[svn-r29041] Patch from Steffen Kiess that allows a relative path to be specified

for external dataset files set via H5Pset_external().

Resolves HDFFV-8740

Tested on: 64-bit Ubuntu 15.10 (Linux 4.2.0 x86_64) gcc 5.2.1
           autotools serial

13 files changed, 757 insertions, 127 deletions

diff --git a/src/H5.c b/src/H5.c
index 4de5731..41fb3ba 100644
--- a/src/H5.c
+++ b/src/H5.c
@@ -193,7 +193,8 @@ H5_init_library(void)
 
         /* Normal library termination code */
         (void)HDatexit(H5_term_library);
-	H5_dont_atexit_g = TRUE;
+
+        H5_dont_atexit_g = TRUE;
     } /* end if */
 
     /*
diff --git a/src/H5Defl.c b/src/H5Defl.c
index ec96ae7..cf1b36c 100644
--- a/src/H5Defl.c
+++ b/src/H5Defl.c
@@ -28,12 +28,13 @@
 /***********/
 /* Headers */
 /***********/
-#include "H5private.h"		/* Generic Functions			*/
-#include "H5Dpkg.h"		/* Datasets				*/
-#include "H5Eprivate.h"		/* Error handling		  	*/
-#include "H5Fprivate.h"		/* Files				*/
-#include "H5HLprivate.h"	/* Local Heaps				*/
-#include "H5VMprivate.h"		/* Vector and array functions		*/
+#include "H5private.h"          /* Generic Functions                        */
+#include "H5Dpkg.h"             /* Datasets                                 */
+#include "H5Eprivate.h"         /* Error handling                           */
+#include "H5Fprivate.h"         /* Files                                    */
+#include "H5HLprivate.h"        /* Local Heaps                              */
+#include "H5MMprivate.h"        /* Memory management                        */
+#include "H5VMprivate.h"        /* Vector and array functions               */
 
 
 /****************/
@@ -48,12 +49,14 @@
 /* Callback info for readvv operation */
 typedef struct H5D_efl_readvv_ud_t {
     const H5O_efl_t *efl;       /* Pointer to efl info */
+    const H5D_t *dset;          /* The dataset */
     unsigned char *rbuf;        /* Read buffer */
 } H5D_efl_readvv_ud_t;
 
 /* Callback info for writevv operation */
 typedef struct H5D_efl_writevv_ud_t {
     const H5O_efl_t *efl;       /* Pointer to efl info */
+    const H5D_t *dset;          /* The dataset */
     const unsigned char *wbuf;  /* Write buffer */
 } H5D_efl_writevv_ud_t;
 
@@ -75,9 +78,9 @@ static ssize_t H5D__efl_writevv(const H5D_io_info_t *io_info,
     size_t mem_max_nseq, size_t *mem_curr_seq, size_t mem_len_arr[], hsize_t mem_offset_arr[]);
 
 /* Helper routines */
-static herr_t H5D__efl_read(const H5O_efl_t *efl, haddr_t addr, size_t size,
+static herr_t H5D__efl_read(const H5O_efl_t *efl, const H5D_t *dset, haddr_t addr, size_t size,
     uint8_t *buf);
-static herr_t H5D__efl_write(const H5O_efl_t *efl, haddr_t addr, size_t size,
+static herr_t H5D__efl_write(const H5O_efl_t *efl, const H5D_t *dset, haddr_t addr, size_t size,
     const uint8_t *buf);
 
 
@@ -232,25 +235,22 @@ H5D__efl_io_init(const H5D_io_info_t *io_info, const H5D_type_info_t H5_ATTR_UNU
 
