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

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.

There is no platform independent way to do this, different operating systems use different memory management strategies.

These other stack overflow questions will help:

• How to get memory usage at run time in c++?
• C/C++ memory usage API in Linux/Windows

You should watch out though: It is notoriously difficult to get a "real" value for available memory in linux. What the operating system displays as used by a process is no guarantee of what is actually allocated for the process.

This is a common issue when developing embedded linux systems such as routers, where you want to buffer as much as the hardware allows. Here is a link to an example showing how to get this information in a linux (in C):

• http://www.unix.com/programming/25035-determining-free-available-memory-mv-linux.html

Having read through these answers I'm astonished that so many take the stance that OP's computer memory belongs to others. It's his computer and his memory to do with as he sees fit, even if it breaks other systems taking a claim it. It's an interesting question. On a more primitive system I had memavail() which would tell me this. Why shouldn't the OP take as much memory as he wants without upsetting other systems?

Here's a solution that allocates less than half the memory available, just to be kind. Output was:

Required FFFFFFFF

Required 7FFFFFFF

Required 3FFFFFFF

Memory size allocated = 1FFFFFFF

#include <stdio.h>
#include <stdlib.h>

#define MINREQ      0xFFF   // arbitrary minimum

int main(void)
{
    unsigned int required = (unsigned int)-1; // adapt to native uint
    char *mem = NULL; 
    while (mem == NULL) {
        printf ("Required %X\n", required);
        mem = malloc (required);
        if ((required >>= 1) < MINREQ) {
            if (mem) free (mem);
            printf ("Cannot allocate enough memory\n");
            return (1);
        }
    }

    free (mem);
    mem = malloc (required);
    if (mem == NULL) {
        printf ("Cannot enough allocate memory\n");
        return (1);
    }
    printf ("Memory size allocated = %X\n", required);
    free (mem);
    return 0;
}

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.