How to get available memory C++/g++?

I want to allocate my buffers according to memory available. Such that, when I do processing and memory usage goes up, but still remains in available memory limits. Is there a way to get available memory (I don't know will virtual or physical memory status will make any difference ?). Method has to be platform Independent as its going to be used on Windows, OS X, Linux and AIX. (And if possible then I would also like to allocate some of available memory for my application, someone it doesn't change during the execution).

Edit: I did it with configurable memory allocation. I understand it is not good idea, as most OS manage memory for us, but my application was an ETL framework (intended to be used on server, but was also being used on desktop as a plugin for Adobe indesign). So, I was running in to issue of because instead of using swap, windows would return bad alloc and other applications start to fail. And as I was taught to avoid crashes and so, was just trying to degrade gracefully.

Solution 1:

On UNIX-like operating systems, there is sysconf.

#include <unistd.h>

unsigned long long getTotalSystemMemory()
{
    long pages = sysconf(_SC_PHYS_PAGES);
    long page_size = sysconf(_SC_PAGE_SIZE);
    return pages * page_size;
}

On Windows, there is GlobalMemoryStatusEx:

#include <windows.h>

unsigned long long getTotalSystemMemory()
{
    MEMORYSTATUSEX status;
    status.dwLength = sizeof(status);
    GlobalMemoryStatusEx(&status);
    return status.ullTotalPhys;
}

So just do some fancy #ifdefs and you'll be good to go.

Solution 2:

There are reasons to do want to do this in HPC for scientific software. (Not game, web, business or embedded software). Scientific software routinely go through terabytes of data to get through one computation (or run) (and run for hours or weeks) -- all of which cannot be stored in memory (and if one day you tell me a terabyte is standard for any PC or tablet or phone it will be the case that the scientific software will be expected to handle petabytes or more). The amount of memory can also dictate the kind of method/algorithm that makes sense. The user does not always want to decide the memory and method - he/she has other things to worry about. So the programmer should have a good idea of what is available (4Gb or 8Gb or 64Gb or thereabouts these days) to decide whether a method will automatically work or a more laborious method is to be chosen. Disk is used but memory is preferable. And users of such software are not encouraged to be doing too many things on their computer when running such software -- in fact, they often use dedicated machines/servers.