libusb/descriptor.c

   1 /*
   2  * USB descriptor handling functions for libusbx
   3  * Copyright © 2007 Daniel Drake <dsd@gentoo.org>
   4  * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
   5  *
   6  * This library is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU Lesser General Public
   8  * License as published by the Free Software Foundation; either
   9  * version 2.1 of the License, or (at your option) any later version.
  10  *
  11  * This library is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14  * Lesser General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU Lesser General Public
  17  * License along with this library; if not, write to the Free Software
  18  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  19  */
  20
  21 #include <errno.h>
  22 #include <stdint.h>
  23 #include <stdlib.h>
  24 #include <string.h>
  25
  26 #include "libusbi.h"
  27
  28 #define DESC_HEADER_LENGTH              2
  29 #define DEVICE_DESC_LENGTH              18
  30 #define CONFIG_DESC_LENGTH              9
  31 #define INTERFACE_DESC_LENGTH           9
  32 #define ENDPOINT_DESC_LENGTH            7
  33 #define ENDPOINT_AUDIO_DESC_LENGTH      9
  34
  35 /** @defgroup desc USB descriptors
  36  * This page details how to examine the various standard USB descriptors
  37  * for detected devices
  38  */
  39
  40 /* set host_endian if the w values are already in host endian format,
  41  * as opposed to bus endian. */
  42 int usbi_parse_descriptor(unsigned char *source, const char *descriptor,
  43         void *dest, int host_endian)
  44 {
  45         unsigned char *sp = source, *dp = dest;
  46         uint16_t w;
  47         const char *cp;
  48
  49         for (cp = descriptor; *cp; cp++) {
  50                 switch (*cp) {
  51                         case 'b':       /* 8-bit byte */
  52                                 *dp++ = *sp++;
  53                                 break;
  54                         case 'w':       /* 16-bit word, convert from little endian to CPU */
  55                                 dp += ((uintptr_t)dp & 1);      /* Align to word boundary */
  56
  57                                 if (host_endian) {
  58                                         memcpy(dp, sp, 2);
  59                                 } else {
  60                                         w = (sp[1] << 8) | sp[0];
  61                                         *((uint16_t *)dp) = w;
  62                                 }
  63                                 sp += 2;
  64                                 dp += 2;
  65                                 break;
  66                 }
  67         }
  68
  69         return (int) (sp - source);
  70 }
  71
  72 static void clear_endpoint(struct libusb_endpoint_descriptor *endpoint)
  73 {
  74         if (endpoint->extra)
  75                 free((unsigned char *) endpoint->extra);
  76 }
  77
  78 static int parse_endpoint(struct libusb_context *ctx,
  79         struct libusb_endpoint_descriptor *endpoint, unsigned char *buffer,
  80         int size, int host_endian)
  81 {
  82         struct usb_descriptor_header header;
  83         unsigned char *extra;
  84         unsigned char *begin;
  85         int parsed = 0;
  86         int len;
  87
  88         usbi_parse_descriptor(buffer, "bb", &header, 0);
  89
  90         /* Everything should be fine being passed into here, but we sanity */
  91         /*  check JIC */
  92         if (header.bLength > size) {
  93                 usbi_err(ctx, "ran out of descriptors parsing");
  94                 return -1;
  95         }
  96
  97         if (header.bDescriptorType != LIBUSB_DT_ENDPOINT) {
  98                 usbi_err(ctx, "unexpected descriptor %x (expected %x)",
  99                         header.bDescriptorType, LIBUSB_DT_ENDPOINT);
 100                 return parsed;
 101         }
 102
 103         if (header.bLength >= ENDPOINT_AUDIO_DESC_LENGTH)
 104                 usbi_parse_descriptor(buffer, "bbbbwbbb", endpoint, host_endian);
 105         else if (header.bLength >= ENDPOINT_DESC_LENGTH)
 106                 usbi_parse_descriptor(buffer, "bbbbwb", endpoint, host_endian);
 107
 108         buffer += header.bLength;
 109         size -= header.bLength;
 110         parsed += header.bLength;
 111
 112         /* Skip over the rest of the Class Specific or Vendor Specific */
 113         /*  descriptors */
 114         begin = buffer;
 115         while (size >= DESC_HEADER_LENGTH) {
 116                 usbi_parse_descriptor(buffer, "bb", &header, 0);
