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## Tutorial

In this section we will go step by step through the different features of boost.process. For a full description see the reference and the concepts sections.

### Starting a process

We want to start a process, so let's start with a simple process. We will invoke the gcc compiler to compile a simple program.

With the standard library this looks like this.

    int result = std::system("g++ main.cpp");

Which we can write exactly like this in boost.process.

    namespace bp = boost::process; //we will assume this for all further examples
    int result = bp::system("g++ main.cpp");

If a single string (or the explicit form `bp::cmd`), it will be interpreted as a command line. That will cause the execution function to search the `PATH` variable to find the executable. The alternative is the `exe-args` style, where the first string will be interpreted as a filename (including the path), and the rest as arguments passed to said function.

| Note
For more details on the `cmd`/`exe-args` style look here

So as a first step, we'll use the `exe-args` style.

    int result = bp::system("/usr/bin/g++", "main.cpp");

With that syntax we still have "g++" hard-coded, so let's assume we get the string from an external source as `boost::filesystem::path`, we can do this too.

    boost::filesystem::path p = "/usr/bin/g++"; //or get it from somewhere else.
    int result = bp::system(p, "main.cpp");

Now we might want to find the `g++` executable in the `PATH`-variable, as the `cmd` syntax would do. `Boost.process` provides a function to this end: `bp::search_path`.

    boost::filesystem::path p = bp::search_path("g++"); //or get it from somewhere else.
    int result = bp::system(p, "main.cpp");

| Note
`search_path` will search for any executable with that name. This also includes to add a file suffix on windows, such as `.exe` or `.bat`.

### Launch functions

Given that our example used the `system` function, our program will wait until the child process is completed. This may be unwanted, especially since compiling can take a while.

In order to avoid that, boost.process provides several ways to launch a process. Besides the already mentioned `system` function and its asynchronous version `async_system`, we can also use the `spawn` function or the `child` class.

The `spawn` function launches a process and immediately detaches it, so no handle will be returned and the process will be ignored. This is not what we need for compiling, but maybe we want to entertain the user, while compiling:

    bp::spawn(bp::search_path("chrome"), "www.boost.org");

Now for the more sensible approach for compiling: a non-blocking execution. To implement that, we directly call the constructor of `child`.

    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();

So we launch the process, by calling the child constructor. Then we check and do other things while the process is running and afterwards get the exit code. The call to `wait` is necessary, to obtain it and tell the operating system, that no one is waiting for the process anymore.

| Note
You can also wait for a time span or a until a time point with `wait_for` and `wait_until`

| Warning
If you don't call wait on a child object, it will be terminated on destruction. This can be avoided by calling `detach` beforehand

### Error

Until now, we have assumed that everything works out, but it is not impossible, that "g++" is not present. That will cause the launch of the process to fail. The default behaviour of all functions is to throw a std::system_error on failure. As with many other functions in this library, passing an std::error_code will change the behaviour, so that instead of throwing an exception, the error will be assigned to the error code.

    std::error_code ec;
    bp::system("g++ main.cpp", ec);

### Synchronous I/O

In the examples given above, we have only started a program, but did not consider the output. The default depends on the system, but usually this will just write it to the same output as the launching process. If this shall be guaranteed, the streams can be explicitly forwarded like this.

    bp::system("g++ main.cpp", bp::std_out > stdout, bp::std_err > stderr, bp::std_in < stdin);

Now for the first example, we might want to just ignore the output, which can be done by redirecting it to the null-device. This can be achieved this way:

    bp::system("g++ main.cpp", bp::std_out > bp::null);

Alternatively we can also easily redirect the output to a file:

    bp::system("g++ main.cpp", bp::std_out > "gcc_out.log");

Now, let's take a more visual example for reading data. nm is a tool on posix, which reads the outline, i.e. a list of all entry points, of a binary. Every entry point will be put into a single line, and we will use a pipe to read it. At the end an empty line is appended, which we use as the indication to stop reading. Boost.process provides the pipestream (`ipstream`, `opstream`, `pstream`) to wrap around the `pipe` and provide an implementation of the std::istream, std::ostream and std::iostream interface.

    std::vector<std::string> read_outline(std::string & file)
    {
        bp::ipstream is; //reading pipe-stream
        bp::child c(bp::search_path("nm"), file, bp::std_out > is);

        std::vector<std::string> data;
        std::string line;

        while (c.running() && std::getline(is, line) && !line.empty())
            data.push_back(line);

        c.wait();

        return data;
    }

What this does is redirect the `stdout` of the process into a pipe and we read this synchronously.

| Warning
The pipe will cause a deadlock if you try to read after nm exited

| Note
You can do the same thing with `std_err`

Now we get the name from `nm` and we might want to demangle it, so we use input and output. `nm` has a demangle option, but for the sake of the example, we'll use c++filt for this.

