Get USB Drive Serial Number on Os X in C++

Getting the serial number for a USB Serial drive on Os X is tricky just like it is to get a USB serial number on Windows. We want to get the serial number of a USB Flash disk that is associated with a particular mounted volume.

The problem is similar to the problems on Windows, a USB disk device can have multiple volumes on it, so the mapping from USB disk to volume is not easy to find, and there is no direct API to do it. You have to go through a similar exercise to figure out the mapping. In addition, on Os X, there is a delay between when the os tells you that a USB device is available on the USB bus, and when the drive is actually mounted.

Unlike Windows, OsX will require you to dig all the way down to the USB Bus level to get the serial number, vendor and product strings.

Headers You’ll Need

You’ll need to include the following headers, I know it is a lot of them. There are a set of standard unix headers, followed by the headers for the IOKit. All device operations use the IOKit, which you may have to install in addition to XCode.

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <paths.h>
#include <sys/param.h>
#include <mach/mach.h>
#include <mach/error.h>

#include <IOKit/IOKitLib.h>
#include <IOKit/IOCFPlugIn.h>
#include <IOKit/usb/IOUSBLib.h>

#include <IOKit/IOBSD.h>
#include <IOKit/storage/IOCDMedia.h>
#include <IOKit/storage/IOMedia.h>
#include <IOKit/storage/IOCDTypes.h>
#include <IOKit/storage/IOMediaBSDClient.h>

#include <IOKit/serial/IOSerialKeys.h>
#include <IOKit/serial/ioss.h>

Registering for Device Notifications

First let start by detecting the insertion of a USB Disk device. On Os X you can register have your program receive a notification whenever a device is added or removed from the system. In this case we want to be notified whenever any USB device is inserted, and then do something when we determine the device is a USB flash drive.

We start by defining the callbacks the OS will call when devices are inserted or removed. Then we show the function that will register those callbacks.

void USBDeviceCallback( void* refCon, io_iterator_t iterator, bool inserted );

void USBDeviceInserted( void* refCon, io_iterator_t iterator )
   USBDeviceCallback( refCon, iterator, true );

void USBDeviceRemoved( void* refCon, io_iterator_t iterator )
   USBDeviceCallback( refCon, iterator, false );

s32 registerForUSBNotifications()
   // Set up matching dictionary for class IOUSBDevice and its subclasses
   CFMutableDictionaryRef matchingDict = IOServiceMatching( "IOUSBDevice" );       
   if ( !matchingDict )
      // could not create dictionary. Should almost never happen.
      return -1;

   // Create a master port for communication with the I/O Kit
   mach_port_t masterPort;
   if ( IOMasterPort( MACH_PORT_NULL, &masterPort ) || !masterPort )
      // could not create master port - again this would be highly unexpected.
      return -1;

   // To set up asynchronous notifications, create a notification port and
   // add its run loop event source to the program’s run loop
   IONotificationPortRef notifyPort = 
      IONotificationPortCreate( masterPort );
   CFRunLoopSourceRef runLoopSource = 
      IONotificationPortGetRunLoopSource( notifyPort );
   CFRunLoopAddSource( CFRunLoopGetCurrent(), runLoopSource, 
                       kCFRunLoopDefaultMode );

   // Retain additional dictionary references because each call to
   // IOServiceAddMatchingNotification consumes one reference
   matchingDict = (CFMutableDictionaryRef) CFRetain( matchingDict );
   matchingDict = (CFMutableDictionaryRef) CFRetain( matchingDict );
   matchingDict = (CFMutableDictionaryRef) CFRetain( matchingDict );

   // Now set up two notifications: one to be called when a raw device
   // is first matched by the I/O Kit and another to be called when the
   // device is terminated

   // Notification of first match:
   kern_return_t kr;
   io_iterator_t addedIter; 
   kr = IOServiceAddMatchingNotification( notifyPort,
                                          &addedIter );
   USBDeviceInserted( NULL, addedIter );
   if ( kr )
      // Error in IOServiceAddMatchingNotification
      mach_port_deallocate( mach_task_self(), masterPort );
      return -1;

