summaryrefslogtreecommitdiffstats
path: root/src/H5Lexternal.c
blob: b8c7819cbf9ed10c6cf71836c363452e473e9b97 (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
/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * Copyright by The HDF Group.                                               *
 * Copyright by the Board of Trustees of the University of Illinois.         *
 * All rights reserved.                                                      *
 *                                                                           *
 * This file is part of HDF5.  The full HDF5 copyright notice, including     *
 * terms governing use, modification, and redistribution, is contained in    *
 * the COPYING file, which can be found at the root of the source code       *
 * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
 * If you do not have access to either file, you may request a copy from     *
 * help@hdfgroup.org.                                                        *
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

/****************/
/* Module Setup */
/****************/

#define H5G_FRIEND        /*suppress error about including H5Gpkg   */
#include "H5Lmodule.h"          /* This source code file is part of the H5L module */


/***********/
/* Headers */
/***********/
#include "H5private.h"          /* Generic Functions                    */
#include "H5ACprivate.h"        /* Metadata cache                       */
#include "H5CXprivate.h"        /* API Contexts                         */
#include "H5Eprivate.h"         /* Error handling                       */
#include "H5Fprivate.h"         /* Files                                */
#include "H5Gpkg.h"             /* Groups                               */
#include "H5Iprivate.h"         /* IDs                                  */
#include "H5Lpkg.h"             /* Links                                */
#include "H5MMprivate.h"        /* Memory management                    */
#include "H5Opublic.h"          /* File objects                         */
#include "H5Pprivate.h"         /* Property lists                       */
#include "H5VLprivate.h"        /* Virtual Object Layer                 */


/****************/
/* Local Macros */
/****************/

/* Version of external link format */
#define H5L_EXT_VERSION         0

/* Valid flags for external links */
#define H5L_EXT_FLAGS_ALL       0

/* Size of local link name buffer for traversing external links */
#define H5L_EXT_TRAVERSE_BUF_SIZE       256


/******************/
/* Local Typedefs */
/******************/


/********************/
/* Local Prototypes */
/********************/
static hid_t H5L__extern_traverse(const char *link_name, hid_t cur_group,
    const void *udata, size_t udata_size, hid_t lapl_id, hid_t dxpl_id);
static ssize_t H5L__extern_query(const char *link_name, const void *udata,
    size_t udata_size, void * buf /*out*/, size_t buf_size);


/*********************/
/* Package Variables */
/*********************/


/*****************************/
/* Library Private Variables */
/*****************************/


/*******************/
/* Local Variables */
/*******************/

/* Default External Link link class */
static const H5L_class_t H5L_EXTERN_LINK_CLASS[1] = {{
    H5L_LINK_CLASS_T_VERS,      /* H5L_class_t version            */
    H5L_TYPE_EXTERNAL,        /* Link type id number            */
    "external",                 /* Link name for debugging        */
    NULL,                       /* Creation callback              */
    NULL,                       /* Move callback                  */
    NULL,                       /* Copy callback                  */
    H5L__extern_traverse,       /* The actual traversal function  */
    NULL,                       /* Deletion callback              */
    H5L__extern_query           /* Query callback                 */
}};

