Using memory pools

[mukulabdagiri]

How can i use memory pool with a char array?

Is there a way to allocate memory somewhere in the middle of the array?
i.e. allocate memory in chunks.
eg. allocate 24 bytes at location arr[56] in 256 byte array.

Similarly how can i deallocate that memory?

Any help is appreciated.

Thnx!

[AbstractionAnon]

No standard library method of doing that. You certainly can write your own functions to do so. This might help get you started:

class Chunk
{ const int size = 256;  // fixed size of character array
   char m_mem[size];   // memory being allocated
   bitset<size> m_bits;  // bit array to keep track of what's allocated
public:
    char * Allocate (int nbytes);
    void Release (char * mem, int nbytes);
};

[Thumper]

AbstractionAnon wrote:

No standard library method of doing that.

That's not entirely true. std::allocator is used for allocating memory. Not exactly like the OP wants, but it can be done.

Just a rough idea:

std::allocator<unsigned char> alloc;
unsigned char *ptr = alloc.allocate(256) //allocate space for 256 chars (chars are 1 byte in size). 
//returns a pointer to the location of the first allocated memory space.

//deallocate
alloc.deallocate(ptr, 256);

Read up on std::allocator here:
http://www.cplusplus.com/reference/memory/allocator/?kw=allocator

[mukulabdagiri]

Thanks. Is the use of memory pools only to save system calls like malloc/new, because otherwise the memory is always going to get allocated in chunks like you mentioned above:

char * Allocate (int nbytes);

and for releasing that memory, i will have to keep track of what sized chunk was allocated and the pointer to that location:

void Release (char * mem, int nbytes);

So if i allocate memory multiple times and deallocate it separately, how do i keep track of the chunk size every time? Do i have to save that data in some object, like obj.ptr and obj.dataSize?

[andywestken]

Boost includes a pool

Boost.Pool - 1.53.0
http://www.boost.org/doc/libs/1_53_0/libs/pool/doc/html/index.html

Andy

[Thumper]

mukuladbagiri wrote:

Do i have to save that data in some object

That's correct. You're responsible for keeping track of how much you've allocated.

@andywestken
I didn't know boost had that. Awesome :D

[mukulabdagiri]

Thanks for the answers. On similar lines, i was curious to know if there is a way to avoid fragmentation if memory is allocated and deallocated frequently. Right now I am using best fit technique to allocate memory. ie. i look for smallest memory block available in the pool, but it still leaves holes in the pool.

Thnx,
Mukul

[andywestken]

Hmmm... maybe it's too involved a question for here?

But there is quite a bit about the web, if you search for "how to design a low fragmentation heap", "heap allocation strategies", and the like.

While I'm here, this article might be of interest?

A short puzzle about heap expansion
http://blogs.msdn.com/b/oldnewthing/archive/2010/04/29/10004218.aspx

Andy