Monash University > CSSE > CSE1303 > Part A > Lectures > Lecture A06 notes

CSE1303 Computer Science
Semester 2 2003
Part A
Lecture A06 notes: Dynamic Memory

In this lecture

• Kruse 4.5
• Deitel and Deitel 12.3
• Standish 8.6
• King 17

• Dynamic memory is allocated and deallocated during the execution of the program.
• There are two types: the run time stack, and the heap.
• Local variables are automatically provided with memory on the run-time stack.
• Memory allocated to local variables (in a function) is automatically freed on returning from a function.

• Computes the size, in bytes, of any object or type in C.

• #include <stdlib.h>

• A call malloc(n) returns a pointer to n bytes of free memory, or NULL if the request cannot be satisfied.
• Always check if malloc has returned the NULL pointer.
• A type cast should in general be applied to the pointer returned by malloc.
• allocated memory in the heap.

• A call free(pointer) deallocates the space pointed to by pointer.
• It does nothing if pointer is NULL.
• pointer must point to space previously allocated by malloc.

• Referring to memory already freed.
• Not deallocating memory which is no longer required.
• Freeing memory not allocated by malloc.

Code for this lecture

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

#define MAXNAME 80

struct StudentRec
{
   char name[MAXNAME];
   int  mark;
};

typedef struct StudentRec Student;

/* This function reads in a student's record. */
Student readStudent(void)
{
   Student next;
   scanf("%s %d", next.name, &next.mark);
   return next;
}

/* This function prints out a student's record. */
void printStudent(const Student student)
{
   printf("%s %d\n", student.name, student.mark);
}

/* This program reads in the student's records for a
 * class and prints out the first student's record it
 * finds with the highest mark.  */

int main()
{
   Student* class = NULL;
   int n;
   int best = 0;
   int i;

   printf("Enter number of students in the class: ");
   scanf("%d", &n);

  class = (Student*)malloc(n * sizeof(Student));

  if (class != NULL)
  {
      for (i = 0; i < n; i++)
      {
         class[i] = readStudent();
         if (class[best].mark < class[i].mark)
         {
            best = i;
         }
      }
      printf("Best student is: ");
      printStudent(class[best]);
  }
}

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

/* This function creates a n x m matrix of float; and
 * returns a pointer to this matrix.  */

float** makeMatrix(int n, int m)
{
 float*  memoryPtr;
 float** matrixPtr;
 int i;

 memoryPtr = (float*)malloc(n*m*sizeof(float));
 matrixPtr = (float**)malloc(n*sizeof(float*));

 if (memoryPtr == NULL || matrixPtr == NULL)
 {
    fprintf(stderr, "Not enough memory\n");
    exit(1);
 }

 for (i = 0; i < n; i++, memoryPtr += m)
 {
    matrixPtr[i] = memoryPtr;
 }
 return matrixPtr;
}

[ Top | Home ]
Last Updated: Thursday 26 June 2003 11:02:04