Question

I wanna share a pointer of map during 2 processes. So I tried mmap. I tested mmap in a single process. Here is my code:

#include  <vector>
#include  <iostream>
#include  <sys/mman.h>
#include  <unistd.h>
#include  <cstdlib>
#include  <stdio.h>
#include  <map>
using namespace std;

int main(int argc, char *argv[])
{
    map<string,string> a, *b;

    b = (map<string,string> *)mmap(&a,sizeof(map<string,string>),
        PROT_READ|PROT_WRITE,MAP_ANON|MAP_SHARED,-1,0);

    b->insert(map<string,string>::value_type("a","b")); //error 
    cout << b->size() << endl;
}

when it runs to b->insert(), Segmentation fault occured. If I remove b->insert(), there is no error (still have b->size). What’s the problem with my code?

Answer 1

You are allocating new memory with mmap (why? seems like a bad idea…) but you are not initializing your map. Use “placement new” to initialize it.

void *p = mmap(....);
if (p == MAP_FAILED)
    abort();
map<string,string> *b = new(p) map<string,string>();
b->insert(...);

But I suspect something is terribly wrong, and mmap really shouldn’t be involved here…

Edit: From the comments, it sounds like you want to share memory between two processes. Shared memory is probably far beyond your current skill level. You cannot normally place a std::map object inside a shared memory segment, because it will contain references to internal objects on the heap, which will not be shared, unless you can create a custom allocator to only create subobjects inside the shared memory segment.

You can create a shared memory object with shm_open, you can change its size with ftruncate, and you can map it into memory with mmap. Different processes that shm_open the same name will get the same object, and the shared object descriptor can also be passed between processes just like file descriptors.