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 MiB 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. It is worthwhile demonstrating this with a simple program, MELP/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);
   ...
