Introduction To mmap(): Anonymous Memory

mmap(): An Anonymous (Private) Mapping

See also

• Lazy Allocation, Demand Paging, And Overcommit

Documentation

• man -s 2 mmap

• Building the situation from Lazy Allocation, Demand Paging, And Overcommit …

  • Mapping (contents of it) not backed from anywhere (a file, say) ⟶ anonymous mapping (MAP_ANONYMOUS)

  • Mapping is not visible in another address space/process ⟶ private mapping (MAP_PRIVATE)

  • We want the allocated memory to be readble and writable ⟶ PROT_READ|PROT_WRITE

mmap-anon.cpp

#include <sys/mman.h>
#include <stddef.h>
#include <stdio.h>

int main()
{
    void* memory = mmap(
        NULL,                                          // <-- (addr) let kernel choose address
        16*1024*1024,                                  // <-- (length) 16 MiB *virtually contiguous* memory
        PROT_READ|PROT_WRITE,                          // <-- (prot) memory access protection
        MAP_ANONYMOUS|MAP_PRIVATE,                     // <-- (flags) anon, copy-on-write (we don't share anyway though)
        -1,                                            // <-- (fd) no fd, this is an anonymous mapping
        0                                              // <-- (offset) no offset, this is an anonymous mapping
    );
    if (memory == MAP_FAILED) {
        perror("mmap");
        return 1;
    }

    *(((char*)memory)+16) = 'a';                       // <-- access memory in first page of mapping
                                                       //     will cause page to be allocated

    munmap(memory, 16*1024*1024);                      // <-- technically not necessary; done anyway at exit

    return 0;
}

Demo: /proc/PID/maps, /proc/PID/smaps

Documentation

• man -s 5 proc

• man -s 2 getpid

• Modify above program to …

  • output its PID

  • wait before and after memory access

mmap-anon-interactive.cpp

cout << getpid() << '\n';
...
cin.get();
*(((char*)memory)+16) = 'a';
cin.get();
...