How to get USB vendor and product info programmatically on Linux?

Haven't tried this myself, but libudev's udev_device_get_property_value should be it; it is used in pulseaudio's udev-util.c as udev_device_get_property_value(card, "ID_VENDOR_FROM_DATABASE")).

Here is a small example I just put together, based on udev-util.c - note that I've used an Arduino Duemillanove with FTDI FT232 chip, whose udev path I find using udevadm (see comments in code below), and then I hardcoded it in the below program, udevl.c:

// sudo apt-get install libudev-dev
// build with: gcc -o udevl -ludev -Wall -g udevl.c

#include <stdio.h>

#include <libudev.h>

int main( int argc, char **argv )
{
  const char *v;
  char t[256];
  struct udev *udev;
  struct udev_device *card = NULL;

  if (!(udev = udev_new())) {
      fprintf(stderr, "Failed to allocate udev context.\n");
      return -1;
  }

  // $ lsusb | grep FT232
  // Bus 002 Device 002: ID 0403:6001 Future Technology Devices International, Ltd FT232 USB-Serial (UART) IC    
  // $ udevadm info --name=/dev/ttyUSB0 --attribute-walk | grep "looking at device"
  // looking at device '/devices/pci0000:00/0000:00:1d.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0'
  // (that one is under /sys)
  // hardcode that path below:

  // udev_get_sys_path(udev) for me: '/sys'
  sprintf(t, "%s/devices/pci0000:00/0000:00:1d.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0", udev_get_sys_path(udev));
  fprintf(stdout, " path: %s\n", t);

  card = udev_device_new_from_syspath(udev, t);
  fprintf(stdout, " udev_device: 0x%08X\n", (unsigned int)card);

  if ((v = udev_device_get_property_value(card, "ID_MODEL_FROM_DATABASE")) )
    fprintf(stdout, "got ID_MODEL_FROM_DATABASE: %s\n", v);
  else
    fprintf(stdout, "failed getting ID_MODEL_FROM_DATABASE: %s\n", v);

  fprintf(stdout, "Done.\n");

  if (card)
    udev_device_unref(card);

  if (udev)
    udev_unref(udev);

  return 0;
}

This program (with the Arduino attached) outputs:

$ ./udevl 
 path: /sys/devices/pci0000:00/0000:00:1d.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0
 udev_device: 0x09FBF080
got ID_MODEL_FROM_DATABASE: FT232 USB-Serial (UART) IC
Done.

... and "FT232 USB-Serial (UART) IC" is the right entry for VID:PID 0403:6001 in usb.ids.

Hope this helps,
Cheers!

lsusb command queries information about currently plugged USB devices. You can use its -d option to query a certain vendor/product (but it seems to work only for currently plugged devices):

$ lsusb -d 0e21:0750
Bus 001 Device 005: ID 0e21:0750 Cowon Systems, Inc.

You can show information for all devices:

$ lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 004: ID 0421:01c7 Nokia Mobile Phones
Bus 001 Device 003: ID 0bda:8187 Realtek Semiconductor Corp. RTL8187 Wireless Adapter
Bus 001 Device 005: ID 0e21:0750 Cowon Systems, Inc.
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 006 Device 002: ID 046d:c01b Logitech, Inc. MX310 Optical Mouse
Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 008 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

You can also make it be verbose (lsusb -v) and printing a lot of stuff.

Note that when accessing information about the system in Linux OS, it's much preferred to do it via shell commands (such as lsusb) than to directly parse the system files these commands access.