 117
 118                 if (header.bLength < 2) {
 119                         usbi_err(ctx, "invalid descriptor length %d", header.bLength);
 120                         return -1;
 121                 }
 122
 123                 /* If we find another "proper" descriptor then we're done  */
 124                 if ((header.bDescriptorType == LIBUSB_DT_ENDPOINT) ||
 125                                 (header.bDescriptorType == LIBUSB_DT_INTERFACE) ||
 126                                 (header.bDescriptorType == LIBUSB_DT_CONFIG) ||
 127                                 (header.bDescriptorType == LIBUSB_DT_DEVICE))
 128                         break;
 129
 130                 usbi_dbg("skipping descriptor %x", header.bDescriptorType);
 131                 buffer += header.bLength;
 132                 size -= header.bLength;
 133                 parsed += header.bLength;
 134         }
 135
 136         /* Copy any unknown descriptors into a storage area for drivers */
 137         /*  to later parse */
 138         len = (int)(buffer - begin);
 139         if (!len) {
 140                 endpoint->extra = NULL;
 141                 endpoint->extra_length = 0;
 142                 return parsed;
 143         }
 144
 145         extra = malloc(len);
 146         endpoint->extra = extra;
 147         if (!extra) {
 148                 endpoint->extra_length = 0;
 149                 return LIBUSB_ERROR_NO_MEM;
 150         }
 151
 152         memcpy(extra, begin, len);
 153         endpoint->extra_length = len;
 154
 155         return parsed;
 156 }
 157
 158 static void clear_interface(struct libusb_interface *usb_interface)
 159 {
 160         int i;
 161         int j;
 162
 163         if (usb_interface->altsetting) {
 164                 for (i = 0; i < usb_interface->num_altsetting; i++) {
 165                         struct libusb_interface_descriptor *ifp =
 166                                 (struct libusb_interface_descriptor *)
 167                                 usb_interface->altsetting + i;
 168                         if (ifp->extra)
 169                                 free((void *) ifp->extra);
 170                         if (ifp->endpoint) {
 171                                 for (j = 0; j < ifp->bNumEndpoints; j++)
 172                                         clear_endpoint((struct libusb_endpoint_descriptor *)
 173                                                 ifp->endpoint + j);
 174                                 free((void *) ifp->endpoint);
 175                         }
 176                 }
 177                 free((void *) usb_interface->altsetting);
 178                 usb_interface->altsetting = NULL;
 179         }
 180
 181 }
 182
 183 static int parse_interface(libusb_context *ctx,
 184         struct libusb_interface *usb_interface, unsigned char *buffer, int size,
 185         int host_endian)
 186 {
 187         int i;
 188         int len;
 189         int r;
 190         int parsed = 0;
 191         size_t tmp;
 192         struct usb_descriptor_header header;
 193         struct libusb_interface_descriptor *ifp;
 194         unsigned char *begin;
 195
 196         usb_interface->num_altsetting = 0;
 197
 198         while (size >= INTERFACE_DESC_LENGTH) {
 199                 struct libusb_interface_descriptor *altsetting =
 200                         (struct libusb_interface_descriptor *) usb_interface->altsetting;
 201                 altsetting = realloc(altsetting,
 202                         sizeof(struct libusb_interface_descriptor) *
 203                         (usb_interface->num_altsetting + 1));
 204                 if (!altsetting) {
 205                         r = LIBUSB_ERROR_NO_MEM;
 206                         goto err;
 207                 }
 208                 usb_interface->altsetting = altsetting;
 209
 210                 ifp = altsetting + usb_interface->num_altsetting;
 211                 usb_interface->num_altsetting++;
 212                 usbi_parse_descriptor(buffer, "bbbbbbbbb", ifp, 0);
 213                 ifp->extra = NULL;
 214                 ifp->extra_length = 0;
 215                 ifp->endpoint = NULL;
 216
 217                 /* Skip over the interface */
 218                 buffer += ifp->bLength;
 219                 parsed += ifp->bLength;
 220                 size -= ifp->bLength;
 221
 222                 begin = buffer;
 223
 224                 /* Skip over any interface, class or vendor descriptors */
 225                 while (size >= DESC_HEADER_LENGTH) {