 
 /*-------------------------------------------------------------------------
- * Function:	H5D__efl_read
+ * Function:    H5D__efl_read
  *
- * Purpose:	Reads data from an external file list.  It is an error to
- *		read past the logical end of file, but reading past the end
- *		of any particular member of the external file list results in
- *		zeros.
+ * Purpose:     Reads data from an external file list.  It is an error to
+ *              read past the logical end of file, but reading past the end
+ *              of any particular member of the external file list results in
+ *              zeros.
  *
- * Return:	Non-negative on success/Negative on failure
+ * Return:      SUCCEED/FAIL
  *
- * Programmer:	Robb Matzke
+ * Programmer:  Robb Matzke
  *              Wednesday, March  4, 1998
  *
- * Modifications:
- *		Robb Matzke, 1999-07-28
- *		The ADDR argument is passed by value.
  *-------------------------------------------------------------------------
  */
 static herr_t
-H5D__efl_read(const H5O_efl_t *efl, haddr_t addr, size_t size, uint8_t *buf)
+H5D__efl_read(const H5O_efl_t *efl, const H5D_t *dset, haddr_t addr, size_t size, uint8_t *buf)
 {
     int		fd = -1;
     size_t	to_read;
@@ -261,6 +261,7 @@ H5D__efl_read(const H5O_efl_t *efl, haddr_t addr, size_t size, uint8_t *buf)
     haddr_t     cur;
     ssize_t	n;
     size_t      u;                      /* Local index variable */
+    char *full_name = NULL;             /* File name with prefix */
     herr_t      ret_value = SUCCEED;    /* Return value */
 
     FUNC_ENTER_STATIC
@@ -273,44 +274,49 @@ H5D__efl_read(const H5O_efl_t *efl, haddr_t addr, size_t size, uint8_t *buf)
 
     /* Find the first efl member from which to read */
     for (u=0, cur=0; u<efl->nused; u++) {
-	if(H5O_EFL_UNLIMITED == efl->slot[u].size || addr < cur + efl->slot[u].size) {
-	    skip = addr - cur;
-	    break;
-	} /* end if */
-  	cur += efl->slot[u].size;
+        if(H5O_EFL_UNLIMITED == efl->slot[u].size || addr < cur + efl->slot[u].size) {
+            skip = addr - cur;
+            break;
+        } /* end if */
+        cur += efl->slot[u].size;
     } /* end for */
 
     /* Read the data */
     while(size) {
         HDassert(buf);
-	if(u >= efl->nused)
-	    HGOTO_ERROR(H5E_EFL, H5E_OVERFLOW, FAIL, "read past logical end of file")
-	if(H5F_OVERFLOW_HSIZET2OFFT(efl->slot[u].offset + skip))
-	    HGOTO_ERROR(H5E_EFL, H5E_OVERFLOW, FAIL, "external file address overflowed")
-	if((fd = HDopen(efl->slot[u].name, O_RDONLY, 0)) < 0)
-	    HGOTO_ERROR(H5E_EFL, H5E_CANTOPENFILE, FAIL, "unable to open external raw data file")
-	if(HDlseek(fd, (HDoff_t)(efl->slot[u].offset + skip), SEEK_SET) < 0)
-	    HGOTO_ERROR(H5E_EFL, H5E_SEEKERROR, FAIL, "unable to seek in external raw data file")
+        if(u >= efl->nused)
+            HGOTO_ERROR(H5E_EFL, H5E_OVERFLOW, FAIL, "read past logical end of file")
+        if(H5F_OVERFLOW_HSIZET2OFFT(efl->slot[u].offset + skip))
+            HGOTO_ERROR(H5E_EFL, H5E_OVERFLOW, FAIL, "external file address overflowed")
+        if(H5_combine_path(dset->shared->extfile_prefix, efl->slot[u].name, &full_name) < 0)
+            HGOTO_ERROR(H5E_EFL, H5E_NOSPACE, FAIL, "can't build external file name")
+        if((fd = HDopen(full_name, O_RDONLY, 0)) < 0)
+            HGOTO_ERROR(H5E_EFL, H5E_CANTOPENFILE, FAIL, "unable to open external raw data file")
+        if(HDlseek(fd, (HDoff_t)(efl->slot[u].offset + skip), SEEK_SET) < 0)
+            HGOTO_ERROR(H5E_EFL, H5E_SEEKERROR, FAIL, "unable to seek in external raw data file")
 #ifndef NDEBUG
-	tempto_read = MIN(efl->slot[u].size-skip, (hsize_t)size);
+        tempto_read = MIN((size_t)(efl->slot[u].size-skip), (hsize_t)size);
         H5_CHECK_OVERFLOW(tempto_read, hsize_t, size_t);
-	to_read = (size_t)tempto_read;
+        to_read = (size_t)tempto_read;
 #else /* NDEBUG */
-	to_read = MIN((size_t)(efl->slot[u].size - skip), size);
+        to_read = MIN((size_t)(efl->slot[u].size - skip), (hsize_t)size);
 #endif /* NDEBUG */
-	if((n = HDread(fd, buf, to_read)) < 0)
-	    HGOTO_ERROR(H5E_EFL, H5E_READERROR, FAIL, "read error in external raw data file")
-	else if((size_t)n < to_read)
-	    HDmemset(buf + n, 0, to_read - (size_t)n);
-	HDclose(fd);
-	fd = -1;
-	size -= to_read;
-	buf += to_read;
-	skip = 0;
-	u++;
+        if((n = HDread(fd, buf, to_read)) < 0)
+            HGOTO_ERROR(H5E_EFL, H5E_READERROR, FAIL, "read error in external raw data file")
+        else if((size_t)n < to_read)
+            HDmemset(buf + n, 0, to_read - (size_t)n);
+        full_name = (char *)H5MM_xfree(full_name);
+        HDclose(fd);
+        fd = -1;
+        size -= to_read;
+        buf += to_read;
+        skip = 0;
+        u++;
     } /* end while */
 
