Re: allocate memory in userspace

[William Lee Irwin III]

On Mon, Jul 01, 2002 at 11:49:13AM -0600, Erik Andersen wrote:
> void *malloc(size_t size)
> {
>     void *result;
>     if (size == 0)
> 	return NULL;
>     result = mmap((void *) 0, size + sizeof(size_t), PROT_READ | PROT_WRITE, 
> 	    MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
>     if (result == MAP_FAILED)
> 	return 0;
>     * (size_t *) result = size;
>     return(result + sizeof(size_t));
> }

This looks like a very bad idea. Userspace allocators should make some
attempt at avoiding diving into the kernel at every allocation like this.
Also, they should be intelligent enough to get around TASK_UNMAPPED_BASE.
Wilson's allocator survey has an excellent discussion of the issues.
Doug Lea has a better example of a userspace memory allocator.

Cheers,
Bill