summaryrefslogtreecommitdiff
path: root/libusb/descriptor.c
blob: da4dbbe7f0d691a0c8f289171d7bee897c16cc2b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
/*
 * USB descriptor handling functions for libusb
 * Copyright (C) 2007 Daniel Drake <dsd@gentoo.org>
 * Copyright (c) 2001 Johannes Erdfelt <johannes@erdfelt.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#include <errno.h>
#include <stdlib.h>
#include <string.h>

#include "libusbi.h"

#define DESC_HEADER_LENGTH		2
#define DEVICE_DESC_LENGTH		18
#define CONFIG_DESC_LENGTH		9
#define INTERFACE_DESC_LENGTH		9
#define ENDPOINT_DESC_LENGTH		7
#define ENDPOINT_AUDIO_DESC_LENGTH	9

/** @defgroup desc USB descriptors
 * This page details how to examine the various standard USB descriptors
 * for detected devices
 */

/* set host_endian if the w values are already in host endian format,
 * as opposed to bus endian. */
int usbi_parse_descriptor(unsigned char *source, char *descriptor, void *dest,
	int host_endian)
{
	unsigned char *sp = source, *dp = dest;
	uint16_t w;
	char *cp;

	for (cp = descriptor; *cp; cp++) {
		switch (*cp) {
			case 'b':	/* 8-bit byte */
				*dp++ = *sp++;
				break;
			case 'w':	/* 16-bit word, convert from little endian to CPU */
				dp += ((uintptr_t)dp & 1);	/* Align to word boundary */

				if (host_endian) {
					memcpy(dp, sp, 2);
				} else {
					w = (sp[1] << 8) | sp[0];
					*((uint16_t *)dp) = w;
				}
				sp += 2;
				dp += 2;
				break;
		}
	}

	return (int)(sp - source);
}

static void clear_endpoint(struct libusb_endpoint_descriptor *endpoint)
{
	if (endpoint->extra)
		free((unsigned char *) endpoint->extra);
}

static int parse_endpoint(struct libusb_context *ctx,
	struct libusb_endpoint_descriptor *endpoint, unsigned char *buffer,
	int size, int host_endian)
{
	struct usb_descriptor_header header;
	unsigned char *extra;
	unsigned char *begin;
	int parsed = 0;
	int len;

	usbi_parse_descriptor(buffer, "bb", &header, 0);

	/* Everything should be fine being passed into here, but we sanity */
	/*  check JIC */
	if (header.bLength > size) {
		usbi_err(ctx, "ran out of descriptors parsing");
		return -1;
	}

	if (header.bDescriptorType != LIBUSB_DT_ENDPOINT) {
		usbi_err(ctx, "unexpected descriptor %x (expected %x)",
			header.bDescriptorType, LIBUSB_DT_ENDPOINT);
		return parsed;
	}

	if (header.bLength >= ENDPOINT_AUDIO_DESC_LENGTH)
		usbi_parse_descriptor(buffer, "bbbbwbbb", endpoint, host_endian);
	else if (header.bLength >= ENDPOINT_DESC_LENGTH)
		usbi_parse_descriptor(buffer, "bbbbwb", endpoint, host_endian);

	buffer += header.bLength;
	size -= header.bLength;
	parsed += header.bLength;

	/* Skip over the rest of the Class Specific or Vendor Specific */
	/*  descriptors */
	begin = buffer;
	while (size >= DESC_HEADER_LENGTH) {
		usbi_parse_descriptor(buffer, "bb", &header, 0);

		if (header.bLength < 2) {
			usbi_err(ctx, "invalid descriptor length %d", header.bLength);
			return -1;
		}

		/* If we find another "proper" descriptor then we're done  */
		if ((header.bDescriptorType == LIBUSB_DT_ENDPOINT) ||
				(header.bDescriptorType == LIBUSB_DT_INTERFACE) ||
				(header.bDescriptorType == LIBUSB_DT_CONFIG) ||
				(header.bDescriptorType == LIBUSB_DT_DEVICE))
			break;

		usbi_dbg("skipping descriptor %x", header.bDescriptorType);
		buffer += header.bLength;
		size -= header.bLength;
		parsed += header.bLength;
	}

