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

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 arguments are ignored; however, some implementations require
fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable applications should ensure this. The use of MAP_ANONYMOUS in conjunction with
MAP_SHARED is only supported on Linux since kernel 2.4.
What is the purpose of using MAP_ANONYMOUS? Any example would be good. Also From where the memory will be mapped?
It is written on man page that The use of MAP_ANONYMOUS in conjunction with MAP_SHARED is only supported on Linux since kernel 2.4.
How can i share the memory mapped with MAP_ANONYMOUS with other process?

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