 226                         usbi_parse_descriptor(buffer, "bb", &header, 0);
 227                         if (header.bLength < 2) {
 228                                 usbi_err(ctx, "invalid descriptor of length %d",
 229                                         header.bLength);
 230                                 r = LIBUSB_ERROR_IO;
 231                                 goto err;
 232                         }
 233
 234                         /* If we find another "proper" descriptor then we're done */
 235                         if ((header.bDescriptorType == LIBUSB_DT_INTERFACE) ||
 236                                         (header.bDescriptorType == LIBUSB_DT_ENDPOINT) ||
 237                                         (header.bDescriptorType == LIBUSB_DT_CONFIG) ||
 238                                         (header.bDescriptorType == LIBUSB_DT_DEVICE))
 239                                 break;
 240
 241                         buffer += header.bLength;
 242                         parsed += header.bLength;
 243                         size -= header.bLength;
 244                 }
 245
 246                 /* Copy any unknown descriptors into a storage area for */
 247                 /*  drivers to later parse */
 248                 len = (int)(buffer - begin);
 249                 if (len) {
 250                         ifp->extra = malloc(len);
 251                         if (!ifp->extra) {
 252                                 r = LIBUSB_ERROR_NO_MEM;
 253                                 goto err;
 254                         }
 255                         memcpy((unsigned char *) ifp->extra, begin, len);
 256                         ifp->extra_length = len;
 257                 }
 258
 259                 /* Did we hit an unexpected descriptor? */
 260                 if (size >= DESC_HEADER_LENGTH) {
 261                         usbi_parse_descriptor(buffer, "bb", &header, 0);
 262                         if ((header.bDescriptorType == LIBUSB_DT_CONFIG) ||
 263                             (header.bDescriptorType == LIBUSB_DT_DEVICE)) {
 264                                 return parsed;
 265                         }
 266                 }
 267
 268                 if (ifp->bNumEndpoints > USB_MAXENDPOINTS) {
 269                         usbi_err(ctx, "too many endpoints (%d)", ifp->bNumEndpoints);
 270                         r = LIBUSB_ERROR_IO;
 271                         goto err;
 272                 }
 273
 274                 if (ifp->bNumEndpoints > 0) {
 275                         struct libusb_endpoint_descriptor *endpoint;
 276                         tmp = ifp->bNumEndpoints * sizeof(struct libusb_endpoint_descriptor);
 277                         endpoint = malloc(tmp);
 278                         ifp->endpoint = endpoint;
 279                         if (!endpoint) {
 280                                 r = LIBUSB_ERROR_NO_MEM;
 281                                 goto err;
 282                         }
 283
 284                         memset(endpoint, 0, tmp);
 285                         for (i = 0; i < ifp->bNumEndpoints; i++) {
 286                                 usbi_parse_descriptor(buffer, "bb", &header, 0);
 287
 288                                 if (header.bLength > size) {
 289                                         usbi_err(ctx, "ran out of descriptors parsing");
 290                                         r = LIBUSB_ERROR_IO;
 291                                         goto err;
 292                                 }
 293
 294                                 r = parse_endpoint(ctx, endpoint + i, buffer, size,
 295                                         host_endian);
 296                                 if (r < 0)
 297                                         goto err;
 298
 299                                 buffer += r;
 300                                 parsed += r;
 301                                 size -= r;
 302                         }
 303                 }
 304
 305                 /* We check to see if it's an alternate to this one */
 306                 ifp = (struct libusb_interface_descriptor *) buffer;
 307                 if (size < LIBUSB_DT_INTERFACE_SIZE ||
 308                                 ifp->bDescriptorType != LIBUSB_DT_INTERFACE ||
 309                                 !ifp->bAlternateSetting)
 310                         return parsed;
 311         }
 312
 313         return parsed;
 314 err:
 315         clear_interface(usb_interface);
 316         return r;
 317 }
 318
 319 static void clear_configuration(struct libusb_config_descriptor *config)
