What is the purpose of MAP_ANONYMOUS flag in mmap system call?

Question

From the man page,

MAP_ANONYMOUS
              The mapping is not backed by any file; its contents are initialized to zero.  The fd and offset a         


        

Answer 1

Anonymous mappings can be pictured as a zeroized virtual file. Anonymous mappings are simply large, zero-filled blocks of memory ready for use. These mappings reside outside of the heap, thus do not contribute to data segment fragmentation.

MAP_ANONYMOUS + MAP_PRIVATE:

• every call creates a distinct mapping
• children inherit parent's mappings
• childrens' writes on the inherited mapping are catered in copy-on-write manner
• the main purpose of using this kind of mapping is to allocate a new zeroized memory
• malloc employs anonymous private mappings to serve memory allocation requests larger than MMAP_THRESHOLD bytes.
  typically, MMAP_THRESHOLD is 128kB.

MAP_ANONYMOUS + MAP_SHARED:

• each call creates a distinct mapping that doesn't share pages with any other mapping
• children inherit parent's mappings
• no copy-on-write when someone else sharing the mapping writes on the shared mapping
• shared anonymous mappings allow IPC in a manner similar to System V memory segments, but only between related processes

On Linux, there are two ways to create anonymous mappings:

• specify MAP_ANONYMOUS flag and pass -1 for fd

      addr = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0); 
      if (addr == MAP_FAILED)
          exit(EXIT_FAILURE);  
  

• open /dev/zero and pass this opened fd

      fd = open("/dev/zero", O_RDWR);   
      addr = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
  

  (this method is typically used on systems like BSD, that do not have MAP_ANONYMOUS flag)

Advantages of anonymous mappings:
- no virtual address space fragmentation; after unmapping, the memory is immediately returned to the system
- they are modifiable in terms of allocation size, permissions and they can also receive advice just like normal mappings
- each allocation is a distinct mapping, separate from global heap

Disadvantages of anonymous mappings:
- size of each mapping is an integer multiple of system's page size, thus it can lead to wastage of address space
- creating and returning mappings incur more overhead than that of from the pre-allocated heap

if a program containing such mapping, forks a process, the child inherits the mapping. The following program demonstrates this kinda inheritance:

#ifdef USE_MAP_ANON
#define _BSD_SOURCE
#endif  
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    /*Pointer to shared memory region*/    
    int *addr;   

#ifdef USE_MAP_ANON      /*Use MAP_ANONYMOUS*/           
     addr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);     
     if (addr == MAP_FAILED) {     
         fprintf(stderr, "mmap() failed\n");     
         exit(EXIT_FAILURE);
     }      

#else        /*Map /dev/zero*/     
    int fd;    
    fd = open("/dev/zero", O_RDWR);      
    if (fd == -1) {    
        fprintf(stderr, "open() failed\n");
        exit(EXIT_FAILURE);
    }    

    addr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);    
    if (addr == MAP_FAILED) {    
        fprintf(stderr, "mmap() failed\n");    
        exit(EXIT_FAILURE);    
    }     

    if (close(fd) == -1) {          /*No longer needed*/    
        fprintf(stderr, "close() failed\n");    
        exit(EXIT_FAILURE);    
    }
#endif    
    *addr = 1;      /*Initialize integer in mapped region*/    

    switch(fork()) {        /*Parent and child share mapping*/     
    case -1:    
        fprintf(stderr, "fork() failed\n");
        exit(EXIT_FAILURE);    

    case 0:         /*Child: increment shared integer and exit*/     
        printf("Child started, value = %d\n", *addr);    
        (*addr)++;    

        if (munmap(addr, sizeof(int)) == -1) {    
            fprintf(stderr, "munmap()() failed\n");    
            exit(EXIT_FAILURE);    
        }     
        exit(EXIT_SUCCESS);     

    default:        /*Parent: wait for child to terminate*/      
        if (wait(NULL) == -1) {    
            fprintf(stderr, "wait() failed\n");    
            exit(EXIT_FAILURE);      
        }     

        printf("In parent, value = %d\n", *addr);         
        if (munmap(addr, sizeof(int)) == -1) {       
            fprintf(stderr, "munmap()() failed\n");      
            exit(EXIT_FAILURE);       
        }        
        exit(EXIT_SUCCESS);
}

Sources:
The Linux Programming Interface
Chapter 49: Memory Mappings,
Author: Michael Kerrisk

Linux System Programming (3rd edition)
Chapter 8: Memory Management,
Author: Robert Love