 done:
+    if(full_name)
+        full_name = (char *)H5MM_xfree(full_name);
     if(fd >= 0)
         HDclose(fd);
 
@@ -337,7 +343,7 @@ done:
  *-------------------------------------------------------------------------
  */
 static herr_t
-H5D__efl_write(const H5O_efl_t *efl, haddr_t addr, size_t size, const uint8_t *buf)
+H5D__efl_write(const H5O_efl_t *efl, const H5D_t *dset, haddr_t addr, size_t size, const uint8_t *buf)
 {
     int		fd = -1;
     size_t	to_write;
@@ -346,7 +352,8 @@ H5D__efl_write(const H5O_efl_t *efl, haddr_t addr, size_t size, const uint8_t *b
 #endif /* NDEBUG */
     haddr_t     cur;
     hsize_t     skip = 0;
-    size_t	u;                      /* Local index variable */
+    size_t	u;                          /* Local index variable */
+    char *full_name = NULL;             /* File name with prefix */
     herr_t      ret_value = SUCCEED;    /* Return value */
 
     FUNC_ENTER_STATIC
@@ -359,46 +366,51 @@ H5D__efl_write(const H5O_efl_t *efl, haddr_t addr, size_t size, const uint8_t *b
 
     /* Find the first efl member in which to write */
     for(u = 0, cur = 0; u < efl->nused; u++) {
-	if(H5O_EFL_UNLIMITED == efl->slot[u].size || addr < cur + efl->slot[u].size) {
-	    skip = addr - cur;
-	    break;
-	} /* end if */
-	cur += efl->slot[u].size;
+        if(H5O_EFL_UNLIMITED == efl->slot[u].size || addr < cur + efl->slot[u].size) {
+            skip = addr - cur;
+            break;
+        } /* end if */
+        cur += efl->slot[u].size;
     } /* end for */
 