 320 {
 321         if (config->interface) {
 322                 int i;
 323                 for (i = 0; i < config->bNumInterfaces; i++)
 324                         clear_interface((struct libusb_interface *)
 325                                 config->interface + i);
 326                 free((void *) config->interface);
 327         }
 328         if (config->extra)
 329                 free((void *) config->extra);
 330 }
 331
 332 static int parse_configuration(struct libusb_context *ctx,
 333         struct libusb_config_descriptor *config, unsigned char *buffer,
 334         int host_endian)
 335 {
 336         int i;
 337         int r;
 338         int size;
 339         size_t tmp;
 340         struct usb_descriptor_header header;
 341         struct libusb_interface *usb_interface;
 342
 343         usbi_parse_descriptor(buffer, "bbwbbbbb", config, host_endian);
 344         size = config->wTotalLength;
 345
 346         if (config->bNumInterfaces > USB_MAXINTERFACES) {
 347                 usbi_err(ctx, "too many interfaces (%d)", config->bNumInterfaces);
 348                 return LIBUSB_ERROR_IO;
 349         }
 350
 351         tmp = config->bNumInterfaces * sizeof(struct libusb_interface);
 352         usb_interface = malloc(tmp);
 353         config->interface = usb_interface;
 354         if (!config->interface)
 355                 return LIBUSB_ERROR_NO_MEM;
 356
 357         memset(usb_interface, 0, tmp);
 358         buffer += config->bLength;
 359         size -= config->bLength;
 360
 361         config->extra = NULL;
 362         config->extra_length = 0;
 363
 364         for (i = 0; i < config->bNumInterfaces; i++) {
 365                 int len;
 366                 unsigned char *begin;
 367
 368                 /* Skip over the rest of the Class Specific or Vendor */
 369                 /*  Specific descriptors */
 370                 begin = buffer;
 371                 while (size >= DESC_HEADER_LENGTH) {
 372                         usbi_parse_descriptor(buffer, "bb", &header, 0);
 373
 374                         if ((header.bLength > size) ||
 375                                         (header.bLength < DESC_HEADER_LENGTH)) {
 376                                 usbi_err(ctx, "invalid descriptor length of %d",
 377                                         header.bLength);
 378                                 r = LIBUSB_ERROR_IO;
 379                                 goto err;
 380                         }
 381
 382                         /* If we find another "proper" descriptor then we're done */
 383                         if ((header.bDescriptorType == LIBUSB_DT_ENDPOINT) ||
 384                                         (header.bDescriptorType == LIBUSB_DT_INTERFACE) ||
 385                                         (header.bDescriptorType == LIBUSB_DT_CONFIG) ||
 386                                         (header.bDescriptorType == LIBUSB_DT_DEVICE))
 387                                 break;
 388
 389                         usbi_dbg("skipping descriptor 0x%x\n", header.bDescriptorType);
 390                         buffer += header.bLength;
 391                         size -= header.bLength;
 392                 }
 393
 394                 /* Copy any unknown descriptors into a storage area for */
 395                 /*  drivers to later parse */
 396                 len = (int)(buffer - begin);
 397                 if (len) {
 398                         /* FIXME: We should realloc and append here */
 399                         if (!config->extra_length) {
 400                                 config->extra = malloc(len);
 401                                 if (!config->extra) {
 402                                         r = LIBUSB_ERROR_NO_MEM;
 403                                         goto err;
 404                                 }
 405
 406                                 memcpy((unsigned char *) config->extra, begin, len);
 407                                 config->extra_length = len;
 408                         }
 409                 }
 410
 411                 r = parse_interface(ctx, usb_interface + i, buffer, size, host_endian);
 412                 if (r < 0)
 413                         goto err;
 414
 415                 buffer += r;
 416                 size -= r;
 417         }
 418
 419         return size;
 420
 421 err:
 422         clear_configuration(config);
 423         return r;
 424 }
 425
 426 /** \ingroup desc
 427  * Get the USB device descriptor for a given device.