   // Notification of termination:
   io_iterator_t removedIter; 
   kr = IOServiceAddMatchingNotification( notifyPort,
                                          &removedIter );
   USBDeviceRemoved( NULL, removedIter );
   if ( kr )
      // Error in IOServiceAddMatchingNotification
      mach_port_deallocate( mach_task_self(), masterPort );
      return -1;

   mach_port_deallocate( mach_task_self(), masterPort );

   return 0;

You call registerForUSBNotifications() at the start of your program, and from that point forward the OS will call back whenever any USB device is plugged in – this will include not just disks but other devices – like mice, keyboards etc.

Iterating over USB Interfaces to Find Mounted Volumes

Lets look at the prototype for the device callback:

void USBDeviceInserted( void* refCon, io_iterator_t iterator ) { }

The callback will return a list of any devices that were inserted or removed since the last callback. Normally this will be just one, but sometimes it can be multiple interfaces. There are many USB devices which export multiple device interfaces, and when you insert them several interfaces are returned.

There are two ways to iterate over the devices.

refCon is an array of pointers to device interfaces. In the case of USB devices it can be cast to an IOUSBInterfaceInterface, like this:

IOUSBInterfaceInterface **interface = (IOUSBInterfaceInterface **) refCon;

iterator is an io_iterator_t. This is a container that holds io_service_t‘s, one for each interface.

We’re going to ignore refCon and just use the io_iterator_t

void USBDeviceCallback( void* refCon, io_iterator_t iterator, bool inserted )
   io_service_t usbDevice;
   while (( usbDevice = IOIteratorNext( iterator )))
      // this is redundant since we only registered for IOUSBDevices, but 
      // it's a good check, and something you might need if you had registered for 
      // more device types. Eg. If you had registered for notifications to 
      // kIOMediaClass, which includes USB disks, but also other non-usb media -
      // like other disk types, CD-Roms etc.
      if ( !IOObjectConformsTo( usbDevice, "IOUSBDevice" )) continue; 

      // you could also do this check by the device's class name:
      io_name_t className;
      IOObjectGetClass( usbDevice, className );
      // fprintf( stderr, "This device's className is %s\n", 
      //          (const char*)className );
      if ( strcmp( className, "IOUSBDevice" )) continue;     

      // this is how you get the device name. For USB disks the default
      // name is "USB DISK", but many manufacturers will substitute in their
      // own brand name here. If you wanted to filter by brand you can do it 
      // with the device name.
      io_name_t deviceName;
      IORegistryEntryGetName( usbDevice, deviceName );
      // fprintf( stderr, "Device Name is %s\n", (const char*)deviceName );
      if ( strcmp( deviceName, "Super Disk Brand" ) && // a fake brand example
           strcmp( deviceName, "USB DISK" )) continue; // or any disk.    

      // now we look for the mount point for the disk. This will be the unix
      // mount point for the device. It can take some time before when you get
      // the notification and the device is actually mounted, so you have to 
      // check for this device name in a loop, waiting for it to mount.
      CFStringRef bsdName = NULL;
      for ( u32 i = 0; i < 200; i++ )
         bsdName = ( CFStringRef ) IORegistryEntrySearchCFProperty( 
            CFSTR( kIOBSDNameKey ),
            kIORegistryIterateRecursively );

        if ( bsdName )
            // The bsdName is a CFStringRef, we'll need to convert it to 
            // a c string, using one of our utility functions, which we can
            // show later.
            char bsdNameBuf[4096];
            sprintf( bsdNameBuf, "/dev/%ss1", cfStringRefToCString( bsdName ));
            const char* bsdNameC = &bsdNameBuf][0];