     /* Write the data */
     while(size) {
         HDassert(buf);
-	if(u >= efl->nused)
-	    HGOTO_ERROR(H5E_EFL, H5E_OVERFLOW, FAIL, "write past logical end of file")
-	if(H5F_OVERFLOW_HSIZET2OFFT(efl->slot[u].offset + skip))
-	    HGOTO_ERROR(H5E_EFL, H5E_OVERFLOW, FAIL, "external file address overflowed")
-	if((fd = HDopen(efl->slot[u].name, O_CREAT | O_RDWR, 0666)) < 0) {
-	    if(HDaccess(efl->slot[u].name, F_OK) < 0)
-		HGOTO_ERROR(H5E_EFL, H5E_CANTOPENFILE, FAIL, "external raw data file does not exist")
-	    else
-		HGOTO_ERROR(H5E_EFL, H5E_CANTOPENFILE, FAIL, "unable to open external raw data file")
-	} /* end if */
-	if(HDlseek(fd, (HDoff_t)(efl->slot[u].offset + skip), SEEK_SET) < 0)
-	    HGOTO_ERROR(H5E_EFL, H5E_SEEKERROR, FAIL, "unable to seek in external raw data file")
+        if(u >= efl->nused)
+            HGOTO_ERROR(H5E_EFL, H5E_OVERFLOW, FAIL, "write past logical end of file")
+        if(H5F_OVERFLOW_HSIZET2OFFT(efl->slot[u].offset + skip))
+            HGOTO_ERROR(H5E_EFL, H5E_OVERFLOW, FAIL, "external file address overflowed")
+        if(H5_combine_path(dset->shared->extfile_prefix, efl->slot[u].name, &full_name) < 0)
+            HGOTO_ERROR(H5E_EFL, H5E_NOSPACE, FAIL, "can't build external file name")
+        if((fd = HDopen(full_name, O_CREAT | O_RDWR, 0666)) < 0) {
+            if(HDaccess(full_name, F_OK) < 0)
+                HGOTO_ERROR(H5E_EFL, H5E_CANTOPENFILE, FAIL, "external raw data file does not exist")
+            else
+                HGOTO_ERROR(H5E_EFL, H5E_CANTOPENFILE, FAIL, "unable to open external raw data file")
+            } /* end if */
+        if(HDlseek(fd, (HDoff_t)(efl->slot[u].offset + skip), SEEK_SET) < 0)
+            HGOTO_ERROR(H5E_EFL, H5E_SEEKERROR, FAIL, "unable to seek in external raw data file")
 #ifndef NDEBUG
-	tempto_write = MIN(efl->slot[u].size - skip, (hsize_t)size);
+        tempto_write = MIN(efl->slot[u].size - skip, (hsize_t)size);
         H5_CHECK_OVERFLOW(tempto_write, hsize_t, size_t);
         to_write = (size_t)tempto_write;
 #else /* NDEBUG */
-	to_write = MIN((size_t)(efl->slot[u].size - skip), size);
+        to_write = MIN((size_t)(efl->slot[u].size - skip), size);
 #endif /* NDEBUG */
-	if((size_t)HDwrite(fd, buf, to_write) != to_write)
-	    HGOTO_ERROR(H5E_EFL, H5E_READERROR, FAIL, "write error in external raw data file")
-	HDclose (fd);
-	fd = -1;
-	size -= to_write;
-	buf += to_write;
-	skip = 0;
-	u++;
+        if((size_t)HDwrite(fd, buf, to_write) != to_write)
+            HGOTO_ERROR(H5E_EFL, H5E_READERROR, FAIL, "write error in external raw data file")
+        full_name = (char *)H5MM_xfree(full_name);
+        HDclose (fd);
+        fd = -1;
+        size -= to_write;
+        buf += to_write;
+        skip = 0;
+        u++;
     } /* end while */
 
 done:
+    if(full_name)
+        full_name = (char *)H5MM_xfree(full_name);
     if(fd >= 0)
         HDclose(fd);
 
@@ -427,7 +439,7 @@ H5D__efl_readvv_cb(hsize_t dst_off, hsize_t src_off, size_t len, void *_udata)
     FUNC_ENTER_STATIC
 
     /* Read data */
-    if(H5D__efl_read(udata->efl, dst_off, len, (udata->rbuf + src_off)) < 0)
+    if(H5D__efl_read(udata->efl, udata->dset, dst_off, len, (udata->rbuf + src_off)) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_READERROR, FAIL, "EFL read failed")
 
 done:
@@ -464,6 +476,9 @@ H5D__efl_readvv(const H5D_io_info_t *io_info,
     HDassert(io_info);
     HDassert(io_info->store->efl.nused > 0);
     HDassert(io_info->u.rbuf);
+    HDassert(io_info->dset);
+    HDassert(io_info->dset->shared);
+    HDassert(io_info->dset->shared->extfile_prefix);
     HDassert(dset_curr_seq);
     HDassert(dset_len_arr);
     HDassert(dset_off_arr);
@@ -473,6 +488,7 @@ H5D__efl_readvv(const H5D_io_info_t *io_info,
 
     /* Set up user data for H5VM_opvv() */
     udata.efl = &(io_info->store->efl);
+    udata.dset = io_info->dset;
     udata.rbuf = (unsigned char *)io_info->u.rbuf;
 
     /* Call generic sequence operation routine */
@@ -507,7 +523,7 @@ H5D__efl_writevv_cb(hsize_t dst_off, hsize_t src_off, size_t len, void *_udata)
     FUNC_ENTER_STATIC
 