 428  *
 429  * This is a non-blocking function; the device descriptor is cached in memory.
 430  *
 431  * \param dev the device
 432  * \param desc output location for the descriptor data
 433  * \returns 0 on success or a LIBUSB_ERROR code on failure
 434  */
 435 int API_EXPORTED libusb_get_device_descriptor(libusb_device *dev,
 436         struct libusb_device_descriptor *desc)
 437 {
 438         unsigned char raw_desc[DEVICE_DESC_LENGTH];
 439         int host_endian = 0;
 440         int r;
 441
 442         usbi_dbg("");
 443         r = usbi_backend->get_device_descriptor(dev, raw_desc, &host_endian);
 444         if (r < 0)
 445                 return r;
 446
 447         memcpy((unsigned char *) desc, raw_desc, sizeof(raw_desc));
 448         if (!host_endian) {
 449                 desc->bcdUSB = libusb_le16_to_cpu(desc->bcdUSB);
 450                 desc->idVendor = libusb_le16_to_cpu(desc->idVendor);
 451                 desc->idProduct = libusb_le16_to_cpu(desc->idProduct);
 452                 desc->bcdDevice = libusb_le16_to_cpu(desc->bcdDevice);
 453         }
 454         return 0;
 455 }
 456
 457 /** \ingroup desc
 458  * Get the USB configuration descriptor for the currently active configuration.
 459  * This is a non-blocking function which does not involve any requests being
 460  * sent to the device.
 461  *
 462  * \param dev a device
 463  * \param config output location for the USB configuration descriptor. Only
 464  * valid if 0 was returned. Must be freed with libusb_free_config_descriptor()
 465  * after use.
 466  * \returns 0 on success
 467  * \returns LIBUSB_ERROR_NOT_FOUND if the device is in unconfigured state
 468  * \returns another LIBUSB_ERROR code on error
 469  * \see libusb_get_config_descriptor
 470  */
 471 int API_EXPORTED libusb_get_active_config_descriptor(libusb_device *dev,
 472         struct libusb_config_descriptor **config)
 473 {
 474         struct libusb_config_descriptor *_config = malloc(sizeof(*_config));
 475         unsigned char tmp[8];
 476         unsigned char *buf = NULL;
 477         int host_endian = 0;
 478         int r;
 479
 480         usbi_dbg("");
 481         if (!_config)
 482                 return LIBUSB_ERROR_NO_MEM;
 483
 484         r = usbi_backend->get_active_config_descriptor(dev, tmp, sizeof(tmp),
 485                 &host_endian);
 486         if (r < 0)
 487                 goto err;
 488
 489         usbi_parse_descriptor(tmp, "bbw", _config, host_endian);
 490         buf = malloc(_config->wTotalLength);
 491         if (!buf) {
 492                 r = LIBUSB_ERROR_NO_MEM;
 493                 goto err;
 494         }
 495
 496         r = usbi_backend->get_active_config_descriptor(dev, buf,
 497                 _config->wTotalLength, &host_endian);
 498         if (r < 0)
 499                 goto err;
 500
 501         r = parse_configuration(dev->ctx, _config, buf, host_endian);
 502         if (r < 0) {
 503                 usbi_err(dev->ctx, "parse_configuration failed with error %d", r);
 504                 goto err;
 505         } else if (r > 0) {
 506                 usbi_warn(dev->ctx, "descriptor data still left");
 507         }
 508
 509         free(buf);
 510         *config = _config;
 511         return 0;
 512
 513 err:
 514         free(_config);
 515         if (buf)
 516                 free(buf);
 517         return r;
 518 }
 519
 520 /** \ingroup desc
 521  * Get a USB configuration descriptor based on its index.