            DASessionRef daSession = DASessionCreate( kCFAllocatorDefault );
            DADiskRef disk = DADiskCreateFromBSDName( kCFAllocatorDefault,
                                                      aSession, bsdNameC );
            if ( disk )
               // once you get here, the device is now mounted, but you
               // have to wait for any associated disk volumes to mount.
               for ( u32 i = 0; i < 200; i++ )
                  CFDictionaryRef desc = DADiskCopyDescription( disk );
                  if ( desc )
                     // at this point you finally have the volume name associated
                     // with a USB Flash disk. But you get it as a CFTypeRef.
                     // You'll need to convert it to a c string.
                     // CFShow( desc );
                     CFTypeRef str = CFDictionaryGetValue( 
                        kDADiskDescriptionVolumeNameKey );

                     // cfTypeToCString will convert it- this is one of our
                     // utility functions that I will show later.
                     char* volumeName = cfTypeToCString( str );
                     if ( volumeName && strlen( volumeName ))
                        // now that we have the mapping from USB device
                        // to mounted volume, we can finally extract the 
                        // serial number from the USB device, and report it as 
                        // the serial number of a mounted volume. We'll 
                        // show the function to extract this data later.
                        // UsbDeviceInfo is one of our classes that holds 
                        // the data, that we can also describe later.
                        UsbDeviceInfo info;
                        getUSBDeviceInfo( usbDevice, info );
                        info.setVolumeName( volumeName );

                        // store the name of the mounted volume with the 
                        // serial number.
                        char* volumePath = malloc( 4096 );
                        sprintf( volumePath, "/Volumes/%s", volumeName );
                        info.setMountPath( volumePath );

                        // this calls back to your own code with the volume 
                        // name, serial number and other information.
                        doStuff( info );

                        CFRelease( desc );
                     } else {
                        CFRelease( desc );
                  } else {
                     // we didn't get a mounted volume yet - so keep waiting.
                     kxSleep( 100000 );
               CFRelease( disk );   
            CFRelease( daSession );     
         } else {
            // we didn't get a bsd name, so keep waiting for the device to
            // get mounted.
            kxSleep( 10000 );

And here are some quick utility functions to extract c strings out of CFStrings and similar:

char* cfStringRefToCString( CFStringRef cfString )
  if ( !cfString ) return NULL;

   static char string[2048];
   string[0] = '\0';
   // CFShow( CFCopyDescription( cfString ));
   CFStringGetCString( cfString,
                       string, MAXPATHLEN, GetApplicationTextEncoding() );
   return &string[0];

char* cfTypeToCString( CFTypeRef cfString )
   if ( !cfString ) return NULL;

   static char deviceFilePath[2048];
   deviceFilePath[0] = '\0';
   // CFShow( CFCopyDescription( cfString ));
   CFStringGetCString( CFCopyDescription( cfString ),
                       deviceFilePath, MAXPATHLEN, 
                       GetApplicationTextEncoding() );
   char* p = deviceFilePath;
   while ( *p != '\"' ) p++; p++;
   char* pp = p;
   while ( *pp != '\"' ) pp++;
   *pp = '\0';
   if ( isdigit( *p )) *p = 'x';
   return p;

Getting USB Serial Number for a USB Device

And finally, now that we have an association from volume name to serial number, we can actually extract the serial number from the USB bus. On OsX this is done using low-level calls to the USB bus.

First lets define a class to hold the USB information. I’ll just describe the interface to this object, you can fill in the implementation in a way that works for you.

class UsbDeviceInfo
   void setVid( const char* str );
   void setPid( const char* str );
   void setRev( int rev );
   void setSerialNumber( const char* str );
   void setMountPath( const char* str );
   void setVolumeName( const char* str );

And we’re going to define getUSBDeviceInfo() that will extract the device information.

void getUSBDeviceInfo( io_service_t usbDevice, UsbDeviceInfo& info )
   // Just as ugly as the stuff you have to do for windows
   SInt32                        score; 
   IOCFPlugInInterface**         plugin; 
   IOUSBDeviceInterface182**     usbDevice182 = NULL; 
   kern_return_t                 err; 
   err = IOCreatePlugInInterfaceForService( usbDevice, 
                                            &score ); 
   if ( err != 0 )
      // error getting the plugin interface. You can get this if you somehow
      // found your way here before the device and volumes are fully mounted.

