Writing my own malloc.

[eXeCuTeR]

Hey guys,
I'm trying to think of a way to write my own malloc & free functions and I have no idea how to do it.
I can't use the original malloc & free functions (it's not a memory pool if you thought so!) and as you probably guessed, the APIs (HeapAlloc, etc')

Any ideas? (I'm not asking for the code, just for your assistant)

And yeah, I know there's a simple example in the K & R, I haven't seen that one though.

[cpjust]

Hey guys,
I'm trying to think of a way to write my own malloc & free functions and I have no idea how to do it.
I can't use the original malloc & free functions (it's not a memory pool if you thought so!) and as you probably guessed, the APIs (HeapAlloc, etc')

Any ideas? (I'm not asking for the code, just for your assistant)

And yeah, I know there's a simple example in the K & R, I haven't seen that one though.

What OS are you using? I'm assuming Windows since you mentioned HeapAlloc.

Why can't you use the Windows API functions? That's the only way to safely get memory from the Windows. After that, what you do with the memory is up to you.
If you don't get the memory from Windows, then Windows could allocate that memory to another program and overwrite your data (although you'd probably get an IPF long before that happens).

[brewbuck]

I can't use the original malloc & free functions (it's not a memory pool if you thought so!) and as you probably guessed, the APIs (HeapAlloc, etc')

Then you're screwed, because you can't just pick an address and stick data there without asking the OS for access to that address. And that request, by definition, is done through some "API."

[Salem]

Unless you want to do something like

char pool[1000000];
Then write some alloc/free code to chop that up into handy slices.

[foxman]

As far as I can see, you have 2 remaining choice:

• Use a big global array
• Allocate a big piece of the stack at the start of main

The first solution would be a little bit cleaner than the second, since the second would need that you add some code at the start and end of main.

[MacGyver]

Use the Windows API.

[eXeCuTeR]

I have both Windows XP and Debian Lenny (testing) on my computer so im flexible.
---
Again, I can't use the API functions, I've mentioned that, it's supposed to be done by "pure" code (I assume I'm supposed to include only stdio.h)

EDIT:

And that request, by definition, is done through some "API."

Could you give an example to such function?

[MacGyver]

Yes, you've mentioned you "can't" use the Windows API, but how do you think malloc() actually works? On Windows, it can't work without using it unless you use some work around by using a memory pool which you said you didn't want to do. This is the year 2008. Operating systems rule the systems they are on. If you want to use the system, you must show respect to the OS. It is a king on your system.

If I was to write an implementation of malloc(), I would have it call HeapAlloc() with GetProcessHeap() as the heap it uses. Depending on how advanced I would want to get, I would also consider trying to create a separate heap (ie. HeapCreate()) and manage all of the heaps together, some for internal malloc() usage, and some for pure user needs.

[brewbuck]

As far as I can see, you have 2 remaining choice:

• Use a big global array
• Allocate a big piece of the stack at the start of main

The first solution would be a little bit cleaner than the second, since the second would need that you add some code at the start and end of main.

That sort of defeats the purpose of dynamic allocation, which is to not actually allocate memory until you need it.

[foxman]

That sort of defeats the purpose of dynamic allocation, which is to not actually allocate memory until you need it.

I agree with you, but it looks like that's what eXeCuTeR wants.

[eXeCuTeR]

I agree with you, but it looks like that's what eXeCuTeR wants.

Nope, what I want is to create my own malloc; a function that does the same as malloc, and not some kind of a function that also creates extra space which might not be in use.

[MacGyver]

Then you must use the Windows API on Windows if you want the exact behavior as malloc().

[cpjust]

Nope, what I want is to create my own malloc; a function that does the same as malloc, and not some kind of a function that also creates extra space which might not be in use.

But why do you need to create your own malloc()? What are you trying to do that makes you think you need your own malloc()?

If you don't want to call an OS API, your only other option is to create a big global or static variable and start slicing it up when someone calls your malloc()...

[grumpy]

But why do you need to create your own malloc()? What are you trying to do that makes you think you need your own malloc()?

Good question. The only reason I can think of is a wish for additional features (eg tracking allocations and deallocations)

If you don't want to call an OS API, your only other option is to create a big global or static variable and start slicing it up when someone calls your malloc()...

Which, if you think about it, is essentially what the OS API functions do, albeit on a grander scale; talk about recreating a wheel.

[eXeCuTeR]

But why do you need to create your own malloc()? What are you trying to do that makes you think you need your own malloc()?

If you don't want to call an OS API, your only other option is to create a big global or static variable and start slicing it up when someone calls your malloc()...

It's not that I'm gonna use my own malloc now, it's just that I've been given this challenge and I'm trying to solve it now.
---
Well, so, you're saying that using HeapCreate, and HeapAlloc, etc', is the only way? If so, it's pretty stupid to give such challenge which can only be solved using API functions.