     /* Write data */
-    if(H5D__efl_write(udata->efl, dst_off, len, (udata->wbuf + src_off)) < 0)
+    if(H5D__efl_write(udata->efl, udata->dset, dst_off, len, (udata->wbuf + src_off)) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_WRITEERROR, FAIL, "EFL write failed")
 
 done:
@@ -544,6 +560,9 @@ H5D__efl_writevv(const H5D_io_info_t *io_info,
     HDassert(io_info);
     HDassert(io_info->store->efl.nused > 0);
     HDassert(io_info->u.wbuf);
+    HDassert(io_info->dset);
+    HDassert(io_info->dset->shared);
+    HDassert(io_info->dset->shared->extfile_prefix);
     HDassert(dset_curr_seq);
     HDassert(dset_len_arr);
     HDassert(dset_off_arr);
@@ -553,6 +572,7 @@ H5D__efl_writevv(const H5D_io_info_t *io_info,
 
     /* Set up user data for H5VM_opvv() */
     udata.efl = &(io_info->store->efl);
+    udata.dset = io_info->dset;
     udata.wbuf = (const unsigned char *)io_info->u.wbuf;
 
     /* Call generic sequence operation routine */
diff --git a/src/H5Dint.c b/src/H5Dint.c
index a58ca56..d35e9f5 100644
--- a/src/H5Dint.c
+++ b/src/H5Dint.c
@@ -63,6 +63,8 @@ static herr_t H5D__cache_dataspace_info(const H5D_t *dset);
 static herr_t H5D__init_space(H5F_t *file, const H5D_t *dset, const H5S_t *space);
 static herr_t H5D__update_oh_info(H5F_t *file, hid_t dxpl_id, H5D_t *dset,
     hid_t dapl_id);
+static herr_t H5D_build_extfile_prefix(const H5D_t *dset, hid_t dapl_id,
+    char **extfile_prefix);
 static herr_t H5D__open_oid(H5D_t *dataset, hid_t dapl_id, hid_t dxpl_id);
 static herr_t H5D__init_storage(const H5D_t *dataset, hbool_t full_overwrite,
     hsize_t old_dim[], hid_t dxpl_id);
@@ -982,6 +984,84 @@ done:
 } /* end H5D__update_oh_info() */
 
 
+/*--------------------------------------------------------------------------
+ * Function:    H5D_build_extfile_prefix
+ *
+ * Purpose:     Determine the external file prefix to be used and store
+ *              it in extfile_prefix. Stores an empty string if no prefix
+ *              should be used.
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ * Programmer:  Steffen Kiess
+ *              October 16, 2015
+ *--------------------------------------------------------------------------
+ */
+static herr_t
+H5D_build_extfile_prefix(const H5D_t *dset, hid_t dapl_id, char **extfile_prefix /*out*/)
+{
+    char            *prefix = NULL;       /* prefix used to look for the file               */
+    char            *extpath = NULL;      /* absolute path of directory the HDF5 file is in */
+    size_t          extpath_len;          /* length of extpath                              */
+    size_t          prefix_len;           /* length of prefix                               */
+    size_t          extfile_prefix_len;   /* length of expanded prefix                      */
+    H5P_genplist_t  *plist = NULL;        /* Property list pointer                          */
+    herr_t          ret_value = SUCCEED;  /* Return value                                   */
+    
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    HDassert(dset);
+    HDassert(dset->oloc.file);
+
+    extpath = H5F_EXTPATH(dset->oloc.file);
+    HDassert(extpath);
+
+    /* XXX: Future thread-safety note - getenv is not required
+     *      to be reentrant.
+     */
+    prefix = HDgetenv("HDF5_EXTFILE_PREFIX");
+
+    if(prefix == NULL || *prefix == '\0') {
+        /* Set prefix to value of H5D_ACS_EFILE_PREFIX_NAME property */
+        if(NULL == (plist = H5P_object_verify(dapl_id, H5P_DATASET_ACCESS)))
+            HGOTO_ERROR(H5E_ATOM, H5E_BADATOM, FAIL, "can't find object for ID")
+        if(H5P_peek(plist, H5D_ACS_EFILE_PREFIX_NAME, &prefix) < 0)
+            HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, FAIL, "can't get external file prefix")
+    } /* end if */
+
+    /* Prefix has to be checked for NULL / empty string again because the
+     * code above might have updated it.
+     */
+    if(prefix == NULL || *prefix == '\0' || HDstrcmp(prefix, ".") == 0) {
+        /* filename is interpreted as relative to the current directory,
+         * does not need to be expanded
+         */
+        if(NULL == (*extfile_prefix = (char *)H5MM_strdup("")))
+            HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "memory allocation failed")
+    } /* end if */
+    else {
+        if (HDstrncmp(prefix, "${ORIGIN}", HDstrlen("${ORIGIN}")) == 0) {
+            /* Replace ${ORIGIN} at beginning of prefix by directory of HDF5 file */
+            extpath_len = HDstrlen(extpath);
+            prefix_len = HDstrlen(prefix);
+            extfile_prefix_len = extpath_len + prefix_len - HDstrlen("${ORIGIN}") + 1;
+        
+            if(NULL == (*extfile_prefix = (char *)H5MM_malloc(extfile_prefix_len)))
+                HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "unable to allocate buffer")
+            HDsnprintf(*extfile_prefix, extfile_prefix_len, "%s%s", extpath, prefix + HDstrlen("${ORIGIN}"));
+        } /* end if */
+        else {
+            if(NULL == (*extfile_prefix = (char *)H5MM_strdup(prefix)))
+                HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "memory allocation failed")
+        } /* end else */
+    } /* end else */
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* H5D_build_extfile_prefix() */
+
+
 /*-------------------------------------------------------------------------
  * Function:	H5D__create
  *
@@ -1151,6 +1231,10 @@ H5D__create(H5F_t *file, hid_t type_id, const H5S_t *space, hid_t dcpl_id,
     /* Indicate that the layout information was initialized */
     layout_init = TRUE;
 