/*-------------------------------------------------------------------------
 * Function:	H5L__extern_traverse
 *
 * Purpose:    Default traversal function for external links. This can
 *              be overridden using H5Lregister().
 *
 *              Given a filename and path packed into the link udata,
 *              attempts to open an object within an external file.
 *              If the H5L_ELINK_PREFIX_NAME property is set in the
 *              link access property list, appends that prefix to the
 *              filename being opened.
 *
 * Return:    ID of the opened object on success/H5I_INVALID_HID on failure
 *
 * Programmer:    James Laird
 *              Monday, July 10, 2006
 *
 *-------------------------------------------------------------------------
 */
static hid_t
H5L__extern_traverse(const char H5_ATTR_UNUSED *link_name, hid_t cur_group,
    const void *_udata, size_t H5_ATTR_UNUSED udata_size, hid_t lapl_id,
    hid_t H5_ATTR_UNUSED dxpl_id)
{
    H5P_genplist_t *plist;              /* Property list pointer */
    H5G_loc_t   root_loc;               /* Location of root group in external file */
    H5G_loc_t   loc;                    /* Location of object */
    H5F_t    *ext_file = NULL;    /* File struct for external file */
    const uint8_t *p = (const uint8_t *)_udata;  /* Pointer into external link buffer */
    const char *file_name;              /* Name of file containing external link's object */
    const char  *obj_name;              /* Name external link's object */
    size_t      fname_len;              /* Length of external link file name */
    unsigned    intent;                 /* File access permissions */
    H5L_elink_cb_t cb_info;             /* Callback info struct */
    hid_t       fapl_id = H5I_INVALID_HID;           /* File access property list for external link's file */
    void       *ext_obj = NULL;         /* External link's object */
    hid_t       ext_obj_id = H5I_INVALID_HID;   /* ID for external link's object */
    H5I_type_t  opened_type;            /* ID type of external link's object */
    char        *parent_group_name = NULL;/* Temporary pointer to group name */
    char        local_group_name[H5L_EXT_TRAVERSE_BUF_SIZE];  /* Local buffer to hold group name */
    H5P_genplist_t  *fa_plist;          /* File access property list pointer */
    H5F_close_degree_t      fc_degree = H5F_CLOSE_WEAK;  /* File close degree for target file */
    char        *elink_prefix = NULL;   /* Pointer to elink prefix */
    hid_t ret_value = H5I_INVALID_HID;  /* Return value */

    FUNC_ENTER_STATIC

    /* Sanity checks */
    HDassert(p);

    /* Check external link version & flags */
    if(((*p >> 4) & 0x0F) > H5L_EXT_VERSION)
        HGOTO_ERROR(H5E_LINK, H5E_CANTDECODE, H5I_INVALID_HID, "bad version number for external link")
    if((*p & 0x0F) & ~H5L_EXT_FLAGS_ALL)
        HGOTO_ERROR(H5E_LINK, H5E_CANTDECODE, H5I_INVALID_HID, "bad flags for external link")
    p++;

    /* Gather some information from the external link's user data */
    file_name = (const char *)p;
    fname_len = HDstrlen(file_name);
    obj_name = (const char *)p + fname_len + 1;

    /* Get the plist structure */
    if(NULL == (plist = H5P_object_verify(lapl_id, H5P_LINK_ACCESS)))
        HGOTO_ERROR(H5E_ATOM, H5E_BADATOM, H5I_INVALID_HID, "can't find object for ID")

    /* Get the fapl_id set for lapl_id if any */
    if(H5P_get(plist, H5L_ACS_ELINK_FAPL_NAME, &fapl_id) < 0)
        HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, H5I_INVALID_HID, "can't get fapl for links")

    /* Get the location for the group holding the external link */
    if(H5G_loc(cur_group, &loc) < 0)
        HGOTO_ERROR(H5E_LINK, H5E_CANTGET, H5I_INVALID_HID, "can't get object location")

    /* get the access flags set for lapl_id if any */
    if(H5P_get(plist, H5L_ACS_ELINK_FLAGS_NAME, &intent) < 0)
        HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, H5I_INVALID_HID, "can't get elink file access flags")

    /* get the file access mode flags for the parent file, if they were not set
     * on lapl_id */
    if(intent == H5F_ACC_DEFAULT)
        intent = H5F_INTENT(loc.oloc->file);

    if((fapl_id == H5P_DEFAULT) && ((fapl_id = H5F_get_access_plist(loc.oloc->file, FALSE)) < 0))
        HGOTO_ERROR(H5E_LINK, H5E_CANTGET, H5I_INVALID_HID, "can't get parent's file access property list")

    /* Get callback_info */
    if(H5P_get(plist, H5L_ACS_ELINK_CB_NAME, &cb_info) < 0)
        HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, H5I_INVALID_HID, "can't get elink callback info")

