C++ best way to launch another process? - c++

Its been a while since I've had to do this and in the past I've used "spawn" to create processes.
Now I want to launch processes from my application asynchronously so my application continues to execute in the background and does not get held up by launching the process.
I also want to be able to communicate with the launched processes. When I launch the process I will send it the launchers process id so that the launched process can communicate with the launcher using it's pid.
What is the best method to use that is not specific to any platform / operating system, I'm looking for a solution that is multi-platform?
I'm writing this in C++, I don't want a solution that ties me to any third party licensed product.
I don't want to use threads, the solution must be for creating new processes.

A portable to launch a new process is std::system.
#include <cstdlib>
int main() {
std::system("./myapp");
return 0;
}
if you use linux and you want to share handles/memory between processes, fork is what you are looking for

Try Boost.Process.
Boost.Process provides a flexible framework for the C++ programming language to manage running programs, also known as processes. It empowers C++ developers to do what Java developers can do with java.lang.Runtime/java.lang.Process and .NET developers can do with System.Diagnostics.Process. Among other functionality, this includes the ability to manage the execution context of the currently running process, the ability to spawn new child processes, and a way to communicate with them them using standard C++ streams and asynchronous I/O.
The library is designed in a way to transparently abstract all process management details to the user, allowing for painless development of cross-platform applications. However, as such abstractions often restrict what the developer can do, the framework allows direct access to operating system specific functionality - obviously losing the portability features of the library.
Example code to run and wait to finish for child process from the site:
bp::child c(bp::search_path("g++"), "main.cpp");
while (c.running())
do_some_stuff();
c.wait(); //wait for the process to exit
int result = c.exit_code();

I'll plug my own little (single header) library:
PStreams allows you to run another program from your C++ application and to transfer data between the two programs similar to shell pipelines.
In the simplest case, a PStreams class is like a C++ wrapper for the POSIX.2 functions popen(3) and pclose(3), using C++ iostreams instead of C's stdio library.
The library provides class templates in the style of the standard iostreams that can be used with any ISO C++ compiler on a POSIX platform. The classes use a streambuf class that uses fork(2) and the exec(2) family of functions to create a new process and creates up to three pipes to write/read data to/from the process.

Related

Can fibers migrate between threads?

Can a fiber created in thread A switch to another fiber created in thread B? To make the question more specific, some operating systems have fibers natively implemented (windows fibers),
other need to implement it themselves (using setjump longjump in linux etc.).
Libcoro for example wraps this all up in a single API (for windows it’s just a wrapper for native fibers, for Linux it implements it itself etc.)
So, if it's possible to migrate fibers between threads, can you give me an example usage in windows (linux) in c/c++?
I found something about fiber migration in the boost library documentation, but it's not specific enough about it's implementation and platform dependence. I still want to understand how to do it myself using only windows fibers for example (or using Libcoro on linux).
If it's not possible in a general way, why so?
I understand that fibers are meant to be used as lightweight threads for cooperative multitasking over a single thread, they have cheap context switching compared to regular threads, and they simplify the programming.
An example usage is a system with several threads, each having several fibers doing some kind of work hierarchy on their parent thread (never leaving the parent thread).
Even though it's not the intended use I still want to learn how to do it if it's possible in a general way, because I think I can optimize the work load on my job system by migrating fibers between threads.
The mentioned boost.fiber uses boost.context (callcc/continuation) to implement context switching.
Till boost-1.64 callcc was implemented in assembler only, boost-1.65 enables you to choose between assembler, Windows Fibers (Windows) or ucontext (POSIX if available; deprecated API by POSIX).
The assembler implementation is faster that the other two (2 orders of magnitude compared to ucontext).
boost.fiber uses callcc to implement lightweight threads/fibers - the library provides fiber schedulers that allow to migrate fibers between threads.
For instance one provided scheduler steals fibers from other threads if its run-queue goes out of work (fibers that are ready/that can be resumed).
(so you can choose Windows Fibers that get migrated between threads).

How to load a shared object multiple times independently

I'm tasked with designing a small daemon (on Debian Linux) which will use a blackbox libfoo.so to communicate with an external EFT terminal. There are several, identical EFT terminals (around 100), and one libfoo.so instance can only communicate with a single terminal. There is an init call which essentially binds the instance to a terminal.
We're mainly using Java in our company, but this probably calls for a C++ implementation. The programming language is not yet defined.
As we'll need to handle concurrent communication with multiple terminals (around maybe 10 concurrent threads), we'll need to load several instances of the libfoo.so. I'm looking for design principles how to solve such a requirement (dlopen will only load an SO once, same thing for JNI). Do I need to spawn child processes? Copy/paste the SO and call it libfoo_1.so, libfoo_2.so etc. (aargh!) Are there other solutions?
Thanks
Simon
If the library has no API, meaning it runs its code using the .init mechanism, then you have no better choice than forking a parent process and dlopen the library in the child processes.
This is pretty simple actually, as long as you remember to wait for your child processes to terminate when needed.
If you need to handle communication between your parent and child processes, there are several Inter-process Communication methods available such as pipes.

How to run a C++ program inside another C++ program?