	/* Copy any unknown descriptors into a storage area for drivers */
	/*  to later parse */
	len = (int)(buffer - begin);
	if (!len) {
		endpoint->extra = NULL;
		endpoint->extra_length = 0;
		return parsed;
	}

	extra = malloc(len);
	endpoint->extra = extra;
	if (!extra) {
		endpoint->extra_length = 0;
		return LIBUSB_ERROR_NO_MEM;
	}

	memcpy(extra, begin, len);
	endpoint->extra_length = len;

	return parsed;
}

static void clear_interface(struct libusb_interface *usb_interface)
{
	int i;
	int j;

	if (usb_interface->altsetting) {
		for (i = 0; i < usb_interface->num_altsetting; i++) {
			struct libusb_interface_descriptor *ifp =
				(struct libusb_interface_descriptor *)
				usb_interface->altsetting + i;
			if (ifp->extra)
				free((void *) ifp->extra);
			if (ifp->endpoint) {
				for (j = 0; j < ifp->bNumEndpoints; j++)
					clear_endpoint((struct libusb_endpoint_descriptor *)
						ifp->endpoint + j);
				free((void *) ifp->endpoint);
			}
		}
		free((void *) usb_interface->altsetting);
		usb_interface->altsetting = NULL;
	}
	
}

static int parse_interface(libusb_context *ctx,
	struct libusb_interface *usb_interface, unsigned char *buffer, int size,
	int host_endian)
{
	int i;
	int len;
	int r;
	int parsed = 0;
	int tmp;
	struct usb_descriptor_header header;
	struct libusb_interface_descriptor *ifp;
	unsigned char *begin;

	usb_interface->num_altsetting = 0;

	while (size >= INTERFACE_DESC_LENGTH) {
		struct libusb_interface_descriptor *altsetting =
			(struct libusb_interface_descriptor *) usb_interface->altsetting;
		altsetting = realloc(altsetting,
			sizeof(struct libusb_interface_descriptor) *
			(usb_interface->num_altsetting + 1));
		if (!altsetting) {
			r = LIBUSB_ERROR_NO_MEM;
			goto err;
		}
		usb_interface->altsetting = altsetting;

		ifp = altsetting + usb_interface->num_altsetting;
		usb_interface->num_altsetting++;
		usbi_parse_descriptor(buffer, "bbbbbbbbb", ifp, 0);
		ifp->extra = NULL;
		ifp->extra_length = 0;
		ifp->endpoint = NULL;

		/* Skip over the interface */
		buffer += ifp->bLength;
		parsed += ifp->bLength;
		size -= ifp->bLength;

		begin = buffer;

		/* Skip over any interface, class or vendor descriptors */
		while (size >= DESC_HEADER_LENGTH) {
			usbi_parse_descriptor(buffer, "bb", &header, 0);
			if (header.bLength < 2) {
				usbi_err(ctx, "invalid descriptor of length %d",
					header.bLength);
				r = LIBUSB_ERROR_IO;
				goto err;
			}

			/* If we find another "proper" descriptor then we're done */
			if ((header.bDescriptorType == LIBUSB_DT_INTERFACE) ||
					(header.bDescriptorType == LIBUSB_DT_ENDPOINT) ||
					(header.bDescriptorType == LIBUSB_DT_CONFIG) ||
					(header.bDescriptorType == LIBUSB_DT_DEVICE))
				break;

			buffer += header.bLength;
			parsed += header.bLength;
			size -= header.bLength;
		}

		/* Copy any unknown descriptors into a storage area for */
		/*  drivers to later parse */
		len = (int)(buffer - begin);
		if (len) {
			ifp->extra = malloc(len);
			if (!ifp->extra) {
				r = LIBUSB_ERROR_NO_MEM;
				goto err;
			}
			memcpy((unsigned char *) ifp->extra, begin, len);
			ifp->extra_length = len;
		}

		/* Did we hit an unexpected descriptor? */
		usbi_parse_descriptor(buffer, "bb", &header, 0);
		if ((size >= DESC_HEADER_LENGTH) &&
				((header.bDescriptorType == LIBUSB_DT_CONFIG) ||
				 (header.bDescriptorType == LIBUSB_DT_DEVICE)))
			return parsed;

		if (ifp->bNumEndpoints > USB_MAXENDPOINTS) {
			usbi_err(ctx, "too many endpoints (%d)", ifp->bNumEndpoints);
			r = LIBUSB_ERROR_IO;
			goto err;
		}

		if (ifp->bNumEndpoints > 0) {
			struct libusb_endpoint_descriptor *endpoint;
			tmp = ifp->bNumEndpoints * sizeof(struct libusb_endpoint_descriptor);
			endpoint = malloc(tmp);
			ifp->endpoint = endpoint;
			if (!endpoint) {
				r = LIBUSB_ERROR_NO_MEM;
				goto err;
			}