    /* Get file access property list */
    if(NULL == (fa_plist = H5P_object_verify(fapl_id, H5P_FILE_ACCESS)))
        HGOTO_ERROR(H5E_ATOM, H5E_BADATOM, H5I_INVALID_HID, "can't find object for ID")

    /* Make callback if it exists */
    if(cb_info.func) {
        const char  *parent_file_name;  /* Parent file name */
        ssize_t group_name_len;         /* Length of parent group name */

        /* Get parent file name */
        parent_file_name = H5F_OPEN_NAME(loc.oloc->file);

        /* Query length of parent group name */
        if((group_name_len = H5G_get_name(&loc, NULL, (size_t) 0, NULL)) < 0)
            HGOTO_ERROR(H5E_LINK, H5E_CANTGET, H5I_INVALID_HID, "unable to retrieve length of group name")

        /* Account for null terminator */
        group_name_len++;

        /* Check if we need to allocate larger buffer */
        if((size_t)group_name_len > sizeof(local_group_name)) {
            if(NULL == (parent_group_name = (char *)H5MM_malloc((size_t)group_name_len)))
                HGOTO_ERROR(H5E_LINK, H5E_CANTALLOC, H5I_INVALID_HID, "can't allocate buffer to hold group name, group_name_len = %zd", group_name_len)
        } /* end if */
        else
            parent_group_name = local_group_name;

        /* Get parent group name */
        if(H5G_get_name(&loc, parent_group_name, (size_t) group_name_len, NULL) < 0)
            HGOTO_ERROR(H5E_LINK, H5E_CANTGET, H5I_INVALID_HID, "unable to retrieve group name")

        /* Make callback */
        if((cb_info.func)(parent_file_name, parent_group_name, file_name, obj_name, &intent, fapl_id, cb_info.user_data) < 0)
            HGOTO_ERROR(H5E_LINK, H5E_CALLBACK, H5I_INVALID_HID, "traversal operator failed")

        /* Check access flags */
        if((intent & H5F_ACC_TRUNC) || (intent & H5F_ACC_EXCL))
            HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, H5I_INVALID_HID, "invalid file open flags")
    } /* end if */

    /* Set file close degree for new file to "weak" */
    if(H5P_set(fa_plist, H5F_ACS_CLOSE_DEGREE_NAME, &fc_degree) < 0)
        HGOTO_ERROR(H5E_PLIST, H5E_CANTSET, H5I_INVALID_HID, "can't set file close degree")

    /* Get the current elink prefix */
    if(H5P_peek(plist, H5L_ACS_ELINK_PREFIX_NAME, &elink_prefix) < 0)
        HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, H5I_INVALID_HID, "can't get external link prefix")

    /* Search for the target file */
    if(NULL == (ext_file = H5F_prefix_open_file(loc.oloc->file, H5F_PREFIX_ELINK, elink_prefix, file_name, intent, fapl_id)))
        HGOTO_ERROR(H5E_LINK, H5E_CANTOPENFILE, H5I_INVALID_HID, "unable to open external file, external link file name = '%s'", file_name)

    /* Retrieve the "group location" for the file's root group */
    if(H5G_root_loc(ext_file, &root_loc) < 0)
        HGOTO_ERROR(H5E_LINK, H5E_BADVALUE, H5I_INVALID_HID, "unable to create location for file")

    /* Open the object referenced in the external file */
    if(NULL == (ext_obj = H5O_open_name(&root_loc, obj_name, &opened_type)))
        HGOTO_ERROR(H5E_LINK, H5E_CANTOPENOBJ, H5I_INVALID_HID, "unable to open object")

    /* Get an ID for the external link's object */
    if((ext_obj_id = H5VL_wrap_register(opened_type, ext_obj, TRUE)) < 0)
        HGOTO_ERROR(H5E_ATOM, H5E_CANTREGISTER, H5I_INVALID_HID, "unable to register external link object")

