I am writing a program in openFrameworks a c++ framework. I want to start another app and communicate with it over stdin and stdout. I can start a new thread conveniently using the ofThread class. I had planned on creating two pipes and redirecting the std in and out of the thread to the pipes (using dup2), but unfortunately, this redirects the pipes for the whole app, not just the thread.
Is there a way I can start another app and be able to reads its output and provide it input?
Instead of another thread you'll need to create a child process using the fork() function (which might involve another thread intrinsically).
The difference is, that fork creates a complete copy of the parent process environment that should be shown on an exec() call within scope of the child process, while just exec() from a thread tries to share all the resource from it's parent process (thread) and thus might lead to unexpected concurrency (race conditon) problems.
If your "another app" is implemented as a subthread within your existing program, you don't need to redirect stdin and stdout to communicate with it over pipes. Just pass the pipe file descriptors to the subthread when you start it up. (You can use fdopen to wrap file descriptors in FILE objects. If you have dup2 and pipe, you have fdopen as well.)
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For my computer science class final project, I need to interact with a SQL database. Only problem is, my prof won't install the SQL c++ API for me. Is there a way I can still interact with SQL without the API?
If I'm understanding your question correctly, you want your program to be able to launch a child process (an SQL command line program in this case), and then be able to read the text it receives from the child process's stdout and/or stderr, and write text to the child process's stdin, the same way a user would if he/she were running that program interactively.
The answer is yes, it is possible to do this, although it takes some work. Under Linux/Unix/MacOSX, you can call forkpty() to spawn a child process -- the parent process will get a socket (via forkpty's first argument) that you can use to communicate with the child process's stdin and stdout. In the child process, you can then call execvp (or one of its variants) to run the SQL program in that process;
Under Windows, it's a bit more complex -- you'll need to set up some pipes and then call CreateProcess() to launch the child process, and communicate with it through those pipes. Microsoft has a page on the topic (including example code) here.
In my c++ windows app I start multiple child processes and I want them to inherit parent's stdout/stderr, so that if output of my app is redirected to some file then that file would also contain output of all child processes that my app creates.
Currently I do that using CreateProcess without output redirection. MSDN has a sample how to redirect output: Creating a Child Process with Redirected Input and Output, but I want to see what alternative do I have. Simplest is to use system and call it from a blocking thread that waits for child to exit. All output is then piped back to parent's stdout/stderr, however in parent process I do not have a chance to process stdout data that comes from child.
There are also other functions to start processes on windows: spawn, exec, which might be easier to port to posix systems.
What should I use if I want it to work on linux/osx? What options do I have if I want it to work on UWP aka WinRT? I might be totally ok with system called from a blocking thread, but perhaps I'd prefer to be able to have more control on process PID (to be able to terminate it) and process stdout/stderr, to prepend each line with child##: for example.
The boost libraries recently released version 1.64 which includes a new boost::process library.
In it, you're given a C++ way to be able to redirect output to a pipe or asio::streambuf, from which you can create a std::string or std::istream to read whatever your child process wrote.
You can read up on boost::process tutorials here, which shows some simple examples of reading child output. It does make heavy use of boost::asio, so I highly recommend you read up on that too.
I'm new to working with forking and I am having a trouble understanding how to achieve what I want. I'll try to explain as best I can.
I have Process A which is a functional Berkeley socket server running on Linux.
I need Process A to load a program from the disk into a separate non-blocking process (Process B) in a background state. Then Process A needs to pass Process B control of Process A's sockets. Lastly Process A needs to end, leaving process B running.
I'm unclear on whats needed to pass the sockets to a new process if the old one ends, and the best way to create a non-blocking new process that allows the original process to end.
There's nothing special you need to do. Just make sure the close on exec flag is cleared for any file descriptors you want process B to inherit and set for any file descriptors you don't want process B to inherit. Then call exec to replace process A with process B. Process B will start with all inheritable file descriptors intact.
If you need to pass an open file (such as a socket) without using inheritance-through-fork, you use ioctl with I_SENDFD. Here is a very detailed description. (There is a corresponding mechanism for receiving it.) You can do this with a named pipe which connects the processes, or via a variation, with a Unix domain socket.
I need to learn how to create a pipe and use fork, and also how to write to a pipe and read, in VC++ 2010.
Are there any tutorials on how to do that?
This question is already answered in detail here.
Quoting verbatim from the same answer
A pipe is a mechanism for interprocess communication. Data written to the pipe by one process can be read by another process. The primitive for creating a pipe is the pipe function. This creates both the reading and writing ends of the pipe. It is not very useful for a single process to use a pipe to talk to itself. In typical use, a process creates a pipe just before it forks one or more child processes. The pipe is then used for communication either between the parent or child processes, or between two sibling processes. A familiar example of this kind of communication can be seen in all operating system shells. When you type a command at the shell, it will spawn the executable represented by that command with a call to fork. A pipe is opened to the new child process and its output is read and printed by the shell. This page has a full example of the fork and pipe functions...
I need to spawn a child process and talk to it over STD IO. I want to use named pipes to do so. The application that I am writing (which spawns the process) is in C++.
Microsoft gives a good example (http://msdn.microsoft.com/en-us/library/ms682499%28v=vs.85%29.aspx) of how to do the above using anonymous pipes and mentions that redirecting can also be done using named pipes.
But just replacing anonymous pipes with named pipes in the example they have provided does not work. (I am assigning the handles returned by CreateNamedPipe() API to the hStdInput, hStdOutput members of the STARTUPINFO structure used to spawn the child process.)
What is the right way to do this? An example code snippet would be great.
NOTE: I cannot modify the child process behavior.