			memset(endpoint, 0, tmp);
			for (i = 0; i < ifp->bNumEndpoints; i++) {
				usbi_parse_descriptor(buffer, "bb", &header, 0);

				if (header.bLength > size) {
					usbi_err(ctx, "ran out of descriptors parsing");
					r = LIBUSB_ERROR_IO;
					goto err;
				}

				r = parse_endpoint(ctx, endpoint + i, buffer, size,
					host_endian);
				if (r < 0)
					goto err;

				buffer += r;
				parsed += r;
				size -= r;
			}
		}

		/* We check to see if it's an alternate to this one */
		ifp = (struct libusb_interface_descriptor *) buffer;
		if (size < LIBUSB_DT_INTERFACE_SIZE ||
				ifp->bDescriptorType != LIBUSB_DT_INTERFACE ||
				!ifp->bAlternateSetting)
			return parsed;
	}

	return parsed;
err:
	clear_interface(usb_interface);
	return r;
}

static void clear_configuration(struct libusb_config_descriptor *config)
{
	if (config->interface) {
		int i;
		for (i = 0; i < config->bNumInterfaces; i++)
			clear_interface((struct libusb_interface *)
				config->interface + i);
		free((void *) config->interface);
	}
	if (config->extra)
		free((void *) config->extra);
}

static int parse_configuration(struct libusb_context *ctx,
	struct libusb_config_descriptor *config, unsigned char *buffer,
	int host_endian)
{
	int i;
	int r;
	int size;
	int tmp;
	struct usb_descriptor_header header;
	struct libusb_interface *usb_interface;

	usbi_parse_descriptor(buffer, "bbwbbbbb", config, host_endian);
	size = config->wTotalLength;

	if (config->bNumInterfaces > USB_MAXINTERFACES) {
		usbi_err(ctx, "too many interfaces (%d)", config->bNumInterfaces);
		return LIBUSB_ERROR_IO;
	}

	tmp = config->bNumInterfaces * sizeof(struct libusb_interface);
	usb_interface = malloc(tmp);
	config->interface = usb_interface;
	if (!config->interface)
		return LIBUSB_ERROR_NO_MEM;

	memset(usb_interface, 0, tmp);
	buffer += config->bLength;
	size -= config->bLength;

	config->extra = NULL;
	config->extra_length = 0;

	for (i = 0; i < config->bNumInterfaces; i++) {
		int len;
		unsigned char *begin;

		/* Skip over the rest of the Class Specific or Vendor */
		/*  Specific descriptors */
		begin = buffer;
		while (size >= DESC_HEADER_LENGTH) {
			usbi_parse_descriptor(buffer, "bb", &header, 0);

			if ((header.bLength > size) ||
					(header.bLength < DESC_HEADER_LENGTH)) {
				usbi_err(ctx, "invalid descriptor length of %d",
					header.bLength);
				r = LIBUSB_ERROR_IO;
				goto err;
			}

			/* If we find another "proper" descriptor then we're done */
			if ((header.bDescriptorType == LIBUSB_DT_ENDPOINT) ||
					(header.bDescriptorType == LIBUSB_DT_INTERFACE) ||
					(header.bDescriptorType == LIBUSB_DT_CONFIG) ||
					(header.bDescriptorType == LIBUSB_DT_DEVICE))
				break;

			usbi_dbg("skipping descriptor 0x%x\n", header.bDescriptorType);
			buffer += header.bLength;
			size -= header.bLength;
		}

		/* Copy any unknown descriptors into a storage area for */
		/*  drivers to later parse */
		len = (int)(buffer - begin);
		if (len) {
			/* FIXME: We should realloc and append here */
			if (!config->extra_length) {
				config->extra = malloc(len);
				if (!config->extra) {
					r = LIBUSB_ERROR_NO_MEM;
					goto err;
				}

				memcpy((unsigned char *) config->extra, begin, len);
				config->extra_length = len;
			}
		}

		r = parse_interface(ctx, usb_interface + i, buffer, size, host_endian);
		if (r < 0)
			goto err;

		buffer += r;
		size -= r;
	}

	return size;

err:
	clear_configuration(config);
	return r;
}

/** \ingroup desc
 * Get the USB device descriptor for a given device.
 *
 * This is a non-blocking function; the device descriptor is cached in memory.
 *
 * \param dev the device
 * \param desc output location for the descriptor data
 * \returns 0 on success or a LIBUSB_ERROR code on failure
 */
API_EXPORTED int libusb_get_device_descriptor(libusb_device *dev,
	struct libusb_device_descriptor *desc)
{
	unsigned char raw_desc[DEVICE_DESC_LENGTH];
	int host_endian = 0;
	int r;