    /* Set return value */
    ret_value = ext_obj_id;

done:
/* XXX (VOL MERGE): Probably also want to consider closing ext_obj here on failures */
    /* Release resources */
    if(fapl_id > 0 && H5I_dec_ref(fapl_id) < 0)
        HDONE_ERROR(H5E_ATOM, H5E_CANTRELEASE, H5I_INVALID_HID, "unable to close atom for file access property list")
    if(ext_file && H5F_efc_close(loc.oloc->file, ext_file) < 0)
        HDONE_ERROR(H5E_LINK, H5E_CANTCLOSEFILE, H5I_INVALID_HID, "problem closing external file")
    if(parent_group_name && parent_group_name != local_group_name)
        parent_group_name = (char *)H5MM_xfree(parent_group_name);
    if(ret_value < 0) {
        /* Close object if it's open and something failed */
        if(ext_obj_id >= 0 && H5I_dec_ref(ext_obj_id) < 0)
            HDONE_ERROR(H5E_ATOM, H5E_CANTRELEASE, H5I_INVALID_HID, "unable to close atom for external object")
    } /* end if */

    FUNC_LEAVE_NOAPI(ret_value)
} /* end H5L__extern_traverse() */


/*-------------------------------------------------------------------------
 * Function:	H5L__extern_query
 *
 * Purpose:    Default query function for external links. This can
 *              be overridden using H5Lregister().
 *
 *              Returns the size of the link's user data. If a buffer of
 *              is provided, copies at most buf_size bytes of the udata
 *              into it.
 *
 * Return:    Size of buffer on success/Negative on failure
 *
 * Programmer:    James Laird
 *              Monday, July 10, 2006
 *
 *-------------------------------------------------------------------------
 */
static ssize_t
H5L__extern_query(const char H5_ATTR_UNUSED * link_name, const void *_udata, size_t udata_size,
    void *buf /*out*/, size_t buf_size)
{
    const uint8_t *udata = (const uint8_t *)_udata;      /* Pointer to external link buffer */
    ssize_t     ret_value = SUCCEED;    /* Return value */

    FUNC_ENTER_STATIC

    /* Check external link version & flags */
    if(((*udata >> 4) & 0x0F) != H5L_EXT_VERSION)
        HGOTO_ERROR(H5E_LINK, H5E_CANTDECODE, FAIL, "bad version number for external link")
    if((*udata & 0x0F) & ~H5L_EXT_FLAGS_ALL)
        HGOTO_ERROR(H5E_LINK, H5E_CANTDECODE, FAIL, "bad flags for external link")

    /* If the buffer is NULL, skip writing anything in it and just return
     * the size needed */
    if(buf) {
        if(udata_size < buf_size)
            buf_size = udata_size;

        /* Copy the udata verbatim up to buf_size */
        H5MM_memcpy(buf, udata, buf_size);
    } /* end if */

    /* Set return value */
    ret_value = (ssize_t)udata_size;

done:
    FUNC_LEAVE_NOAPI(ret_value)
} /* end H5L__extern_query() */


/*-------------------------------------------------------------------------
 * Function:    H5Lcreate_external
 *
 * Purpose:     Creates an external link from LINK_NAME to OBJ_NAME.
 *
 *              External links are links to objects in other HDF5 files.  They
 *              are allowed to "dangle" like soft links internal to a file.
 *              FILE_NAME is the name of the file that OBJ_NAME is is contained
 *              within.  If OBJ_NAME is given as a relative path name, the
 *              path will be relative to the root group of FILE_NAME.
 *              LINK_NAME is interpreted relative to LINK_LOC_ID, which is
 *              either a file ID or a group ID.
 *
 * Return:      Non-negative on success/Negative on failure
 *
 * Programmer:  Quincey Koziol
 *              Wednesday, May 18, 2005
 *
 *-------------------------------------------------------------------------
 */
herr_t
H5Lcreate_external(const char *file_name, const char *obj_name,
    hid_t link_loc_id, const char *link_name, hid_t lcpl_id, hid_t lapl_id)
{
    H5VL_object_t    *vol_obj = NULL;   /* Object of loc_id */
    H5VL_loc_params_t loc_params;
    char       *norm_obj_name = NULL;   /* Pointer to normalized current name */
    void       *ext_link_buf = NULL;    /* Buffer to contain external link */
    size_t      buf_size;               /* Size of buffer to hold external link */
    size_t      file_name_len;          /* Length of file name string */
    size_t      norm_obj_name_len;      /* Length of normalized object name string */
    uint8_t    *p;                      /* Pointer into external link buffer */
    H5L_type_t link_type = H5L_TYPE_EXTERNAL;
    herr_t      ret_value = SUCCEED;    /* Return value */

