Allocating Memory

We already know about the heap and stack. Now we want to allocate memory that can persist function calls and does not only live on the stack.

int* ptr_to_pi() {
    int pi = 3;
    return π
    //pi gets dealloced here!
}

int* ptr = ptr_to_pi();
int x = 3 + *ptr; // fails

Malloc

There are four functions for memory management in stdlib.h.

• malloc lets us allocate some uninitialized memore
• calloc like malloc but zeroes the memory
• realloc changes the size of an allocated block of memory
• free gives the memory back (meaning we cannot use it anymore)

Now we can initialize a value on the heap and return a pointer to it.

int* to_four() {
    int* mem = (int*) malloc(sizeof(int));
    if(mem == NULL)
        exit(1); // malloc failed
    *mem = 4;
    return mem;
}

int* my_four = to_four();
int three = *my_four - 1;
free(my_four); // never forget to free!

It is very important that memory is freed again at some point. Otherwise we have a memory leak and the program keeps taking up more and more RAM over time.

Some languages have a garbage collector which does the freeing for us. This eliminates (almost all) memory leaks.

Structs and Unions

If we want to store some data in a structured way, we can use structs. Note that structs are not like objects in Java. They are copied on assignment and pass-by-value.

struct person {
    int age;
    int height;
};

struct person tom = {24, 180}
struct person* p = &tom;

tom.age = 25;
p->height = 182; // same as (*p).height

Unions only ever hold one of the values declared in it. They only have one memory space for all the values (which means they will override each other).

union fandi {
    float f;
    int i;
};

union fandi u;
u.i = 10;
// u.i = 10 and u.f = 0.0
u.f = 0.5;
// u.i = 1056964608 and u.f = 0.5

Typedef

Say we do not want to type “union” or “struct” every time or we want to simplify complicated type declarations. typedef allows us to rename types.

typedef unsigned int natural_number;
typedef struct point {
    int x, y;
} point;

natural_number x = 10;
point my_point;

C has different namespaces for labels (goto), tags (for all structs, unions, enums), member names (one per struct, …) and everything else.