	usbi_dbg("");
	r = usbi_backend->get_device_descriptor(dev, raw_desc, &host_endian);
	if (r < 0)
		return r;

	memcpy((unsigned char *) desc, raw_desc, sizeof(raw_desc));
	if (!host_endian) {
		desc->bcdUSB = libusb_le16_to_cpu(desc->bcdUSB);
		desc->idVendor = libusb_le16_to_cpu(desc->idVendor);
		desc->idProduct = libusb_le16_to_cpu(desc->idProduct);
		desc->bcdDevice = libusb_le16_to_cpu(desc->bcdDevice);
	}
	return 0;
}

/** \ingroup desc
 * Get the USB configuration descriptor for the currently active configuration.
 * This is a non-blocking function which does not involve any requests being
 * sent to the device.
 *
 * \param dev a device
 * \param config output location for the USB configuration descriptor. Only
 * valid if 0 was returned. Must be freed with libusb_free_config_descriptor()
 * after use.
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if the device is in unconfigured state
 * \returns another LIBUSB_ERROR code on error
 * \see libusb_get_config_descriptor
 */
API_EXPORTED int libusb_get_active_config_descriptor(libusb_device *dev,
	struct libusb_config_descriptor **config)
{
	struct libusb_config_descriptor *_config = malloc(sizeof(*_config));
	unsigned char tmp[8];
	unsigned char *buf = NULL;
	int host_endian = 0;
	int r;

	usbi_dbg("");
	if (!_config)
		return LIBUSB_ERROR_NO_MEM;

	r = usbi_backend->get_active_config_descriptor(dev, tmp, sizeof(tmp),
		&host_endian);
	if (r < 0)
		goto err;

	usbi_parse_descriptor(tmp, "bbw", _config, host_endian);
	buf = malloc(_config->wTotalLength);
	if (!buf) {
		r = LIBUSB_ERROR_NO_MEM;
		goto err;
	}

	r = usbi_backend->get_active_config_descriptor(dev, buf,
		_config->wTotalLength, &host_endian);
	if (r < 0)
		goto err;

	r = parse_configuration(dev->ctx, _config, buf, host_endian);
	if (r < 0) {
		usbi_err(dev->ctx, "parse_configuration failed with error %d", r);
		goto err;
	} else if (r > 0) {
		usbi_warn(dev->ctx, "descriptor data still left");
	}

	free(buf);
	*config = _config;
	return 0;

err:
	free(_config);
	if (buf)
		free(buf);
	return r;
}

/** \ingroup desc
 * Get a USB configuration descriptor based on its index.
 * This is a non-blocking function which does not involve any requests being
 * sent to the device.
 *
 * \param dev a device
 * \param config_index the index of the configuration you wish to retrieve
 * \param config output location for the USB configuration descriptor. Only
 * valid if 0 was returned. Must be freed with libusb_free_config_descriptor()
 * after use.
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
 * \returns another LIBUSB_ERROR code on error
 * \see libusb_get_active_config_descriptor()
 * \see libusb_get_config_descriptor_by_value()
 */
API_EXPORTED int libusb_get_config_descriptor(libusb_device *dev,
	uint8_t config_index, struct libusb_config_descriptor **config)
{
	struct libusb_config_descriptor *_config;
	unsigned char tmp[8];
	unsigned char *buf = NULL;
	int host_endian = 0;
	int r;

	usbi_dbg("index %d", config_index);
	if (config_index >= dev->num_configurations)
		return LIBUSB_ERROR_NOT_FOUND;

	_config = malloc(sizeof(*_config));
	if (!_config)
		return LIBUSB_ERROR_NO_MEM;

	r = usbi_backend->get_config_descriptor(dev, config_index, tmp,
		sizeof(tmp), &host_endian);
	if (r < 0)
		goto err;

	usbi_parse_descriptor(tmp, "bbw", _config, host_endian);
	buf = malloc(_config->wTotalLength);
	if (!buf) {
		r = LIBUSB_ERROR_NO_MEM;
		goto err;
	}

	host_endian = 0;
	r = usbi_backend->get_config_descriptor(dev, config_index, buf,
		_config->wTotalLength, &host_endian);
	if (r < 0)
		goto err;

	r = parse_configuration(dev->ctx, _config, buf, host_endian);
	if (r < 0) {
		usbi_err(dev->ctx, "parse_configuration failed with error %d", r);
		goto err;
	} else if (r > 0) {
		usbi_warn(dev->ctx, "descriptor data still left");
	}

