Can't read complete string when writing into IPC SHM in c++ - c++

Question

I'm attempting to build a simple interface to use shm ipc in c++. For that, I've written the following code:
sharedmem.h:
#pragma once
#include <iostream>
#include <sstream>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
using namespace std;
namespace IPC
{
void Send(const string filename, std::string msg,int size=1024 )
{
key_t key = ftok(filename.c_str(),65);
std::stringstream ss;
ss << msg.c_str();
int shmid = shmget(key,size,0666|IPC_CREAT);
char *str = (char*) shmat(shmid,(void*)0,0);
ss >> str;
shmdt(str);
}
string Receive(const string filename, int size=1024 )
{
key_t key = ftok(filename.c_str(),65);
int shmid = shmget(key,size,0666|IPC_CREAT);
char *str = (char*) shmat(shmid,(void*)0,0);
string ret(str);
shmdt(str);
shmctl(shmid,IPC_RMID,NULL);
return ret;
}
};
Outside, I use it like:
sender.cpp
#include "sharedmem.h"
int main()
{
IPC::Send("fila1", "hello ipc");
return 0;
}
receiver.cpp
#include "sharedmem.h"
int main()
{
std::string ret = IPC::Receive("fila1");
cout << "Recebi na fila: " << ret;
return 0;
}
CMakeLists.txt:
set (CMAKE_CXX_STANDARD 17)
add_executable(sender sender.cpp)
add_executable(receiver receiver.cpp)
and built with cmake . && make
In this example I write "hello ipc" but the other process reads only "hello". What could be wrong here? Thanks in advance.

Answer 1

In your send function:
void Send(const string filename, std::string msg,int size=1024 )
{
key_t key = ftok(filename.c_str(),65);
std::stringstream ss;
ss << msg.c_str();
int shmid = shmget(key,size,0666|IPC_CREAT); // this call could fail, what happens next is
// a likely a segmentation error.
// ... or worse.
char *str = (char*) shmat(shmid,(void*)0,0);
ss >> str; // <-- error is here. You extract from ss until the first whitespace character.
// what happens if input string is larger than the size of the allocated block?
shmdt(str);
}
The stringstream ss has no functional use in your function, except for adding a bug. I suggest you try this instead:
int Send(const string& filename, const std::string& msg) noexcept // if you have no return value,
// you should throw on error,
// let's avoid that
{
key_t key = ftok(filename.c_str(), 65); // you should maybe consider using a named constant
// for your project ID
if (key == -1)
return errno;
int shmid = shmget(key, msg.length() + 1, 0666 | IPC_CREAT); // allocate enough memory for the
// message, plus its NULL terminator
if (shmid == -1)
return errno;
void *shared_mem = shmat(shmid, nullptr, 0);
if (shared_mem == (void*)-1)
{
// the systeml failed to lock the allocated memory.
// do some cleanup by de-allocating the shared memory block.
int ret = errno; // keep original error for return.
shmctl(shmid , IPC_RMID, nullptr);
return ret;
}
// copy message string with its NULL terminator to shared memory
memcpy(shared_mem, msg.c_str(), msg.length() + 1); // using length() + 1 is ok here, result of
// c_str() always has a NULL terminator.
shmdt(shared_mem);
return 0;
}
Your receive function also lacks in error checking. That should be very similar to the Send() function.
Note that the strings are passed by const reference, that's to avoid copying them (and the potential errors associated with those unneeded memory allocations)