CODEWITHSUNDEEP
clinuxshared-memorymmapvirtual-memory
boiler96
boiler96

Reputation: 1177

Linux mapping virtual memory range to existing virtual memory range?

In Linux, is there a way (in user space) to map a virtual address range to the physical pages that back an existing virtual address range? The mmap() function only allows one to map files or "new" physical pages. I need to be able to do something like this:

int* addr1 = malloc(SIZE);
int* addr2 = 0x60000;      // Assume nothing is allocated here
fancy_map_function(addr1, addr2, SIZE);
assert(*addr1 == *addr2);  // Should succeed
assert(addr1 != addr2);    // Should succeed

Upvotes: 3

Views: 3016

Answers (3)

Chris Stratton
Chris Stratton

Reputation: 40357

I was curious so I tested the shared memory idea suggested in question comments, and it seems to work:

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <assert.h>

#define SIZE 256
int main (int argc, char ** argv) {
  int fd;
  int *addr1, *addr2;

  fd = shm_open("/example_shm", O_RDWR | O_CREAT, 0777);
  ftruncate( fd, SIZE);
  addr1 = mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
  addr2 = mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

  printf("addr1 = %p addr2 = %p\n", addr1, addr2);
  *addr1 = 0x12345678;
  assert(*addr1 == *addr2);  // Should succeed
  assert(addr1 != addr2);    // Should succeed

  return 0;
}

(Obviously real code will want to check the return value of the syscalls for errors and clean up after itself)

Upvotes: 3

R.. GitHub STOP HELPING ICE
R.. GitHub STOP HELPING ICE

Reputation: 215259

Open /proc/self/mem and mmap the range of virtual addresses you need from it.

Upvotes: -1

ephemient
ephemient

Reputation: 204718

If you have the fd for the file mapped at addr1, you can simply mmap it again at addr2.

Otherwise, the Linux-specific remap_file_pages can modify the virtual address ⇆ file offset translation within a single VMA, with page-sized granularity, including mapping the same file offset to multiple addresses.

Upvotes: 1

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