	free(buf);
	*config = _config;
	return 0;

err:
	free(_config);
	if (buf)
		free(buf);
	return r;
}

/* iterate through all configurations, returning the index of the configuration
 * matching a specific bConfigurationValue in the idx output parameter, or -1
 * if the config was not found.
 * returns 0 or a LIBUSB_ERROR code
 */
int usbi_get_config_index_by_value(struct libusb_device *dev,
	uint8_t bConfigurationValue, int *idx)
{
	uint8_t i;

	usbi_dbg("value %d", bConfigurationValue);
	for (i = 0; i < dev->num_configurations; i++) {
		unsigned char tmp[6];
		int host_endian;
		int r = usbi_backend->get_config_descriptor(dev, i, tmp, sizeof(tmp),
			&host_endian);
		if (r < 0)
			return r;
		if (tmp[5] == bConfigurationValue) {
			*idx = i;
			return 0;
		}
	}

	*idx = -1;
	return 0;
}

/** \ingroup desc
 * Get a USB configuration descriptor with a specific bConfigurationValue.
 * This is a non-blocking function which does not involve any requests being
 * sent to the device.
 *
 * \param dev a device
 * \param bConfigurationValue the bConfigurationValue of the configuration you
 * wish to retrieve
 * \param config output location for the USB configuration descriptor. Only
 * valid if 0 was returned. Must be freed with libusb_free_config_descriptor()
 * after use.
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
 * \returns another LIBUSB_ERROR code on error
 * \see libusb_get_active_config_descriptor()
 * \see libusb_get_config_descriptor()
 */
API_EXPORTED int libusb_get_config_descriptor_by_value(libusb_device *dev,
	uint8_t bConfigurationValue, struct libusb_config_descriptor **config)
{
	int idx;
	int r = usbi_get_config_index_by_value(dev, bConfigurationValue, &idx);
	if (r < 0)
		return r;
	else if (idx == -1)
		return LIBUSB_ERROR_NOT_FOUND;
	else
		return libusb_get_config_descriptor(dev, (uint8_t)idx, config);
}

/** \ingroup desc
 * Free a configuration descriptor obtained from
 * libusb_get_active_config_descriptor() or libusb_get_config_descriptor().
 * It is safe to call this function with a NULL config parameter, in which
 * case the function simply returns.
 *
 * \param config the configuration descriptor to free
 */
API_EXPORTED void libusb_free_config_descriptor(
	struct libusb_config_descriptor *config)
{
	if (!config)
		return;

	clear_configuration(config);
	free(config);
}

/** \ingroup desc
 * Retrieve a string descriptor in C style ASCII.
 *
 * Wrapper around libusb_get_string_descriptor(). Uses the first language 
 * supported by the device.
 *
 * \param dev a device handle
 * \param desc_index the index of the descriptor to retrieve
 * \param data output buffer for ASCII string descriptor
 * \param length size of data buffer
 * \returns number of bytes returned in data, or LIBUSB_ERROR code on failure
 */
API_EXPORTED int libusb_get_string_descriptor_ascii(libusb_device_handle *dev,
	uint8_t desc_index, unsigned char *data, int length)
{
	unsigned char tbuf[255]; /* Some devices choke on size > 255 */
	int r, si, di;
	uint16_t langid;

	/* Asking for the zero'th index is special - it returns a string
	 * descriptor that contains all the language IDs supported by the device.
	 * Typically there aren't many - often only one. The language IDs are 16
	 * bit numbers, and they start at the third byte in the descriptor. See
	 * USB 2.0 specification section 9.6.7 for more information. */
	r = libusb_get_string_descriptor(dev, 0, 0, tbuf, sizeof(tbuf));
	if (r < 0)
		return r;

	if (r < 4)
		return LIBUSB_ERROR_IO;

	langid = tbuf[2] | (tbuf[3] << 8);

	r = libusb_get_string_descriptor(dev, desc_index, langid, tbuf,
		sizeof(tbuf));
	if (r < 0)
		return r;

	if (tbuf[1] != LIBUSB_DT_STRING)
		return LIBUSB_ERROR_IO;

	if (tbuf[0] > r)
		return LIBUSB_ERROR_IO;

	for (di = 0, si = 2; si < tbuf[0]; si += 2) {
		if (di >= (length - 1))
			break;

		if (tbuf[si + 1]) /* high byte */
			data[di++] = '?';
		else
			data[di++] = tbuf[si];
	}

	data[di] = 0;
	return di;
}