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Memory Management in C

Core Functions for Memory Management

Functions defined in header <stdlib.h>:

  • malloc:
    • allocates memory
  • calloc:
    • allocates and zeroes memory
  • realloc:
    • expands previously allocated memory block
  • free:
    • deallocates previously allocated memory
  • free_sized:
    • deallocates previously allocated sized memory
  • free_aligned_sized:
    • deallocates previously allocated sized and aligned memory
  • aligned_alloc:
    • allocates aligned memory

Memory Allocation in C

The most basic function to allocate memory is malloc() 

void *malloc( size_t size );
This allocates size butes of uninitialized storage (memory).
If the allocation succeeds, it returns a pointer to the allocated memory.

  • The pointer is a void* type, which means it can be cast to any other type.

  • If size is zero, the behavior of malloc is implementation-defined.

    • For example, a null pointer may be returned.
    • Alternatively, a non-null pointer may be returned; but such a pointer should not be dereferenced, and should be passed to free to avoid memory leaks.

Alignment

Every complete object type has a property called alignment requirement

  • [Obj]
// Designated initializer
coordinate_t c1 = {.x = 1, .y = 1, .z = 1}
coordinate_t c2 = {.x = 2, .y = 2, .z = 2}
int count = 5;
  • A designated initializer is a way to initialize a variable with multiple values.

Getting Memory Back in C

// Designated initializer
coordinate_t c1 = {.x = 1, .y = 1, .z = 1}
coordinate_t c2 = {.x = 2, .y = 2, .z = 2}
int count = 5;

char *repeated_str = malloc(strlen(str) * count);
// Gotcha moment: This is not big enough!


char x[] = "hello";
char *repeated = repeat_string(x, count);

// THIS IS BAD. REALLY BAD.
for (int i = 0; i < 1000000; i++) {
    repeat_string(x, count)
}
// In a language with a garbage collector, this wouldn't be a problem.  

// But... How does the computer get the memory back?
//      How can we do what happens in Python?
//      In Python, we don't have to do anything explicitly to get the memory back. 


/* We need to free the memory. */
char *allocated_string = repeat_string("<This is allocated>", 3);
free(allocated_string);
// Always double check your work!
free(allocated_string);