Thread: Create malloc and free function in C?

Any updates?

How can I allocate memory? I've told you why I need it and what OS I'm using? Could you guys at least give me a breakdown of the steps involved?

Thanks

I haven't done anything except a printf, OSDEV wiki recommends memory management as the next step.

Hmmm -- so where is the kernel going to run? Inside a virtual machine on ubuntu running inside a virtual machine on windows?

Yes, I think its weird too, but that's how I'm running it

hey as far of my knowledge,if you want to create such fnctns then it's better to read few in functions.......

Originally Posted by krishnaswathi

hey as far of my knowledge,if you want to create such fnctns then it's better to read few in functions.......

Don't talk with your mouth full and/or your mind away for a vacation.

actually, i am making an OS kernel, on Ubuntu 11 running as a Virtual Machine on windows...

Search results - OSDev Wiki

And you should not be surprised if it doesn't work when you un-nest it from all that virtualization...

Virtual machines are pretty good, but they're not perfect...

Okay...
But how do I allocate memory? I mean there are various tutorials on the net on how the process should be done, and the dangers of making one(which is kind of why I'm using a Virtual Machine), but not one line of code. What function/command in C allocates memory? Or do I need to use assembly's mov command?

Originally Posted by guestcheap

Okay...
But how do I allocate memory? I mean there are various tutorials on the net on how the process should be done, and the dangers of making one(which is kind of why I'm using a Virtual Machine), but not one line of code. What function/command in C allocates memory? Or do I need to use assembly's mov command?

The only C function that allocates memory is malloc() and it is dependent upon the underlying operating system.

If you are working on an OS kernel you need to write a complete memory manager, that is, the underlying stuff behind malloc()...

Usually memory is allocated by placing frames around blocks of memory... they form a sort of linked list (loose analogy, but close enough for illustration purposes) of allocated blocks, each pointing to the next. You start at the bottom (or top) of memory and allocate 100 bytes, so you put a frame there, the frame contains a pointer to the next block and the size of the current block and you return a pointer to the first byte of the block to the program... to allocate another block you simply add a nother frame...

Code:

*
*allocated block
*
size of this block
pointer to next block
===================
*
*  allocated block
*
Size of this block
pointer to next block
====================

Then to deallocate a block you fix the pointers and remove the frame...

You know where each block begins and ends, so as you add and remove blocks you can "manage" the memory by filling gaps.

Of course it's a little bit more complicated than this --well, more than a little actually-- but that's the jist of it... If you understand linked lists you understand memory management.

Originally Posted by CommonTater

The only C function that allocates memory is malloc() and it is dependent upon the underlying operating system.

If you are working on an OS kernel you need to write a complete memory manager, that is, the underlying stuff behind malloc()...

Usually memory is allocated by placing frames around blocks of memory... they form a sort of linked list (loose analogy, but close enough for illustration purposes) of allocated blocks, each pointing to the next. You start at the bottom (or top) of memory and allocate 100 bytes, so you put a frame there, the frame contains a pointer to the next block and the size of the current block and you return a pointer to the first byte of the block to the program... to allocate another block you simply add a nother frame...

Code:

*
*allocated block
*
size of this block
pointer to next block
===================
*
*  allocated block
*
Size of this block
pointer to next block
====================

Then to deallocate a block you fix the pointers and remove the frame...

You know where each block begins and ends, so as you add and remove blocks you can "manage" the memory by filling gaps.

Of course it's a little bit more complicated than this --well, more than a little actually-- but that's the jist of it... If you understand linked lists you understand memory management.

One problem I had with the script here:i386/boot/cdboot/malloc.c Source is that I have no idea what &end is. It hasn't been declared in this piece of code, so I assume its in boot.h. How do I define end?

Originally Posted by tabstop

end is probably the end of the allocated block of memory; but as you say, it is not defined here (nor is it changed here!) so if you want to know what it is you'll have to find it elsewhere.

Actually I think end is a marker set by the linker (see below) for where the memory occupied by the bootloader code section ends, so everything between &end and MAXBRK is available for use by the allocator.

Originally Posted by guestcheap

One problem I had with the script here:i386/boot/cdboot/malloc.c Source is that I have no idea what &end is. It hasn't been declared in this piece of code, so I assume its in boot.h. How do I define end?

You could, maybe, look at boot.h?

Code:

/* linker stuff */
extern void end;

Originally Posted by guestcheap

Okay...
But how do I allocate memory? I mean there are various tutorials on the net on how the process should be done, and the dangers of making one(which is kind of why I'm using a Virtual Machine), but not one line of code. What function/command in C allocates memory? Or do I need to use assembly's mov command?

Because they assume that if you think you are capable of writing your own kernel you shouldn't need to be hand held by copying finished code.
The OSDev page you yourself linked mentions two different approaches: Bitmaps or headers. The same site also has explanations of what those two words mean.

Originally Posted by _Mike

Because they assume that if you think you are capable of writing your own kernel you shouldn't need to be hand held by copying finished code.
The OSDev page you yourself linked mentions two different approaches: Bitmaps or headers. The same site also has explanations of what those two words mean.

First of all I didn't 'myself' link this page AndrewHunter did. That being said, I found help here I understand how the code works, but I can't identify the line of code which actually assigns the memory. Any help?