[SOLVED] fork(), shared memory and pointers in C

Cristiano Urban · 10-30-2019, 10:26 AM

Hi folks,
I'm trying to share a struct like the one below among the various processes:

Code:

// Data to be shared among processes
struct Shared_data {
    int setPointCounter;
    struct Point old_point;
    pthread_mutex_t my_mutex;
} *shd;

The struct is declared as global (it is located before the main(), let's say...).

I initialize the shared memory into the main:

Code:

shd = (struct Shared_data *) (mmap(NULL, sizeof *shd, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0));

and then at a certain point I do a fork(). It works fine.

The problem arises when I have to share something like this:

Code:

// Data to be shared among processes
struct Shared_data {
    int setPointCounter;
    struct Point old_point;
    MyObject *obj;
    pthread_mutex_t my_mutex;
} *shd;

In the main function I call a function which returns a pointer to an object of MyObject type and I would like to share it in some way. How can I do this?
By searching on the web I've found something related to relative pointers but I'm not sure it will work.

I'm working on Linux (Slackware 64 14.2) and the language is C/C++ (mostly is C, actually).

Thank you in advance for your suggestions.

NevemTeve · 10-30-2019, 11:18 AM

The pointed object has to be in the shared memory, too. (Note: there is no point in storing pthread_mutex in shared memory: it works only with threads, not with processes.)

Edit: sorry, I have been wrong here regarding mutexes.

Cristiano Urban · 10-30-2019, 12:54 PM

Hi,
thank you for the answer.

Quote:

Originally Posted by NevemTeve

The pointed object has to be in the shared memory, too.

The function in the main which returns the pointer to the object is part of a third-party library. So, I think the only way to access to the object is through a pointer. :-(

Quote:

Originally Posted by NevemTeve

(Note: there is no point in storing pthread_mutex in shared memory: it works only with threads, not with processes.)

In the main I've set the following:

Code:

pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_mutex_init(&shd->my_mutex, &attr);

NevemTeve · 10-30-2019, 01:16 PM

The thing you should use is called deep copy -- if you know the complete structure, and it doesn't contain native resources (files, pipes, sockets, any sort of handles).

Note: Sorry, I was wrong regarding mutexes in post #2

Cristiano Urban · 10-31-2019, 02:50 PM

Quote:

Originally Posted by NevemTeve

The thing you should use is called deep copy -- if you know the complete structure, and it doesn't contain native resources (files, pipes, sockets, any sort of handles).

The class seems to not have a copy constructor and to be honest I would not like to change the code of that library, so I think I will go through another way to solve my problem.

Quote:

Originally Posted by NevemTeve

Note: Sorry, I was wrong regarding mutexes in post #2

Hey! It's ok, don't worry!

Thank you for the hints!

bigearsbilly · 11-07-2019, 03:45 AM

uuugh shared memory, what a hideous concept.

whatever happened to "high cohesion" and "low coupling"?

I have used OpenMP, compile it on linux, compile the same code on windows

https://www.openmp.org/wp-content/up...P3.1-CCard.pdf

Cristiano Urban · 11-07-2019, 05:36 AM

Quote:

Originally Posted by bigearsbilly

https://www.openmp.org/wp-content/up...P3.1-CCard.pdf

What a wonderful cheatsheet!

Thank you for the suggestion but at the end I've solved my problem with the classical "blocking" way (no processes, no threads).

bigearsbilly · 11-07-2019, 08:47 AM

I remember i was on a project before when someone thought sharing memory between processes was a good idea.
So I cut the brake lines on his car and he went over a cliff.
Consequently we had a much simpler design.