 522  * This is a non-blocking function which does not involve any requests being
 523  * sent to the device.
 524  *
 525  * \param dev a device
 526  * \param config_index the index of the configuration you wish to retrieve
 527  * \param config output location for the USB configuration descriptor. Only
 528  * valid if 0 was returned. Must be freed with libusb_free_config_descriptor()
 529  * after use.
 530  * \returns 0 on success
 531  * \returns LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
 532  * \returns another LIBUSB_ERROR code on error
 533  * \see libusb_get_active_config_descriptor()
 534  * \see libusb_get_config_descriptor_by_value()
 535  */
 536 int API_EXPORTED libusb_get_config_descriptor(libusb_device *dev,
 537         uint8_t config_index, struct libusb_config_descriptor **config)
 538 {
 539         struct libusb_config_descriptor *_config;
 540         unsigned char tmp[8];
 541         unsigned char *buf = NULL;
 542         int host_endian = 0;
 543         int r;
 544
 545         usbi_dbg("index %d", config_index);
 546         if (config_index >= dev->num_configurations)
 547                 return LIBUSB_ERROR_NOT_FOUND;
 548
 549         _config = malloc(sizeof(*_config));
 550         if (!_config)
 551                 return LIBUSB_ERROR_NO_MEM;
 552
 553         r = usbi_backend->get_config_descriptor(dev, config_index, tmp,
 554                 sizeof(tmp), &host_endian);
 555         if (r < 0)
 556                 goto err;
 557
 558         usbi_parse_descriptor(tmp, "bbw", _config, host_endian);
 559         buf = malloc(_config->wTotalLength);
 560         if (!buf) {
 561                 r = LIBUSB_ERROR_NO_MEM;
 562                 goto err;
 563         }
 564
 565         host_endian = 0;
 566         r = usbi_backend->get_config_descriptor(dev, config_index, buf,
 567                 _config->wTotalLength, &host_endian);
 568         if (r < 0)
 569                 goto err;
 570
 571         r = parse_configuration(dev->ctx, _config, buf, host_endian);
 572         if (r < 0) {
 573                 usbi_err(dev->ctx, "parse_configuration failed with error %d", r);
 574                 goto err;
 575         } else if (r > 0) {
 576                 usbi_warn(dev->ctx, "descriptor data still left");
 577         }
 578
 579         free(buf);
 580         *config = _config;
 581         return 0;
 582
 583 err:
 584         free(_config);
 585         if (buf)
 586                 free(buf);
 587         return r;
 588 }
 589
 590 /* iterate through all configurations, returning the index of the configuration
 591  * matching a specific bConfigurationValue in the idx output parameter, or -1
 592  * if the config was not found.
 593  * returns 0 or a LIBUSB_ERROR code
 594  */
 595 int usbi_get_config_index_by_value(struct libusb_device *dev,
 596         uint8_t bConfigurationValue, int *idx)
 597 {
 598         uint8_t i;
 599
 600         usbi_dbg("value %d", bConfigurationValue);
 601         for (i = 0; i < dev->num_configurations; i++) {
 602                 unsigned char tmp[6];
 603                 int host_endian;
 604                 int r = usbi_backend->get_config_descriptor(dev, i, tmp, sizeof(tmp),
 605                         &host_endian);
 606                 if (r < 0)
 607                         return r;
 608                 if (tmp[5] == bConfigurationValue) {
 609                         *idx = i;
 610                         return 0;
 611                 }
 612         }
 613
 614         *idx = -1;
 615         return 0;
 616 }
 617
 618 /** \ingroup desc
 619  * Get a USB configuration descriptor with a specific bConfigurationValue.
 620  * This is a non-blocking function which does not involve any requests being
 621  * sent to the device.
 622  *
 623  * \param dev a device
 624  * \param bConfigurationValue the bConfigurationValue of the configuration you
 625  * wish to retrieve
 626  * \param config output location for the USB configuration descriptor. Only
 627  * valid if 0 was returned. Must be freed with libusb_free_config_descriptor()
 628  * after use.