   // Now get the USB device interface. This will allow you to make the low-level
   // USB bus calls to extract device information. This is just a wrapper to
   // the low level USB interface. Once you have one of these, you can manipulate
   // the device at the bus level. BTW "182" means the version of this function.
   // OsX will keep updating this function as their support for new USB bus
   // features is implemented. For our purposes we need 182 or higher.
   err = (*plugin)->QueryInterface( 
      (void**)&usbDevice182 ); 

   // we just needed the plugin interface to get the USB interface, so we can 
   // release it now.
   IODestroyPlugInInterface( plugin ); 

   if ( err != 0 )
      // failed to get the bus interface.

   // To understand this you should look at the USB bus interface spec. 
   // But basically were getting byte offsets into the USB device descriptor
   // record so we can copy out the strings.
   UInt8 vidIdx;
   UInt8 pidIdx;
   UInt8 snIdx;
   UInt16 rev;

   (*usbDevice182)->USBGetManufacturerStringIndex( usbDevice182, &vidIdx );
   (*usbDevice182)->USBGetProductStringIndex     ( usbDevice182, &pidIdx );
   (*usbDevice182)->USBGetSerialNumberStringIndex( usbDevice182, &snIdx );
   (*usbDevice182)->GetDeviceReleaseNumber       ( usbDevice182, &rev );

   // copy out the strings and set them in our info class.
   info.setVid          ( getUSBStringDescriptor( usbDevice182, vidIdx ));
   info.setPid          ( getUSBStringDescriptor( usbDevice182, pidIdx ));
   info.setSerialNumber ( getUSBStringDescriptor( usbDevice182, snIdx ));

   info.setRev( rev );

There is just one more thing to do. Implementing a function that copies out strings from USB device descriptors. Again this is low level USB bus stuff that makes more sense if you read the USB bus spec.

const char* getUSBStringDescriptor( IOUSBDeviceInterface182** usbDevice, u8 idx )
   assert( usbDevice );
   UInt16 buffer[64];

   // wow... we're actually forced to make hard coded bus requests. Its like 
   // hard disk programming in the 80's!
   IOUSBDevRequest request;

   request.bmRequestType = USBmakebmRequestType( 
      kUSBDevice );
   request.bRequest = kUSBRqGetDescriptor;
   request.wValue = (kUSBStringDesc << 8) | idx;
   request.wIndex = 0x409; // english
   request.wLength = sizeof( buffer );
   request.pData = buffer;

   kern_return_t err = (*usbDevice)->DeviceRequest( usbDevice, &request );
   if ( err != 0 )
      // the request failed... fairly uncommon for the USB disk driver, but not
      // so uncommon for other devices. This can also be less reliable if your 
      // disk is mounted through an external USB hub. At this level we actually
      // have to worry about hardware issues like this.
      return NULL;
   // we're mallocing this string just as an example. But you probably will want 
   // to do something smarter, like pre-allocated buffers in the info class, or 
   // use a string class.
   char* string = malloc( 128 );

   u32 count = ( request.wLenDone - 1 ) / 2;
   u32 i;
   for ( i = 0; i < count; i++ )
      string[i] = buffer[i+1];
   string[i] = '\0';  

   return string;

Pretty ugly, but there you have it.


USB Device Interface Guide An Apple USB Programming Guide
IOKit Fundamentals Apple’s reference documents on using IOKit
USB Made Simple A Website about USB Bus Programming
USB In a Nutshell Another Site about USB Bus Programming
Flash Memory Toolkit A program to view the serial number, for debugging