print shared memory address value to command line

Question

I'm attempting to use boost's shared memory library to do some inter-process communication (VS 2015). I found an example online which is very helpful. For sanity's sake I just want to perform a simple check that the value I wrote to the shared memory address is what I wanted. To do this I want to print the value of the shared memory using cout. This is the code I have currently:

#include <boost\interprocess\shared_memory_object.hpp>
#include <boost\interprocess\mapped_region.hpp>
#include <iostream>
#include <stdio.h>
#include <conio.h>
#include <cstring>
#include <cstdlib>
#include <string>

int main()
{
    using namespace boost::interprocess;

    struct shm_remove
    {
        shm_remove() { shared_memory_object::remove("MySharedMemory"); }
        ~shm_remove() { shared_memory_object::remove("MySharedMemory"); }
    } remover;

    //Create a shared memory object
    shared_memory_object shm(create_only, "MySharedMemory", read_write);

    //Set size to 1
    shm.truncate(1);

    //Map the whole shared memory in this process
    mapped_region region(shm, read_write);

    //Write all the memory to 1
    std::memset(region.get_address(), 1, region.get_size());

    //Check that memory was initialized to 1
    char *mem = static_cast<char*>(region.get_address());

    for (std::size_t i = 0; i < region.get_size(); ++i)
    {
        std::cout << "Memory value: " << *mem << "\n";
        if (*mem++ != 1)
        {
            return 1;   //Error checking memory
        }
    }
    std::cout << "press any key to quit";
    _getch();
}

The code works fine, no errors are thrown when it checks that the mapped memory has been set to 1. However when I try to print what I think should be the value at the address, I get a smiley face...

Can anyone point me in the right direction? I have some suspicions (no terminating \0?) but I really don't understand the inner workings here. Any help is appreciated!

Try `std::cout << "Memory value: " << (int) *mem << "\n";` – Jabberwocky Apr 27 '16 at 14:44 — Jabberwocky, Apr 27 '16 at 14:44

score 3 · Accepted Answer · edited May 23 '17 at 12:16

3

Cast mem to int explicitly, then std::cout is going to output the value as a number, not as a letter corresponding to the ASCII code stored inmem (which may be unprintable or look funny, the latter being your case).

See cout not printing unsigned char for a more thorough answer on the problem.

edited May 23 '17 at 12:16

Community

1
1

answered Apr 27 '16 at 14:47

iksemyonov

4,106
1
22
42

Thanks for the explanation! – willpower2727 Apr 27 '16 at 14:54

score 2 · Answer 2 · answered Apr 27 '16 at 14:43

2

The console shows you the ASCII representation of bytes in memory. Any character under 13 is usually unprintable. Try setting the memory to something like 67.

answered Apr 27 '16 at 14:43

Benjamin Racette

172
6

You were right, it was printing ASCII rep of the bytes, I tested with 67 and it printed "C." +1 But I was looking for 1, I didn't realize exactly what I was trying to get. Thanks! – willpower2727 Apr 27 '16 at 14:55
A is 65, B is 66 so 67 = C is great! Your code works fine :) – Benjamin Racette Apr 27 '16 at 14:56

print shared memory address value to command line

2 Answers2