Shared memory for communicating between two programs

[Man4ish]

Hi,

I am working on shared memory to write the map by one program and using the same map by other program. Is it feasible to do that with shared memory. Is there any source code or documentation doing the same.i will be really thankful as i am in great need.

[JosAH]

@Man4ish
It depends on what you want to do exactly and which implementation of the shared memory package you're using. Most implementations have some form of locking and/or event dispatching available, but they're all different. Google is your friend here.

kind regards,

Jos

[Banfa]

My experience using shared memory to communicate has been using dual port RAM to communicate between separate processors. Dual port RAM actually has 2 address and 2 data buses so both processors can connect simultaneously to any memory location.

As with all shared memory this has the problem of making sure you don't read while the other processor is writing. However unlike a software implementation on a single processor there is no locking mechanism between 2 processors (on the whole).

We got round that but implementing a strict state machine that ran on both sides of the RAM, a few memory locations were reserved as control bytes and using these it was possible to implement a locking mechanism that allowed effective communication through the RAM.

The whole point of this storyu is that if it turns out you have no effective locking/event dispatch available in the software solution you find you could always do the same thing.

[weaknessforcats]

This is usually done by using separate processes.

The process with the map is the server process and the processes that use the map are the client processes.

These processes may be separate programs or subprocesses created by a program.

[Man4ish]

I searched in google in as suggested........

    char c;
    int shmid;
    key_t key;
    char *shm, *s;
 
    /*
     * We'll name our shared memory segment
     * "123321".
     */
    key = 123321;
 
    /*
     * Create the segment.
     */
    if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0) {
        perror("shmget");
        exit(1);
    }
 
    /*
     * Now we attach the segment to our data space.
     */
    if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
        perror("shmat");
        exit(1);
    }
 
    /*
     * Now put some things into the memory for the
     * other process to read.
     */
  // map <string, string>
    s = shm;
    strcpy(s,"Bytes  ....");
 
It is working fine for client process and if i print the value of string child process, I am able to print the mesage "Bytes ..... ". But now i am moving step ahead to store the values in map and print the value in child process.
map<int, string> shm;
 
 if ((shm =  shmat(shmid, NULL, 0)) == (char *) -1) {
        perror("shmat");
        exit(1);
    }
 
no match for ‘operator=’ in ‘shm = shmat(shmid, 0u, 0)’
/usr/lib/gcc/ia64-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_map.h:225: note: candidates are: std::map<_Key, _Tp, _Compare, _Alloc>& std::map<_Key, _Tp, _Compare, _Alloc>::operator=(const std::map<_Key, _Tp, _Compare, _Alloc>&) [with _Key = int, _Tp = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >]
 
map <int, string> mapregion;
    mapregion = shm;
    mapregion[1] = "My name is manish";

I think map is not allowed , Then how map can be implemented for such process.Thanks in advance.

[newb16]

You can't assign string ( or char* ) to map - you ned to insert it like
shm[whatever] = shmat(shmid, NULL, 0);
like you do it below with mapregion[1]

[Man4ish]

@newb16
still map is not stored in memory, it will generate a map but it will not be accessible by other process(program).

[newb16]

You can't put class in the shared memory - it can create some internal structures and reference them by pointers, and you can't force it to place them in the shared memory. Even if you write custom allocator, values of pointers to the shared memory will be different for the different processes that share it.

[weaknessforcats]

When you use shared processes, each process has its own memory space so that pointers and such ar valid only in the process that creates them.

Therefore: Do not try to directly access structures in a process from another process.

Instead, you pass a query to your process with the map. That process does the lookup and passes back the results of the query. (called marshaling).

If you were using Windows, there's a nice section onhow to do this is Jeffrey Richter's Windows via C/C++.