c++ - Allocating more memory than there exists using malloc

Question

This code snippet will allocate 2Gb every time it reads the letter 'u' from stdin, and will initialize all the allocated chars once it reads 'a'.

#include <iostream>
#include <stdlib.h>
#include <stdio.h>
#include <vector>
#define bytes 2147483648
using namespace std;
int main()
{
    char input [1];
    vector<char *> activate;
    while(input[0] != 'q')
    {
        gets (input);
        if(input[0] == 'u')
        {
            char *m = (char*)malloc(bytes);
            if(m == NULL) cout << "cant allocate mem" << endl;
            else cout << "ok" << endl;
            activate.push_back(m);
        }
        else if(input[0] == 'a')
        {
            for(int x = 0; x < activate.size(); x++)
            {
                char *m;
                m = activate[x];
                for(unsigned x = 0; x < bytes; x++)
                {
                    m[x] = 'a';
                }
            }
        }
    }
    return 0;
}

I am running this code on a linux virtual machine that has 3Gb of ram. While monitoring the system resource usage using the htop tool, I have realized that the malloc operation is not reflected on the resources.

For example when I input 'u' only once(i.e. allocate 2GB of heap memory), I don't see the memory usage increasing by 2GB in htop. It is only when I input 'a'(i.e. initialize), I see the memory usage increasing.

As a consequence, I am able to "malloc" more heap memory than there exists. For example, I can malloc 6GB(which is more than my ram and swap memory) and malloc would allow it(i.e. NULL is not returned by malloc). But when I try to initialize the allocated memory, I can see the memory and swap memory filling up till the process is killed.

-My questions:

1.Is this a kernel bug?

2.Can someone explain to me why this behavior is allowed?

Answer 1

It is called memory overcommit. You can disable it by running as root:

 echo 2 > /proc/sys/vm/overcommit_memory

and it is not a kernel feature that I like (so I always disable it). See malloc(3) and mmap(2) and proc(5)

^{NB: echo 0 instead of echo 2 often -but not always- works also. Read the docs (in particular proc man page that I just linked to).}