How to use the io_control() method of a Boost.Asio socket with a custom command?

Question

I'm trying to use the Boost.Asio library for C++ to make an ioctl system call.

Here is my equivalent "C style" code, which gets the ESSID for a wireless interface:

int sockfd;
char * id;
struct iwreq wreq;

memset(&wreq, 0, sizeof(struct iwreq));
sprintf(wreq.ifr_name, "wlp3s0");

if((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
    fprintf(stderr, "Cannot open socket: %s\n", strerror(errno));
    exit(1);
}

id = malloc(IW_ESSID_MAX_SIZE+1);
wreq.u.essid.pointer = id;
if (ioctl(sockfd, SIOCGIWESSID, &wreq)) {
    fprintf(stderr, "Get ESSID ioctl failed: %s\n", strerror(errno));
    exit(2);
}

printf("ESSID is %s\n", wreq.u.essid.pointer);

I found a relevant example for Boost.Asio, but I'm not clear how to use it correctly. Here is the code that I tried adding to my main function:

boost::asio::ip::udp::socket socket(io_service);
struct iwreq wreq
memset(&wreq, 0, sizeof(struct iwreq));
sprintf(wreq.ifr_name, "wlp3s0");
id = malloc(IW_ESSID_MAX_SIZE+1);
wreq.u.essid.pointer = id;

boost::asio::detail::io_control::myCommand command;
command.set(&wreq);
boost::system::error_code ec;
socket.io_control(command, ec);
if (ec)
{
  // An error occurred.
}

This is the code for my custom command:

#include <boost/asio/detail/config.hpp>
#include <cstddef>
#include <boost/config.hpp>
#include <boost/asio/detail/socket_types.hpp>
#include <boost/asio/detail/push_options.hpp>

namespace boost {
namespace asio {
namespace detail {
namespace io_control {

class myCommand
{
public:
  // Default constructor.
  myCommand()
    : value_(0)
  {
  }

  // Get the name of the IO control command.
  int name() const
  {
    return static_cast<int>(SIOCGIWESSID);
  }

  // Set the value of the I/O control command.
  void set(struct iwreq* value)
  {
    value_ = static_cast<detail::ioctl_arg_type>(value);
  }

  // Get the current value of the I/O control command.
  std::size_t get() const
  {
    return static_cast<struct iwreq*>(value_);
  }

  // Get the address of the command data.
  detail::ioctl_arg_type* data()
  {
    return &value_;
  }

  // Get the address of the command data.
  const detail::ioctl_arg_type* data() const
  {
    return &value_;
  }

private:
  detail::ioctl_arg_type value_;
};

} // namespace io_control
} // namespace detail
} // namespace asio
} // namespace boost

However, the code does not work. If you have any example code or solution, please let me know. Thank you.

dpward · Accepted Answer · 2025-11-11 17:33:22Z

In Asio, an IoControlCommand class provides support for a specific ioctl command. Asio itself includes one such class, bytes_readable, which handles the FIONREAD command. (It is the example mentioned in the question.)

The argument to FIONREAD is a pointer to an integer¹, as shown here in C-style code:

int fd, value, ret;
/* ... */
ret = ioctl(fd, FIONREAD, &value);

Because of this, Asio's bytes_readable class has a private member variable that holds an integer, and data() returns a pointer to it.

ㅤ

The argument to SIOCGIWESSID, however, is a pointer to an instance of struct iwreq. The custom IoControlCommand class should store and initialize that instance, and data() should return a pointer to it:

#include <linux/wireless.h>
#include <string>

class myCommand
{
public:
  // Construct with a specific command value.
  myCommand(std::string interface)
    : wreq_(), essid_()
  {
    interface.copy(wreq_.ifr_name, sizeof(wreq_.ifr_name));
    wreq_.u.essid.pointer = essid_;
    wreq_.u.essid.length = sizeof(essid_);
  }

  // Get the name of the IO control command.
  int name() const
  {
    return static_cast<int>(SIOCGIWESSID);
  }

  // Get the current value of the I/O control command.
  std::string essid() const
  {
    return essid_;
  }

  // Get the address of the command data.
  void* data()
  {
    return &wreq_;
  }

  // Get the address of the command data.
  const void* data() const
  {
    return &wreq_;
  }

private:
  struct iwreq wreq_;
  char essid_[IW_ESSID_MAX_SIZE + 1];
};

To use this class:

boost::asio::ip::udp::socket socket(io_service);

myCommand cmd("wlp3s0");

boost::system::error_code ec;
socket.io_control(cmd, ec);
if (ec)
{
  // An error occurred.
}

std::string essid = cmd.essid();
std::cout << "ESSID is " << essid << std::endl;

¹ The exact integer type is platform-specific: it is int for Linux, but unsigned long for Windows. For this reason value_ is defined using a type alias, boost::asio::detail::ioctl_arg_type.

Collectives™ on Stack Overflow

How to use the io_control() method of a Boost.Asio socket with a custom command?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related