Dynamic Memory Allocation (Heap)

Dynamic memory allocation gives control over:

• when to allocate
• how much to allocate
• when to free

Used when:

• size unknown at compile time
• need resizable data structures

malloc()

void *malloc(size_t size);

• allocates bytes
• uninitialized memory
• returns NULL if fail

Example:

int *p = malloc(sizeof(int));
if(!p) return;
*p = 10;
free(p);
p = NULL;

calloc()

void *calloc(size_t n, size_t size);

• allocates n blocks
• initializes all to 0

realloc()

void *realloc(void *ptr, size_t newSize);

May move block. Use safe pattern:

int *tmp = realloc(p, newSize);
if(tmp) p = tmp;  // else old p valid

Allocating arrays safely

int n = 10;
int *arr = malloc(n * sizeof(*arr));

free()

free(p);
p = NULL;

Dynamic Memory Allocation (malloc/free lifecycle) Diagram

Shows the lifecycle of dynamic allocation on heap using malloc/calloc, safe usage, and cleanup using free() and setting pointer to NULL.

Dynamic 2D array (important)

Method 1: row pointers

int r=3,c=4;
int **a = malloc(r*sizeof(*a));
for(int i=0;i<r;i++)
    a[i] = malloc(c*sizeof(**a));

Free:

for(int i=0;i<r;i++) free(a[i]);
free(a);

Method 2: contiguous block (faster)

int *data = malloc(r*c*sizeof(int));
int (*a)[c] = (int (*)[c])data;

Common DMA bugs

1) memory leak
2) double free
3) use-after-free
4) forgetting NULL check
5) overflow write