How to correctly allocate a large chunk of RAM with calloc/malloc in Windows 10?

Question

I need to allocate memory for a vector with n=10^9 (1 billion) rows using calloc or malloc but when I try to allocate this amount of memory the system crashes and returns me NULL, which I presumed to be the system not allowing me to allocate this big chunk of memory. I'm using Windows 10 in a 64-bit platform with 16 GB RAM. However, when I ran the same code in a Linux OS (Debian) the system actually allocated the amount I demanded, so now I'm wondering:

How can I allocate this big chunk using Windows 10, once I'm out of time to venture in Linux yet?

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

int main(void) {

    uint32_t * a = calloc(1000000000, 4);
    printf("a = %08x\n", a);

    return 0;
}

Answer 1

The C run time won't let you do this but windows will, use the VirtualAlloc API instead of calloc. Specify NULL for the lpAddress parameter, MEM_COMMIT for flAllocationType and PAGE_READWRITE for flProtect. Also note that even though dwSize uses the "dw" decoration that is usually DWORD in this case the parameter is actually a SIZE_T which is 64-bit for 64-bit builds.

Code would look like:

#include <windows.h>

...

LPVOID pResult = VirtualAlloc(NULL, dwSize, MEM_COMMIT, PAGE_READWRITE);
if(NULL == pResult)
{ /* Handle allocation error */ }

Where dwSize is the number of bytes of memory you wish to allocate, you later use VirtualFree to release the allocated memory : VirtualFree(pResult, 0, MEM_RELEASE);

The dwSize parameter to VirtualFree is for use when you specify MEM_DECOMMIT (rather than MEM_RELEASE), allowing you to put memory back in the reserved but uncommitted state (meaning that actual pages have not yet been found to satisfy the allocation).