+    /* Set the external file prefix */
+    if(H5D_build_extfile_prefix(new_dset, dapl_id, &new_dset->shared->extfile_prefix) < 0)
+        HGOTO_ERROR(H5E_DATASET, H5E_CANTINIT, NULL, "unable to initialize external file prefix")
+
     /* Add the dataset to the list of opened objects in the file */
     if(H5FO_top_incr(new_dset->oloc.file, new_dset->oloc.addr) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_CANTINC, NULL, "can't incr object ref. count")
@@ -1195,6 +1279,7 @@ done:
             } /* end if */
             if(new_dset->shared->dcpl_id != 0 && H5I_dec_ref(new_dset->shared->dcpl_id) < 0)
                 HDONE_ERROR(H5E_DATASET, H5E_CANTDEC, NULL, "unable to decrement ref count on property list")
+            new_dset->shared->extfile_prefix = (char *)H5MM_xfree(new_dset->shared->extfile_prefix);
             new_dset->shared = H5FL_FREE(H5D_shared_t, new_dset->shared);
         } /* end if */
         new_dset->oloc.file = NULL;
@@ -1288,7 +1373,8 @@ H5D_open(const H5G_loc_t *loc, hid_t dapl_id, hid_t dxpl_id)
 {
     H5D_shared_t    *shared_fo = NULL;
     H5D_t           *dataset = NULL;
-    H5D_t           *ret_value = NULL;          /* Return value */
+    char            *extfile_prefix = NULL;  /* Expanded external file prefix */
+    H5D_t           *ret_value = NULL;       /* Return value */
 
     FUNC_ENTER_NOAPI(NULL)
 
@@ -1307,6 +1393,10 @@ H5D_open(const H5G_loc_t *loc, hid_t dapl_id, hid_t dxpl_id)
     if(H5G_name_copy(&(dataset->path), loc->path, H5_COPY_SHALLOW) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_CANTCOPY, NULL, "can't copy path")
 
+    /* Get the external file prefix */
+    if(H5D_build_extfile_prefix(dataset, dapl_id, &extfile_prefix) < 0)
+        HGOTO_ERROR(H5E_DATASET, H5E_CANTINIT, NULL, "unable to initialize external file prefix")
+
     /* Check if dataset was already open */
     if(NULL == (shared_fo = (H5D_shared_t *)H5FO_opened(dataset->oloc.file, dataset->oloc.addr))) {
         /* Clear any errors from H5FO_opened() */
@@ -1326,6 +1416,12 @@ H5D_open(const H5G_loc_t *loc, hid_t dapl_id, hid_t dxpl_id)
 
         /* We're the first dataset to use the the shared info */
         dataset->shared->fo_count = 1;
+
+        /* Set the external file prefix */
+        dataset->shared->extfile_prefix = extfile_prefix;
+        /* Prevent string from being freed during done: */
+        extfile_prefix = NULL;
+
     } /* end if */
     else {
         /* Point to shared info */
@@ -1334,6 +1430,12 @@ H5D_open(const H5G_loc_t *loc, hid_t dapl_id, hid_t dxpl_id)
         /* Increment # of datasets using shared information */
         shared_fo->fo_count++;
 
