problem with memcpy'ing from shared memory in boost.interprocess

Question

This is driving me wild with frustration. I am just trying to create a shared memory buffer class that uses in shared memory created through Boost.Interprocess where I can read/store data. I wrote the following to test the functionality

#include <boost/interprocess/shared_memory_object.hpp>
#include <boost/interprocess/mapped_region.hpp>
#include <iostream>
using namespace std;
using namespace boost::interprocess;

int main( int argc, char* argv[] ) {
    shared_memory_object::remove( "MyName" );
    // Create a shared memory object
    shared_memory_object shm ( create_only, "MyName", read_write );
    // Set size for the shared memory region
    shm.truncate(1000);
    // Map the whole shared memory in this process
    mapped_region region(shm, read_write);
    // Get pointer to the beginning of the mapped shared memory region
    int* start_ptr;
    start_ptr = static_cast<int*>(region.get_address());

    // Write data into the buffer
    int* write_ptr = start_ptr;
    for( int i= 0; i<10; i++ ) {
        cout << "Write data: " << i << endl;
        memcpy( write_ptr, &i, sizeof(int) );
        write_ptr++;
    }

    // Read data from the buffer
    int* read_ptr = start_ptr;
    int* data;
    for( int i= 0; i<10; i++ ) {
        memcpy( data, read_ptr, sizeof(int) );
        cout << "Read data: " << *data << endl;
        read_ptr++;
    }

    shared_memory_object::remove( "MyName" );
    return 0;
}

When I run this, it writes the data OK, but segfaults on the first memcpy in the read loop. gdb says the following:

Program received signal EXC_BAD_ACCESS, Could not access memory. Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000 0x00007fffffe007c5 in __memcpy ()

(gdb) where

#0 0x00007fffffe007c5 in __memcpy () #1 0x0000000100000e45 in main (argc=1, argv=0x7fff5fbff9d0) at try.cpp:36

The functionality is so simple, I don't know what I'm missing. Any help will be much appreciated.

Answer 1

data isn't being set to point at anything. (Make sure the program is being compiled with all warnings enabled.) It looks like it shouldn't be a pointer anyway.

The second loop should perhaps be:

int* read_ptr = start_ptr;
int data;
for( int i= 0; i<10; i++ ) {
    memcpy( &data, read_ptr, sizeof(int) );
    cout << "Read data: " << data << endl;
    read_ptr++;
}

Answer 2

I can't test it here because I don't have boost available but I a have guess. In this example, the shared_memory_object object is first used to write with a flag create_only.

shared_memory_object shm (create_only, "MySharedMemory", read_write);

Then it is read with a second shared_memory_object object with a flag open_only:

shared_memory_object shm (open_only, "MySharedMemory", read_only);

It seems you have to change your shared_memory_object to a proper flag.