== `posix/bind_fd.hpp`

`bind_fd` is a utility class to bind a file descriptor to an explicit file descriptor for the child process.

[source,cpp]
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struct bind_fd
{
  // Inherit file descriptor with the same value.
  /*
  * This will pass descriptor 42 as 42 to the child process:
  * @code
  * process p{"test", {},  posix::bind_fd(42)};
  * @endcode
  */
  bind_fd(int target);

  // Inherit an asio io-object as a given file descriptor to the child process.
  /*
  * This will pass the tcp::socket, as 42 to the child process:
  * @code
  * extern tcp::socket sock;
  * process p{"test", {},  posix::bind_fd(42, sock)};
  * @endcode
  */

  template<typename Stream>
  bind_fd(int target, Stream && str);

  // Inherit a `FILE` as a given file descriptor to the child process.
  /* This will pass the given `FILE*`, as 42 to the child process:

       process p{"test", {},  posix::bind_fd(42, stderr)};

   */
  bind_fd(int target, FILE * f);

  // Inherit a file descriptor with as a different value.
  /* This will pass 24 as 42 to the child process:

       process p{"test", {},  posix::bind_fd(42, 24)};

   */
  bind_fd(int target, int fd);

  // Inherit a null device as a set descriptor.
  /* This will a null device as 42 to the child process:

       process p{"test", {},  posix::bind_fd(42, nullptr)};

   */
  bind_fd(int target, std::nullptr_t);

  // Inherit a newly opened-file as a set descriptor.
  /* This will pass a descriptor to "extra-output.txt" as 42 to the child process:

       process p{"test", {},  posix::bind_fd(42, "extra-output.txt")};

   */
  bind_fd(int target, const filesystem::path & pth, int flags = O_RDWR | O_CREAT);

};
----

Using `bind_fd` can be used to inherit file descriptors explicitly, because no unused one will be.

[source,cpp]
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