boost interprocess message_queue and fork - c++

Question

I am trying to communicate with forked child process using message queue from boost interprocess library. When child process calls receive it causes exception with message
boost::interprocess_exception::library_error
I am using GCC 6.3 on Debian 9 x64.
#include <iostream>
#include <unistd.h>
#include <boost/interprocess/ipc/message_queue.hpp>
#include <memory>
int main(int argc, char* argv[])
{
using namespace boost::interprocess;
const char* name = "foo-552b8ae9-6037-4b77-aa0d-d4dc9dad790b";
const int max_num_msg = 100;
const int max_msg_size = 32;
bool is_child = false;
message_queue::remove(name);
auto mq = std::make_unique<message_queue>(create_only, name, max_num_msg, max_msg_size);
auto child_pid = fork();
if (child_pid == -1)
{
std::cout << "fork failed" << std::endl;
return -1;
}
else if (child_pid == 0)
{
is_child = true;
}
if (is_child)
{
// does child needs to reopen it?
mq.reset( new message_queue(open_only, name) );
}
int send_num = 0;
while(true)
{
unsigned int priority = 0;
if (is_child)
{
message_queue::size_type bytes = 0;
try
{
int num;
// Always throws. What is wrong ???????
mq->receive(&num, sizeof(num), bytes, priority);
std::cout << num << std::endl;
}
catch(const std::exception& e)
{
std::cout << "Receive caused execption " << e.what() << std::endl;
}
sleep(1);
}
else
{
mq->send(&send_num, sizeof(send_num), priority);
send_num++;
sleep(5);
}
}
return 0;
}
Also, in child process is it required to reopen the message queue created by the parent process? I tried it both ways and neither worked. I am getting the same exception on receive.

Answer 1

The problem is that your receive buffer is smaller than max_msg_size. Assuming 4-byte integers, this should work:
int num[8];
mq.receive(num, sizeof(num), bytes, priority);
std::cout << *num << std::endl;
Also, I see no reason to play fast and loose with the actual queue instance. Just create it per process:
#include <boost/interprocess/ipc/message_queue.hpp>
#include <boost/exception/diagnostic_information.hpp>
#include <iostream>
#include <memory>
#include <unistd.h>
int main() {
namespace bip = boost::interprocess;
const char *name = "foo-552b8ae9-6037-4b77-aa0d-d4dc9dad790b";
{
const int max_num_msg = 100;
const int max_msg_size = 32;
bip::message_queue::remove(name);
bip::message_queue mq(bip::create_only, name, max_num_msg, max_msg_size);
}
auto child_pid = fork();
if (child_pid == -1) {
std::cout << "fork failed" << std::endl;
return -1;
}
bip::message_queue mq(bip::open_only, name);
if (bool const is_child = (child_pid == 0)) {
while (true) {
unsigned int priority = 0;
bip::message_queue::size_type bytes = 0;
try {
int num[8];
mq.receive(num, sizeof(num), bytes, priority);
std::cout << *num << std::endl;
} catch (const bip::interprocess_exception &e) {
std::cout << "Receive caused execption " << boost::diagnostic_information(e, true) << std::endl;
}
sleep(1);
}
} else {
// parent
int send_num = 0;
while (true) {
unsigned int priority = 0;
mq.send(&send_num, sizeof(send_num), priority);
send_num++;
sleep(5);
}
}
}