+        /* Check whether the external file prefix of the already open dataset
+         * matches the new external file prefix
+         */
+        if(HDstrcmp(extfile_prefix, dataset->shared->extfile_prefix) != 0)
+            HGOTO_ERROR(H5E_DATASET, H5E_CANTOPENOBJ, NULL, "new external file prefix does not match external file prefix of already open dataset")
+
         /* Check if the object has been opened through the top file yet */
         if(H5FO_top_count(dataset->oloc.file, dataset->oloc.addr) == 0) {
             /* Open the object through this top file */
@@ -1350,11 +1452,15 @@ H5D_open(const H5G_loc_t *loc, hid_t dapl_id, hid_t dxpl_id)
     ret_value = dataset;
 
 done:
+    extfile_prefix = (char *)H5MM_xfree(extfile_prefix);
+
     if(ret_value == NULL) {
         /* Free the location--casting away const*/
         if(dataset) {
-            if(shared_fo == NULL)   /* Need to free shared fo */
+            if(shared_fo == NULL && dataset->shared) {   /* Need to free shared fo */
+                dataset->shared->extfile_prefix = (char *)H5MM_xfree(dataset->shared->extfile_prefix);
                 dataset->shared = H5FL_FREE(H5D_shared_t, dataset->shared);
+            }
 
             H5O_loc_free(&(dataset->oloc));
             H5G_name_free(&(dataset->path));
@@ -1652,6 +1758,9 @@ H5D_close(H5D_t *dataset)
         if(dataset->shared->layout.ops->dest && (dataset->shared->layout.ops->dest)(dataset, H5AC_dxpl_id) < 0)
             HDONE_ERROR(H5E_DATASET, H5E_CANTRELEASE, FAIL, "unable to destroy layout info")
 
+        /* Free the external file prefix */
+        dataset->shared->extfile_prefix = (char *)H5MM_xfree(dataset->shared->extfile_prefix);
+
         /* Release layout, fill-value, efl & pipeline messages */
         if(dataset->shared->dcpl_id != H5P_DATASET_CREATE_DEFAULT)
             free_failed |= (H5O_msg_reset(H5O_PLINE_ID, &dataset->shared->dcpl_cache.pline) < 0) ||
diff --git a/src/H5Dpkg.h b/src/H5Dpkg.h
index 97fc442..f9c585a 100644
--- a/src/H5Dpkg.h
+++ b/src/H5Dpkg.h
@@ -438,6 +438,8 @@ typedef struct H5D_shared_t {
                                          */
         H5D_rdcc_t      chunk;          /* Information about chunked data */
     } cache;
+
+    char                *extfile_prefix; /* expanded external file prefix */
 } H5D_shared_t;
 
 struct H5D_t {
diff --git a/src/H5Dprivate.h b/src/H5Dprivate.h
index 3b43aaf..ab60a50 100644
--- a/src/H5Dprivate.h
+++ b/src/H5Dprivate.h
@@ -54,6 +54,7 @@
 #define H5D_ACS_PREEMPT_READ_CHUNKS_NAME    "rdcc_w0"       /* Preemption read chunks first */
 #define H5D_ACS_VDS_VIEW_NAME               "vds_view"      /* VDS view option */
 #define H5D_ACS_VDS_PRINTF_GAP_NAME         "vds_printf_gap" /* VDS printf gap size */
+#define H5D_ACS_EFILE_PREFIX_NAME           "external file prefix" /* External file prefix */
 
