Get device path based on USB VID:PID in Linux - c++

Question

If I plug in a device, say /dev/ttyUSB0 and I want to get the number 0 based on its VID:PID (found with lsusb), how could I do that in C++ Linux? I have this code to find one printer device, if it's helpful at all:
int printer_open (void)
{
char printer_location[] = "/dev/usb/lpX";
struct stat buf;
// continuously try all numbers until stat returns true for the connected printer
for (int i = 0; i < 10; i++)
{
printer_location[11] = '0' + i;
if (!stat (printer_location, &buf))
break;
}
return 0;
}

Answer 1

You could use libusb
apt-get install build-essential libudev-dev
Here is a good example: http://www.dreamincode.net/forums/topic/148707-introduction-to-using-libusb-10/
and here is the lib description: http://libusb.sourceforge.net/api-1.0/
int main() {
libusb_context *context = NULL;
libusb_device **list = NULL;
int rc = 0;
ssize_t count = 0;
rc = libusb_init(&context);
assert(rc == 0);
count = libusb_get_device_list(context, &list);
assert(count > 0);
for (size_t idx = 0; idx < count; ++idx) {
libusb_device *device = list[idx];
libusb_device_descriptor desc = {0};
rc = libusb_get_device_descriptor(device, &desc);
assert(rc == 0);
printf("Vendor:Device = %04x:%04x\n", desc.idVendor, desc.idProduct);
}
}
And if you compile your code don't forget to add the lib reference -I/usr/include/libusb-1.0/ and - lusb-1.0

Answer 2

libusb can't get it actually. So look at this file instead: /proc/bus/input/devices
Example line from the file:
I: Bus=0003 Vendor=1a2c Product=0c23 Version=0110
N: Name="USB USB Keyboard"
P: Phys=usb-0000:00:14.0-3/input0
S: Sysfs=/devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3:1.0/0003:1A2C:0C23.0015/input/input30
U: Uniq=
H: Handlers=sysrq kbd event10 leds
B: PROP=0
B: EV=120013
B: KEY=1000000000007 ff800000000007ff febeffdff3cfffff fffffffffffffffe
B: MSC=10
B: LED=7
This function gets the event number from the device with the matching VID:PID:
#include <string>
#include <iostream>
#include <fstream>
void open_device (std::string device_vid, std::string device_pid)
{
try
{
std::ifstream file_input;
std::size_t pos;
std::string device_path, current_line, search_str, event_str;
std::string device_list_file = "/proc/bus/input/devices";
bool vid_pid_found = false;
int fd = 0;
bool debug = true;
// 1. open device list file
file_input.open(device_list_file.c_str());
if (!file_input.is_open())
{
std::cerr << "file_input.open >> " << std::strerror(errno) << std::endl;
throw -2;
}
// 2. search for first VID:PID and get event number
search_str = "Vendor=" + device_vid + " Product=" + device_pid;
while (getline(file_input, current_line))
{
if (!vid_pid_found)
{
pos = current_line.find(search_str, 0);
if (pos != std::string::npos)
{
vid_pid_found = true;
search_str = "event";
}
}
else
{
pos = current_line.find(search_str, 0);
if (pos != std::string::npos)
{
event_str = current_line.substr(pos);
// find space and substring event##
pos = event_str.find(' ', 0);
event_str = event_str.substr(0, pos);
break;
}
}
}
// 3. build device path
device_path = "/dev/input/" + event_str;
if (debug) std::cout << "device_path = " << device_path << std::endl;
// 4. connect to device
fd = open (device_path.c_str(), O_RDONLY);
if (fd < 0)
{
std::cerr << "open >> errno = " << std::strerror(errno) << std::endl;
throw -3;
}
}
catch (const std::exception &e)
{
std::cerr << "e.what() = " << e.what() << std::endl;
throw -1;
}
return;
}