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## Global std_in

boost::process::std_in

## Synopsis

    // In header: <boost/process/io.hpp>

    unspecified std_in;

## Description

This property allows to set the input stream for the child process.

## Details

### File Input

The file I/O simple redirects the stream to a file, for which the possible types are

- `boost::filesystem::path`

- `std::basic_string<char_type>`

- `const char_type*`

- `FILE*`

with `char_type` being either `char` or `wchar_t`.

FILE* is explicitly added, so the process can easily redirect the output stream of the child to another output stream of the process. That is:

    system("ls", std_in < stdin);

| Warning
If the launching and the child process use the input, this leads to undefined behaviour.

A syntax like `system("ls", std_out > std::cerr)` is not possible, due to the C++ implementation not providing access to the handle.

The valid expressions for this property are

    std_in < file;
    std_in = file;

### Pipe Input

As explained in the corresponding section, the boost.process library provides a async_pipe class which can be used to communicate with child processes.

| Note
Technically the async_pipe works synchronous here, since no asio implementation is used by the library here. The async-operation will then however not end if the process is finished, since the pipe remains open. You can use the async_close function with on_exit to fix that.

Valid expressions with pipes are these:

    std_in < pipe;
    std_in = pipe;

Where the valid types for `pipe` are the following:

- `basic_pipe`

- `async_pipe`

- `basic_opstream`

- `basic_pstream`

Note that the pipe may also be used between several processes, like this:

    pipe p;
    child c1("nm", "a.out", std_out>p);
    child c2("c++filt", std_in<p);

### Asynchronous Pipe Input

Asynchronous Pipe I/O classifies communication which has automatically handling of the asynchronous operations by the process library. This means, that a pipe will be constructed, the async_read/-write will be automatically started, and that the end of the child process will also close the pipe.

Valid types for pipe I/O are the following:

- `boost::asio::const_buffer` [24]

- `boost::asio::mutable_buffer` [25]

- `boost::asio::streambuf`

Valid expressions with pipes are these:

    std_in < buffer;
    std_in = buffer;
    std_out > buffer;
    std_out = buffer;
    std_err > buffer;
    std_err = buffer;
    (std_out & std_err) > buffer;
    (std_out & std_err) = buffer;

| Note
It is also possible to get a future for std_in, by chaining another `std::future<void>` onto it, so you can wait for the input to be completed. It looks like this:
    std::future<void> fut;
    boost::asio::io_context ios;
    std::string data;
    child c("prog", std_in < buffer(data) >  fut, ios);
    fut.get();
`boost::asio::buffer` is also available in the `boost::process` namespace.

| Warning
This feature requires `boost/process/async.hpp` to be included and a reference to `boost::asio::io_context` to be passed to the launching function.

### Close

The input stream can be closed, so it cannot be read from. This will lead to an error when attempted.

This can be achieved by the following syntax.

    std_in < close;
    std_in = close;
    std_in.close();

### Null

The input stream can be redirected to read from the null-device, which means that only `EOF` is read.

The syntax to achieve that has the following variants:

    std_in < null;
    std_in = null;
    std_in.null();

[24] Constructed with `boost::asio::buffer`

[25] Constructed with `boost::asio::buffer`