C Memory Management

I've always heard that in C you have to really watch how you manage memory. And I'm still beginning to learn C, but thus far, I have not had to do any memory managing related activities at all.. I always imagined having to release variables and do all sorts of ugly things. But this doesn't seem to be the case.

Can someone show me (with code examples) an example of when you would have to do some "memory management" ?

There are two places where variables can be put in memory. When you create a variable like this:

int  a;
char c;
char d[16];

The variables are created in the "stack". Stack variables are automatically freed when they go out of scope (that is, when the code can't reach them anymore). You might hear them called "automatic" variables, but that has fallen out of fashion.

Many beginner examples will use only stack variables.

The stack is nice because it's automatic, but it also has two drawbacks: (1) The compiler needs to know in advance how big the variables are, and (2) the stack space is somewhat limited. For example: in Windows, under default settings for the Microsoft linker, the stack is set to 1 MB, and not all of it is available for your variables.

If you don't know at compile time how big your array is, or if you need a big array or struct, you need "plan B".

Plan B is called the "heap". You can usually create variables as big as the Operating System will let you, but you have to do it yourself. Earlier postings showed you one way you can do it, although there are other ways:

int size;
// ...
// Set size to some value, based on information available at run-time. Then:
// ...
char *p = (char *)malloc(size);

(Note that variables in the heap are not manipulated directly, but via pointers)

Once you create a heap variable, the problem is that the compiler can't tell when you're done with it, so you lose the automatic releasing. That's where the "manual releasing" you were referring to comes in. Your code is now responsible to decide when the variable is not needed anymore, and release it so the memory can be taken for other purposes. For the case above, with:

free(p);

What makes this second option "nasty business" is that it's not always easy to know when the variable is not needed anymore. Forgetting to release a variable when you don't need it will cause your program to consume more memory that it needs to. This situation is called a "leak". The "leaked" memory cannot be used for anything until your program ends and the OS recovers all of its resources. Even nastier problems are possible if you release a heap variable by mistake before you are actually done with it.

In C and C++, you are responsible to clean up your heap variables like shown above. However, there are languages and environments such as Java and .NET languages like C# that use a different approach, where the heap gets cleaned up on its own. This second method, called "garbage collection", is much easier on the developer but you pay a penalty in overhead and performance. It's a balance.

(I have glossed over many details to give a simpler, but hopefully more leveled answer)

Here's an example. Suppose you have a strdup() function that duplicates a string:

char *strdup(char *src)
{
    char * dest;
    dest = malloc(strlen(src) + 1);
    if (dest == NULL)
        abort();
    strcpy(dest, src);
    return dest;
}

And you call it like this:

main()
{
    char *s;
    s = strdup("hello");
    printf("%s\n", s);
    s = strdup("world");
    printf("%s\n", s);
}

You can see that the program works, but you have allocated memory (via malloc) without freeing it up. You have lost your pointer to the first memory block when you called strdup the second time.

This is no big deal for this small amount of memory, but consider the case:

for (i = 0; i < 1000000000; ++i)  /* billion times */
    s = strdup("hello world");    /* 11 bytes */

You have now used up 11 gig of memory (possibly more, depending on your memory manager) and if you have not crashed your process is probably running pretty slowly.

To fix, you need to call free() for everything that is obtained with malloc() after you finish using it:

s = strdup("hello");
free(s);  /* now not leaking memory! */
s = strdup("world");
...

Hope this example helps!