Sum = %d\n”, sum_array(a, n); } Sudeshna Sarkar, CSE, IIT Kharagpur

void read_array (int *a, int n) { int i; for (i=0; i<n; i++) scanf (“%d”, &a[i]) ; } void sum_array (int *a, int n) { int i, sum=0; for (i=0; i<n; i++) sum += a[i] ; return sum; } void wrt_array (int *a, int n) { int i; ........ } Sudeshna Sarkar, CSE, IIT Kharagpur

Arrays of Pointers • Array elements can be of any type • array of structures • array of pointers Sudeshna Sarkar, CSE, IIT Kharagpur

int main (void) { char word[MAXWORD]; char * w[N]; /* an array of pointers */ int i, n; /* n: no of words to sort */ for (i=0; scanf(“%s”, word) == 1); ++i) { w[i] = calloc (strlen(word)+1, sizeof(char)); if (w[i] == NULL) exit(0); strcpy (w[i], word) ; } n = i; sortwords (w, n) ; wrt_words (w, n); return 0; } Sudeshna Sarkar, CSE, IIT Kharagpur

Input : A is for apple or alphabet pie which all get a slice of come taste it and try w 0 A \0 1 i s \0 2 f o r \0 3 a p p l e \0 17 t r y \0 Sudeshna Sarkar, CSE, IIT Kharagpur

void sort_words (char *w[], int n) { int i, j; for (i=0; i<n; ++i) for (j=i+1; j<n; ++j) if (strcmp(w[i], w[j]) > 0) swap (&w[i], &w[j]) ; } void swap (char **p, char **q) { char *tmp ; tmp = *p; *p = *q; *q = tmp; } Sudeshna Sarkar, CSE, IIT Kharagpur

Before swapping w w[i] f o r \0 a p p l e \0 w[j] Sudeshna Sarkar, CSE, IIT Kharagpur

After swapping w w[i] f o r \0 a p p l e \0 w[j] Sudeshna Sarkar, CSE, IIT Kharagpur

Pointers to Structure Sudeshna Sarkar, CSE, IIT Kharagpur

Pointers and Structures • You may recall that the name of an array stands for the address of its zero-th element. • Also true for the names of arrays of structure variables. • Consider the declaration: struct stud { int roll; char dept_code[25]; float cgpa; }class[100], *ptr ; Sudeshna Sarkar, CSE, IIT Kharagpur

The name class represents the address of the zero-th element of the structure array. • ptr is a pointer to data objects of the type struct stud. • The assignment ptr = class ; • will assign the address of class[0] to ptr. • When the pointer ptr is incremented by one (ptr++) : • The value of ptr is actually increased by sizeof(stud). • It is made to point to the next record. Sudeshna Sarkar, CSE, IIT Kharagpur

Once ptr points to a structure variable, the members can be accessed as: ptr –> roll ; ptr –> dept_code ; ptr –> cgpa ; • The symbol “–>” is called the arrow operator. Sudeshna Sarkar, CSE, IIT Kharagpur

Warning • When using structure pointers, we should take care of operator precedence. • Member operator “.” has higher precedence than “*”. • ptr –> roll and (*ptr).roll mean the same thing. • *ptr.roll will lead to error. • The operator “–>” enjoys the highest priority among operators. • ++ptr –> roll will increment roll, not ptr. • (++ptr) –> roll will do the intended thing. Sudeshna Sarkar, CSE, IIT Kharagpur

Program to add two complex numbers using pointers typedef struct { float re; float im; } complex; main() { complex a, b, c; scanf (“%f %f”, &a.re, &a.im); scanf (“%f %f”, &b.re, &b.im); add (&a, &b, &c) ; printf (“\n %f %f”, c,re, c.im); } Sudeshna Sarkar, CSE, IIT Kharagpur

void add (complex * x, complex * y, complex * t){ • t->re = x->re + y->re ; • t->im = x->im + y->im ; • } Sudeshna Sarkar, CSE, IIT Kharagpur

Dynamic Memory Allocation, Structure pointers