User-space memory layout

Linux employs a lazy allocation strategy for user space, only mapping physical pages of memory when the program accesses it. For example, allocating a buffer of 1 MB using malloc(3) returns a pointer to a block of memory addresses but no actual physical memory. A flag is set in the page table entries such that any read or write access is trapped by the kernel. This is known as a page fault. Only at this point does the kernel attempt to find a page of physical memory and add it to the page table mapping for the process. Let’s demonstrate this with a simple program from MELD/Chapter18/pagefault-demo:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/resource.h>
#define BUFFER_SIZE (1024 * 1024)
void print_pgfaults(void)
{
    int ret;
    struct rusage usage;
    ret = getrusage(RUSAGE_SELF, &usage);
    if (ret == -1) {
        perror("getrusage");
    } else {
        printf("Major...