    FUNC_ENTER_API(FAIL)
    H5TRACE6("e", "*s*si*sii", file_name, obj_name, link_loc_id, link_name,
             lcpl_id, lapl_id);

    /* Check arguments */
    if(!file_name || !*file_name)
        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "no file name specified")
    if(!obj_name || !*obj_name)
        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "no object name specified")
    if(!link_name || !*link_name)
        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "no link name specified")

    /* Get the link creation property list */
    if(H5P_DEFAULT == lcpl_id)
        lcpl_id = H5P_LINK_CREATE_DEFAULT;

    /* Set the LCPL for the API context */
    H5CX_set_lcpl(lcpl_id);

    /* Verify access property list and set up collective metadata if appropriate */
    if(H5CX_set_apl(&lapl_id, H5P_CLS_LACC, link_loc_id, TRUE) < 0)
        HGOTO_ERROR(H5E_LINK, H5E_CANTSET, FAIL, "can't set access property list info")

    /* Get normalized copy of the link target */
    if(NULL == (norm_obj_name = H5G_normalize(obj_name)))
        HGOTO_ERROR(H5E_LINK, H5E_BADVALUE, FAIL, "can't normalize object name")

    /* Combine the filename and link name into a single buffer to give to the UD link */
    file_name_len = HDstrlen(file_name) + 1;
    norm_obj_name_len = HDstrlen(norm_obj_name) + 1;
    buf_size = 1 + file_name_len + norm_obj_name_len;
    if(NULL == (ext_link_buf = H5MM_malloc(buf_size)))
        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "unable to allocate udata buffer")

    /* Encode the external link information */
    p = (uint8_t *)ext_link_buf;
    *p++ = (H5L_EXT_VERSION << 4) | H5L_EXT_FLAGS_ALL;  /* External link version & flags */
    HDstrncpy((char *)p, file_name, buf_size - 1);      /* Name of file containing external link's object */
    p += file_name_len;
    HDstrncpy((char *)p, norm_obj_name, buf_size - (file_name_len + 1));       /* External link's object */

    loc_params.type                         = H5VL_OBJECT_BY_NAME;
    loc_params.loc_data.loc_by_name.name    = link_name;
    loc_params.loc_data.loc_by_name.lapl_id = lapl_id;
    loc_params.obj_type                     = H5I_get_type(link_loc_id);

    /* get the location object */
    if(NULL == (vol_obj = (H5VL_object_t *)H5I_object(link_loc_id)))
        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "invalid object identifier")

    /* Create an external link */
    if(H5VL_link_create(H5VL_LINK_CREATE_UD, vol_obj, &loc_params, lcpl_id, lapl_id, H5P_DATASET_XFER_DEFAULT, H5_REQUEST_NULL, (int)link_type, ext_link_buf, buf_size) < 0)
        HGOTO_ERROR(H5E_LINK, H5E_CANTINIT, FAIL, "unable to create external link")

done:
    H5MM_xfree(ext_link_buf);
    H5MM_xfree(norm_obj_name);

