Dynamic Memory Allocation

Simple Example

#include <conio.h>
#include <stdio.h>
#include <alloc.h>

void main()
{
   int i,n;
   int *parray,*p;
   clrscr();
   printf("Enter the size of a an array===>");
   scanf("%d",&n);
   parray=(int *)malloc(n*sizeof(int)) ;
   if (parray==NULL)
      {
	printf("\nNot enough memory");
	return;
      }
   for (p=parray,i=0; i<n ; i++)
       scanf("%d",p++);
   for (i=0; i<n; i++)
       printf("\n%d",*(--p));
}

Memory Initialization

#include <conio.h>
#include <stdio.h>
#include <stdlib.h>

#include <string.h>
#include <mem.h>

void main()
{
   int i;
   char *p;
   clrscr();
   p=(char *)malloc(50);
   printf("Buffer after malloc:\n %s\n", p);

   for(i=0;i<49;i++)
       *(p+i)='*';
   *(p+49)='\0';
   printf("Buffer after loop:\n  %s\n", p);
   exit(0);
}

using memset

#include <conio.h>
#include <stdio.h>
#include <stdlib.h>

#include <string.h>
#include <mem.h>

void main()
{
   char *p;
   clrscr();
   p=(char *)malloc(50);
   printf("Buffer after malloc:\n %s\n", p);


   memset(p, '*',49); *(p+49)='\0';
   printf("Buffer after memset:\n  %s\n", p);
   exit(0);
}

Dynamic sort of strings

/*----------------------------------------------------*/
/* T140.40   dynamic sort of strings                  */
/*----------------------------------------------------*/
#include <conio.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define LINES_SIZE 80
#define LINES_NO   500
void gen_line(char *line)
{
    int i,len;
    char ch;
    len=rand()%(LINES_SIZE-1) +1;
    for (i=0;i<len;i++)
    {
       ch =rand()%26 + 'A';
       *line++=ch ;
    }
    *line='\0';
}
void main()
{
   char *pline[LINES_NO];
   char *swap_line;
   int no_of_lines;
   int i,j;
   clrscr();
   printf("Enter no of lines===>");
   scanf("%d",&no_of_lines);
   /*--------------------------------------
   *   generate and print lines before sort
   *--------------------------------------*/
   for (i=0;i<no_of_lines;i++)
     {
       pline[i]=(char *)malloc(LINES_SIZE);
       if(pline[i]==NULL)
	  { printf("Not enough memory");
	    exit(0);
	  }

       gen_line(pline[i]);
       puts(pline[i]);
     }
   /*-------------------------------------
   *    sort lines
   *-------------------------------------*/
   for(i=0;i<no_of_lines;i++)
    {
       for (j=i+1;j<no_of_lines;j++)
	  if(strcmp(pline[i],pline[j] )>0)
	    {  strcpy(swap_line,pline[i]);
	       strcpy(pline[i],pline[j]);
	       strcpy(pline[j],swap_line);
	    }
    }
    printf("\n the sorted output is:\n\n");
    for (i=0;i<no_of_lines;i++)
      {  puts(pline[i]);
	 free(pline[i]);
      }
    exit(0);
}

Simple malloc

#include <stdio.h>
#include <stdlib.h>

void main(void)
{
   long nbr_students = 0;
   long ctr;

   char *student_name;
   printf("\nHow many students will be entered? ==> ");
   scanf("%D", &nbr_students);
   fflush(stdin);       /* clear out keyboard buffer */

   student_name = (char *) malloc( 35*nbr_students);

   if( student_name == NULL )    /* verify malloc() was successful */
   {
	 printf( "\nError in line %3.3d: Could not allocate memory.",
		    __LINE__);
	 exit(1);
   }

   for( ctr = 0; ctr < nbr_students; ctr++ )
   {
	 printf("\nEnter student %5.5d: ", ctr+1);
	 gets(student_name+(ctr*35));
   }

   printf("\n\nYou entered the following:\n");

   for ( ctr = 0; ctr < nbr_students; ctr++ )
   {
	 printf("\nStudent %5.5d:", ctr+1);
	 printf(" %s", student_name+(ctr*35));
   }

   /* this program does not release allocted memory! */
}

calloc example

#include <stdio.h>
#include <stdlib.h>

void main( void )
{
   int  nbr_grades = 0;
   int  total = 0;
   int  ctr;
   int  *student_grades;

   while( nbr_grades < 1 || nbr_grades >= 10000 )
   {
	 printf("\nHow many grades will be entered? ==> ");
	 scanf("%d", &nbr_grades);
	 fflush(stdin);
   }

   student_grades = (int *) calloc( nbr_grades, sizeof(int));

   if( student_grades == NULL )
   {
	 printf( "\nError in line %3.3d: Could not allocate memory.",
		    __LINE__);
	 exit(1);
   }

   for( ctr = 0; ctr < nbr_grades; ctr++ )
   {
	 printf("\nEnter grade %4.4d: ", ctr+1);
	 scanf("%d", student_grades+ctr);
   }

   printf("\n\nYou entered the following:\n");

   for ( ctr = 0; ctr < nbr_grades; ctr++ )
   {
	 printf("\nGrade %4.4d:", ctr+1);
	 printf(" %d", student_grades[ctr]);

	 total += student_grades[ctr];
   }

   printf("\n\nThe average grade is: %d\n\n", (total/nbr_grades));

   /* Free allocated memory */

   free(student_grades);
}

matrix allocation

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>

void main()
{
int **a,i,j;
clrscr();

a=(int **)malloc(sizeof(int *)*10);

for (i=0;i<10;i++)
	a[i]=(int *)malloc(20);

for (i=0;i<10;i++)
	for(j=0;j<10;j++)
		a[i][j]=(i+1)*(j+1);

for (i=0;i<10;i++)
	{
	for(j=0;j<10;j++)
		printf("%4d",a[i][j]);
	printf("\n");
	}

for (i=0;i<10;i++)
	free(a[i]);
}

realloc example

#include <stdio.h>
#include <stdlib.h>

#define NAME_SIZE  35

void main(void)
{
   long student_ctr = 0;
   long ctr;
   char *student_name = NULL;

   while( (student_name = realloc( student_name, (NAME_SIZE * (student_ctr+1)))) != NULL )
   {
	 printf("\nEnter student %5.5d: ", student_ctr+1);
	 gets(student_name+( student_ctr * NAME_SIZE));
	 if( student_name[student_ctr * NAME_SIZE] == NULL )
	 {

	    break;
	 }
	 else
	 {
	    student_ctr++;
	 }
   }
   printf("\n\nYou entered the following:\n");

   for ( ctr = 0; ctr < student_ctr; ctr++ )
   {
	 printf("\nStudent %5.5d:", ctr+1);
	 printf(" %s", student_name+(ctr*NAME_SIZE));
   }
   free(student_name);
}