 629  * \returns 0 on success
 630  * \returns LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
 631  * \returns another LIBUSB_ERROR code on error
 632  * \see libusb_get_active_config_descriptor()
 633  * \see libusb_get_config_descriptor()
 634  */
 635 int API_EXPORTED libusb_get_config_descriptor_by_value(libusb_device *dev,
 636         uint8_t bConfigurationValue, struct libusb_config_descriptor **config)
 637 {
 638         int idx;
 639         int r = usbi_get_config_index_by_value(dev, bConfigurationValue, &idx);
 640         if (r < 0)
 641                 return r;
 642         else if (idx == -1)
 643                 return LIBUSB_ERROR_NOT_FOUND;
 644         else
 645                 return libusb_get_config_descriptor(dev, (uint8_t) idx, config);
 646 }
 647
 648 /** \ingroup desc
 649  * Free a configuration descriptor obtained from
 650  * libusb_get_active_config_descriptor() or libusb_get_config_descriptor().
 651  * It is safe to call this function with a NULL config parameter, in which
 652  * case the function simply returns.
 653  *
 654  * \param config the configuration descriptor to free
 655  */
 656 void API_EXPORTED libusb_free_config_descriptor(
 657         struct libusb_config_descriptor *config)
 658 {
 659         if (!config)
 660                 return;
 661
 662         clear_configuration(config);
 663         free(config);
 664 }
 665
 666 /** \ingroup desc
 667  * Retrieve a string descriptor in C style ASCII.
 668  *
 669  * Wrapper around libusb_get_string_descriptor(). Uses the first language
 670  * supported by the device.
 671  *
 672  * \param dev a device handle
 673  * \param desc_index the index of the descriptor to retrieve
 674  * \param data output buffer for ASCII string descriptor
 675  * \param length size of data buffer
 676  * \returns number of bytes returned in data, or LIBUSB_ERROR code on failure
 677  */
 678 int API_EXPORTED libusb_get_string_descriptor_ascii(libusb_device_handle *dev,
 679         uint8_t desc_index, unsigned char *data, int length)
 680 {
 681         unsigned char tbuf[255]; /* Some devices choke on size > 255 */
 682         int r, si, di;
 683         uint16_t langid;
 684
 685         /* Asking for the zero'th index is special - it returns a string
 686          * descriptor that contains all the language IDs supported by the
 687          * device. Typically there aren't many - often only one. Language
 688          * IDs are 16 bit numbers, and they start at the third byte in the
 689          * descriptor. There's also no point in trying to read descriptor 0
 690          * with this function. See USB 2.0 specification section 9.6.7 for
 691          * more information.
 692          */
 693
 694         if (desc_index == 0)
 695                 return LIBUSB_ERROR_INVALID_PARAM;
 696
 697         r = libusb_get_string_descriptor(dev, 0, 0, tbuf, sizeof(tbuf));
 698         if (r < 0)
 699                 return r;
 700
 701         if (r < 4)
 702                 return LIBUSB_ERROR_IO;
 703
 704         langid = tbuf[2] | (tbuf[3] << 8);
 705
 706         r = libusb_get_string_descriptor(dev, desc_index, langid, tbuf,
 707                 sizeof(tbuf));
 708         if (r < 0)
 709                 return r;
 710
 711         if (tbuf[1] != LIBUSB_DT_STRING)
 712                 return LIBUSB_ERROR_IO;
 713
 714         if (tbuf[0] > r)
 715                 return LIBUSB_ERROR_IO;
 716
 717         for (di = 0, si = 2; si < tbuf[0]; si += 2) {
 718                 if (di >= (length - 1))
 719                         break;
 720
 721                 if (tbuf[si + 1]) /* high byte */
 722                         data[di++] = '?';
 723                 else
 724                         data[di++] = tbuf[si];
 725         }
 726
 727         data[di] = 0;
 728         return di;
 729 }