I will sketch the scenario I would like to get working below.
I have one main application.
That application, based on user interactions, can load other applications inside a secure environment/shell. This means these child applications cannot interact with the OS anymore, nor with each other.
The parent program can at any time call functions of these child programs.
The child program can at any time call functions of these parent programs.
Does anyone know how to implement this in C++? Preferably both parent and child should be written in C++.
The performance of loading the child applications inside the parent application doesn't matter. The only thing that matters is the performance of the communication between child and parent when calling functions of each other.
You will have to write your own compiler.
Consider: No normal OS supports what you want. You want both executables to run inside a single process, yet that process may or may not make OS calls depending on some weirdness inside the process which the OS doesn't understand at all.
This is no longer a problem with your custom compiler, as it simply will not create the offending instructions. It's similar to Java and .Net, which also prevent such OS calls outside their control.
A portable solution: Google Native Client
One possible Linux solution:
Make AppArmor profile with "hats" (a "hat" is a sandboxing configuration to which the application can switch programmatically with libapparmor),
have the main application create a "pipe",
have the main application "fork",
change into a "hat" corresponding to the child application,
"exec" the child application,
the main application and the child application communicate via the "pipe" created earlier.
If you want a (semi)crossplatform way to do this you can use RPC to call functions in another process. It's going to work on anything that supports the distributed computing environment. It's been around for some time and the msdn documentation states that parts of windows use it for inter process communication so it's probably fast enough. Here's a tutorial on msdn that should get you up and running http://msdn.microsoft.com/en-us/library/windows/desktop/aa379010.aspx The bad part is that I haven't been able to find a tutorial about using it on linux.
If you don't want to use RPC or find it too hard to find good documentation on the subject, you can use the standard IPC(Inter Process Communication) mechanisms from unix systems to signal your process that should call a certain function. I'd recommend a message queue because it's very fast and lightweight. You can find a tutorial here: http://www.cs.cf.ac.uk/Dave/C/node25.html
I am not familiar with OS restrictions in above answers. However, I found an easy way to solve this problem. I hope it helps and does not have a technical issue. I used Linux OS. Suppose I want to call C++ program B inside another C++ program A. I wrote a perl script (such as PerlScript.pl) that contains a system call to run program B. Then in A, I did a system call like system("perl PerlScript.pl") that ask perl to run B for me.

Multiplatform multiprocessing?

I was wondering why in the new C++11 they added threads and not processes.
Couldn't have they done a wrapper around platform specific functions?
Any suggestion about the most portable way to do multiprocessing? fork()? OpenMP?
If you could use Qt, QProcess class could be an elegant platform independent solution.
If you want to do this portably I'd suggest you avoid calling fork() directly and instead write your own library function that can be mapped on to a combination of fork() and exec() on systems where that's available. If you're careful you can make your function have the same or similar semantics as CreateProcess() on Win32.
UNIX systems tend to have a quite different approach to processes and process management compared to Windows based systems so it's non-trivial to make all but the simplest wrappers portable.
Of course if you have C++11 or Boost available I'd just stick with that. If you don't have any globals (which is a good thing generally anyway) and don't set up and shared data any other way then the practical differences between threads and processes on modern systems is slim. All the threads you create can make progress independently of each other in the same way the processes can.
Failing that you could look at An MPI implementation if message passing suits your task, or a batch scheduler system.
I am using Boost Interprocess.
It does not provide the possibility to create new processes, but once they are there, it allows them to communicate.
In this particular case I can create the processes I need from a shell script.

C++ master/worker

I am looking for a cross-platform C++ master/worker library or work queue library. The general idea is that my application would create some sort of Task or Work objects, pass them to the work master or work queue, which would in turn execute the work in separate threads or processes. To provide a bit of context, the application is a CD ripper, and the the tasks that I want to parallelize are things like "rip track", "encode WAV to Mp3", etc.
My basic requirements are:
Must support a configurable number of concurrent tasks.
Must support dependencies between tasks, such that tasks are not executed until all tasks that they depend on have completed.
Must allow for cancellation of tasks (or at least not prevent me from coding cancellation into my own tasks).
Must allow for reporting of status and progress information back to the main application thread.
Must work on Windows, Mac OS X, and Linux
Must be open source.
It would be especially nice if this library also:
Integrated with Qt's signal/slot mechanism.
Supported the use of threads or processes for executing tasks.
By way of analogy, I'm looking for something similar to Java's ExecutorService or some other similar thread pooling library, but in cross-platform C++. Does anyone know of such a beast?
Thanks!
I haven't used it in long enough that I'm not positive whether it exactly meets your needs, but check out the Adaptive Communications Environment (ACE). This library allows you to construct "active objects" which have work queues and execute their main body in their own threads, as well as thread pools that can be shared amoung objects. Then you can pass queue work objects on to active objects for them to process. Objects can be chained in various ways. The library is fairly heavy and has a lot to it to learn, but there have been a couple of books written about it and theres a fair amount of tutorial information available online as well. It should be able to do everything you want plus more, my only concern is whether it possesses the interfaces you are looking for 'out of the box' or if you'd need to build on top of it to get exactly what you are looking for.
I think this calls for intel's Threading Building Blocks, which pretty much does what you want.
Check out Intels' Thread Building Blocks library.
Sounds like you require some kind of "Time Sharing System".
There are some good open source ones out there, but I don't know
if they have built-in QT slot support.
This is probably a huge overkill for what you need but still worth mentioning -
BOINC is a distributed framework for such tasks. There's a main server that gives out tasks to perform and a cloud of workers that do its bidding. It is the framework behind projects like SETI#Home and many others.
See this post for creating threads using the boost library in C++:
Simple example of threading in C++
(it is a c++ thread even though the title says c)
basically, create your own "master" object that takes a "runnable" object and starts it running in a new thread.
Then you can create new classes that implement "runnable" and throw them over to your master runner any old time you want.