Re: [Boost-users] shmem - Allocating enough space. open and allocate.

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2006-10-09 15:55:23

Hi Andrea,

> The domain of the application is centered on the passing of image frames
> from process that acquires the images to the client process via a local
> shared memory segment.

Ok.

> If I open the segment with the exact number of bytes needed:
> 
>>    const int memsize = 320*240;
>>    //Create shared memory
>>    if(!segment.open_or_create(shMemName, memsize)){
>>    	std::cout << "error creating shared memory\n";
>>    	return -1;
>>    };
> 
> Then I cannot use:
> 
>>    unsigned char* uchar_ptr = (unsigned char*)segment.allocate(num_elements);

Because the shared memory segment is not a trivial shm_open + mmap-like 
shared memory. It has dynamic allocation, a named parameter index 
type... so it wastes memory. The size passed in open is the shared 
memory size (well, more or less). Each allocate() call needs to keep 
track of the allocated size, to be able to deallocate it when the user 
calls deallocate().

In short, there is no way to know beforehand how much space it will be 
wasted in bookkeeping, since it depends on the number of allocations, 
the alignment, the number of named parameters, their length, the type of 
index...

If you need a raw, classic shared memory, I would recommend you to use 
Boost.Interprocess:

http://boost-consulting.com/vault/index.php?&direction=0&order=&directory=Concurrent%20Programming

Interprocess is the official Boost name for Shmem. I surely will remove 
Shmem from the vault soon because I want people to use Interprocess. In 
interprocess you can create a classic shared memory segment like this:

http://tinyurl.com/z5gyn

Regards,

Ion