    FUNC_LEAVE_API(ret_value)
} /* end H5Lcreate_external() */


/*-------------------------------------------------------------------------
 * Function: H5L_register_external
 *
 * Purpose: Registers default "External Link" link class.
 *              Use during library initialization or to restore the default
 *              after users change it.
 *
 * Return: Non-negative on success/ negative on failure
 *
 * Programmer:  James Laird
 *              Monday, July 17, 2006
 *
 *-------------------------------------------------------------------------
 */
herr_t
H5L_register_external(void)
{
    herr_t      ret_value = SUCCEED;       /* Return value */

    FUNC_ENTER_NOAPI(FAIL)

    if(H5L_register(H5L_EXTERN_LINK_CLASS) < 0)
        HGOTO_ERROR(H5E_LINK, H5E_NOTREGISTERED, FAIL, "unable to register external link class")

done:
    FUNC_LEAVE_NOAPI(ret_value)
} /* end H5L_register_external() */


/*-------------------------------------------------------------------------
 * Function: H5Lunpack_elink_val
 *
 * Purpose: Given a buffer holding the "link value" from an external link,
 *              gets pointers to the information within the link value buffer.
 *
 *              External link link values contain some flags and
 *              two NULL-terminated strings, one after the other.
 *
 *              The FLAGS value will be filled in and FILENAME and
 *              OBJ_PATH will be set to pointers within ext_linkval (unless
 *              any of these values is NULL).
 *
 *              Using this function on strings that aren't external link
 *              udata buffers can result in segmentation faults.
 *
 * Return: Non-negative on success/ Negative on failure
 *
 * Programmer:  James Laird
 *              Monday, July 17, 2006
 *
 *-------------------------------------------------------------------------
 */
herr_t
H5Lunpack_elink_val(const void *_ext_linkval, size_t link_size,
    unsigned *flags, const char **filename, const char **obj_path)
{
    const uint8_t *ext_linkval = (const uint8_t *)_ext_linkval; /* Pointer to the link value */
    unsigned    lnk_version;            /* External link format version */
    unsigned    lnk_flags;              /* External link flags */
    size_t      len;                    /* Length of the filename in the linkval*/
    herr_t      ret_value = SUCCEED;    /* Return value */

    FUNC_ENTER_API(FAIL)
    H5TRACE5("e", "*xz*Iu**s**s", _ext_linkval, link_size, flags, filename,
             obj_path);

    /* Sanity check external link buffer */
    if(ext_linkval == NULL )
        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "not an external link linkval buffer")
    lnk_version = (*ext_linkval >> 4) & 0x0F;
    lnk_flags = *ext_linkval & 0x0F;
    if(lnk_version > H5L_EXT_VERSION)
        HGOTO_ERROR(H5E_LINK, H5E_CANTDECODE, FAIL, "bad version number for external link")
    if(lnk_flags & (unsigned)~H5L_EXT_FLAGS_ALL)
        HGOTO_ERROR(H5E_LINK, H5E_CANTDECODE, FAIL, "bad flags for external link")
    if(link_size <= 2)
        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "not a valid external link buffer")

    /* Try to do some error checking.  If the last character in the linkval
     * (the last character of obj_path) isn't NULL, then something's wrong.
     */
    if(ext_linkval[link_size - 1] != '\0')
        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "linkval buffer is not NULL-terminated")

    /* We're now guaranteed that HDstrlen won't segfault, since the buffer has
     * at least one NULL in it.
     */
    len = HDstrlen((const char *)ext_linkval + 1);

    /* If the first NULL we found was at the very end of the buffer, then
     * this external link value has no object name and is invalid.
     */
    if((len + 1) >= (link_size - 1))
        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "linkval buffer doesn't contain an object path")

    /* If we got here then the buffer contains (at least) two strings packed
     * in the correct way.  Assume it's correct and return pointers to the
     * filename and object path.
     */
    if(filename)
        *filename = (const char *)ext_linkval + 1;
    if(obj_path)
        *obj_path = ((const char *)ext_linkval + 1) + len + 1;  /* Add one for NULL terminator */

    /* Set the flags to return */
    if(flags)
        *flags = lnk_flags;

done:
    FUNC_LEAVE_API(ret_value)
} /* end H5Lunpack_elink_val() */