    bp::opstream in;
    bp::ipstream out;

    bp::child c("c++filt", std_out > out, std_in < in);

    in << "_ZN5boost7process8tutorialE" << endl;
    std::string value;
    out >> value;

    c.terminate();

Now you might want to forward output from one process to another processes input.

    std::vector<std::string> read_demangled_outline(const std::string & file)
    {
        bp::pipe p;
        bp::ipstream is;

        std::vector<std::string> outline;

        //we just use the same pipe, so the
        bp::child nm(bp::search_path("nm"), file,  bp::std_out > p);
        bp::child filt(bp::search_path("c++filt"), bp::std_in < p, bp::std_out > is);

        std::string line;
        while (filt.running() && std::getline(is, line)) //when nm finished the pipe closes and c++filt exits
            outline.push_back(line);

        nm.wait();
        filt.wait();
    }

This forwards the data from `nm` to `c++filt` without your process needing to do anything.

### Asynchronous I/O

Boost.process allows the usage of boost.asio to implement asynchronous I/O. If you are familiar with boost.asio (which we highly recommend), you can use `async_pipe` which is implemented as an I/O-Object and can be used like `pipe` as shown above.

Now we get back to our compiling example. `nm` we might analyze it line by line, but the compiler output will just be put into one large buffer.

With boost.asio this is what it looks like.

    boost::asio::io_service ios;
    std::vector<char> buf(4096);

    bp::async_pipe ap(ios);

    bp::child c(bp::search_path("g++"), "main.cpp", bp::std_out > ap);

    boost::asio::async_read(ap, boost::asio::buffer(buf),
                    [](const boost::system::error_code &ec, std::size_t size){});

    ios.run();
    int result = c.exit_code();

To make it easier, boost.process provides simpler interface for that, so that the buffer can be passed directly, provided we also pass a reference to an boost::asio::io_service.

    boost::asio::io_service ios;
    std::vector<char> buf(4096);

    bp::child c(bp::search_path("g++"), "main.cpp", bp::std_out > boost::asio::buffer(buf), ios);

    ios.run();
    int result = c.exit_code();

| Note
Passing an instance of boost::asio::io_service to the launching function automatically cause it to wait asynchronously for the exit, so no call of `wait` is needed

To make it even easier, you can use std::future for asynchronous operations (you will still need to pass a reference to a boost::asio::io_service) to the launching function, unless you use `bp::system` or `bp::async_system`.

Now we will revisit our first example and read the compiler output asynchronously:

    boost::asio::boost::asio::io_service ios;

    std::future<std::string> data;

    child c("g++", "main.cpp", //set the input
            bp::std_in.close(),
            bp::std_out > bp::null, //so it can be written without anything
            bp::std_err > data,
            ios);

    ios.run(); //this will actually block until the compiler is finished

    auto err =  data.get();

### Groups

When launching several processes, processes can be grouped together. This will also apply for a child process, that launches other processes, if they do not modify the group membership. E.g. if you call `make` which launches other processes and call terminate on it, it will not terminate all the child processes of the child unless you use a group.

The two main reasons to use groups are:

1. Being able to terminate child processes of the child process
2. Grouping several processes into one, just so they can be terminated at once

If we have program like `make`, which does launch its own child processes, a call of `terminate` might not suffice. I.e. if we have a makefile launching `gcc` and use the following code, the `gcc` process will still run afterwards:

    bp::child c("make");
    if (!c.wait_for(std::chrono::seconds(10)) //give it 10 seconds
        c.terminate(); //then terminate

So in order to also terminate `gcc` we can use a group.

    bp::group g;
    bp::child c("make", g);
    if (!g.wait_for(std::chrono::seconds(10))
        g.terminate();

    c.wait(); //to avoid a zombie process & get the exit code

Now given the example, we still call `wait` to avoid a zombie process. An easier solution for that might be to use `spawn`.

To put two processes into one group, the following code suffices. Spawn already launches a detached process (i.e. without a child-handle), but they can be grouped, to that in the case of a problem, RAII is still a given.

    void f()
    {
        bp::group g;
        bp::spawn("foo", g);
        bp::spawn("bar", g);

        do_something();

        g.wait();
    };

In the example, it will wait for both processes at the end of the function unless an exception occurs. I.e. if an exception is thrown, the group will be terminated.

Please see the `reference` for more information.

### Environment

This library provides access to the environment of the current process and allows setting it for the child process.

    //get a handle to the current environment
    auto env = boost::this_process::environment();
    //add a variable to the current environment
    env["VALUE_1"] = "foo";

    //copy it into an environment separate to the one of this process
    bp::environment env_ = env;
    //append two values to a variable in the new env
    env_["VALUE_2"] += {"bar1", "bar2"};

    //launch a process with `env_`
    bp::system("stuff", env_);

A more convenient way to modify the environment for the child is the `env` property, which the example as following:

    bp::system("stuff", bp::env["VALUE_1"]="foo", bp::env["VALUE_2"]+={"bar1", "bar2"});

Please see to the `reference` for more information.