C: Malloc and Free

Question

I am trying to undestand the C functions malloc and free. I know this has been discussed a lot on StackOverflow. However, I think I kind of know what these functions do by now. I want to know why to use them. Let's take a look at this piece of code:

int n = 10;
char* array;
array = (char*) malloc(n * sizeof(char));
// Check whether memory could be allocated or not...
// Do whatever with array...
free(array);
array = NULL;

I created a pointer of type char which I called array. Then I used malloc to find a chunk of memory that is currently not used and (10 * sizeof(char)) bytes large. That address I casted to type char pointer before assigning it to my previously created char pointer. Now I can work with my char array. When I am done, I'll use free to free that chunk of memory since it's not being used anymore.

I have one question: Why wouldn't I just do char array[10];? Wikipedia has only one small sentence to give to answer that, and that sentence I unfortunately don't understand:

However, the size of the array is fixed at compile time. If one wishes to allocate a similar array dynamically...

The slide from my university is similarily concise:

It is also possible to allocate memory from the heap.

What is the heap? I know a data structure called heap. :)

However, I've someone could explain to me in which case it makes sense to use malloc and free instead of the regular declaration of a variable, that'd be great. :)

Answer 1

C provides three different possible "storage durations" for objects:

• Automatic - local storage that's specific to the invocation of the function it's in. There may be more than one instance of objects created with automatic storage, if a function is called recursively or from multiple threads. Or there may be no instances (if/when the function isn't being called).

• Static - storage that exists, in exactly one instance, for the entire duration of the running program.

• Allocated (dynamic) - created by malloc, and persists until free is called to free it or the program terminates. Allocated storage is the only type of storage with which you can create arbitrarily large or arbitrarily many objects which you can keep even when functions return. This is what malloc is useful for.

Answer 2

First of all there is no need to cast the malloc

array = malloc(n * sizeof(char));   

I have one question: Why wouldn't I just do char array[10];?

What will you do if you don't know how many storage space do you want (Say, if you wanted to have an array of arbitrary size like a stack or linked list for example)?
In this case you have to rely on malloc (in C99 you can use Variable Length Arrays but for small memory size).

The function malloc is used to allocate a certain amount of memory during the execution of a program. The malloc function will request a block of memory from the heap. If the request is granted, the operating system will reserve the requested amount of memory.

When the amount of memory is not needed anymore, you must return it to the operating system by calling the function free.
In simple: you use an array when you know the number of elements the array will need to hold at compile time. you use malloc with pointers when you don't know how many elements the array will need to be at compile time.
For more detail read Heap Management With malloc() and free().