 /* ======== Data transfer properties ======== */
 #define H5D_XFER_MAX_TEMP_BUF_NAME      "max_temp_buf"  /* Maximum temp buffer size */
diff --git a/src/H5Fquery.c b/src/H5Fquery.c
index 05667ac..75fc216 100644
--- a/src/H5Fquery.c
+++ b/src/H5Fquery.c
@@ -158,7 +158,7 @@ H5F_get_actual_name(const H5F_t *f)
  * Function:	H5F_get_extpath
  *
  * Purpose:	Retrieve the file's 'extpath' flags
- *		This is used by H5L_extern_traverse() to retrieve the main file's location
+ *		This is used by H5L_extern_traverse() and H5D_build_extfile_prefix() to retrieve the main file's location
  *		when searching the target file.
  *
  * Return:	'extpath' on success/abort on failure (shouldn't fail)
diff --git a/src/H5MM.c b/src/H5MM.c
index 09e6461..5c2a731 100644
--- a/src/H5MM.c
+++ b/src/H5MM.c
@@ -125,10 +125,10 @@ static size_t H5MM_peak_alloc_blocks_count_s = 0;
  * Function:    H5MM__is_our_block
  *
  * Purpose:     Try to determine if a memory buffer has been allocated through
- *		the H5MM* interface, instead of the system's malloc() routines.
+ *              the H5MM* interface, instead of the system's malloc() routines.
  *
- * Return:	Success:    TRUE/FALSE
- *		Failure:    (Can't fail)
+ * Return:      Success:    TRUE/FALSE
+ *              Failure:    (Can't fail)
  *
  * Programmer:  Quincey Koziol
  *              Dec 30 2015
@@ -149,7 +149,7 @@ H5MM__is_our_block(void *mem)
  *
  * Purpose:     Check a block wrapper around a buffer to validate it.
  *
- * Return:	N/A (void)
+ * Return:      N/A (void)
  *
  * Programmer:  Quincey Koziol
  *              Dec 30 2015
@@ -173,9 +173,9 @@ H5MM__sanity_check_block(const H5MM_block_t *block)
  * Function:    H5MM__sanity_check
  *
  * Purpose:     Check a buffer to validate it (just calls
- *		H5MM__sanity_check_block after finding block for buffer)
+ *              H5MM__sanity_check_block after finding block for buffer)
  *
- * Return:	N/A (void)
+ * Return:      N/A (void)
  *
  * Programmer:  Quincey Koziol
  *              Dec 30 2015
@@ -196,7 +196,7 @@ H5MM__sanity_check(void *mem)
  *
  * Purpose:     Sanity check all current memory allocations.
  *
- * Return:	N/A (void)
+ * Return:      N/A (void)
  *
  * Programmer:  Quincey Koziol
  *              Jan  5 2016
@@ -206,7 +206,7 @@ H5MM__sanity_check(void *mem)
 void
 H5MM_sanity_check_all(void)
 {
-    H5MM_block_t *curr;
+    H5MM_block_t *curr = NULL;
 
     curr = H5MM_block_head_s.next;
     while(curr != &H5MM_block_head_s) {
@@ -221,7 +221,7 @@ H5MM_sanity_check_all(void)
  *
  * Purpose:     Final sanity checks on memory allocation.
  *
- * Return:	N/A (void)
+ * Return:      N/A (void)
  *
  * Programmer:  Quincey Koziol
  *              Jan  1 2016
@@ -257,8 +257,8 @@ H5MM_final_sanity_check(void)
  *              considered an error condition since allocations of zero
  *              bytes usually indicate problems.
  *  
- * Return:  Success:    Pointer new memory
- *          Failure:	NULL
+ * Return:      Success:    Pointer to new memory
+ *              Failure:    NULL
  *
  * Programmer:  Quincey Koziol
  *              Nov  8 2003
@@ -268,7 +268,7 @@ H5MM_final_sanity_check(void)
 void *
 H5MM_malloc(size_t size)
 {
-    void *ret_value;
+    void *ret_value = NULL;
 
     HDassert(size);
 
@@ -346,8 +346,8 @@ H5MM_malloc(size_t size)
  *              bytes usually indicate problems.
  *
  *
- * Return:  Success:    Pointer new memory
- *          Failure:	NULL
+ * Return:      Success:    Pointer to new memory
+ *              Failure:    NULL
  *
  * Programmer:	Quincey Koziol
  *              Nov  8 2003
@@ -357,7 +357,7 @@ H5MM_malloc(size_t size)
 void *
 H5MM_calloc(size_t size)
 {
-    void *ret_value;
+    void *ret_value = NULL;
 
     HDassert(size);
 
@@ -392,8 +392,9 @@ H5MM_calloc(size_t size)
  *              Note that the (NULL, 0) combination is undefined behavior
  *              in the C standard.
  *
- * Return:  Success:    Ptr to new memory if size > 0, NULL if size is zero
- *          Failure:    NULL (input buffer is unchanged on failure)
+ * Return:      Success:    Ptr to new memory if size > 0
+ *                          NULL if size is zero
+ *              Failure:    NULL (input buffer is unchanged on failure)
  *
  * Programmer:  Robb Matzke
  *              Jul 10 1997
@@ -403,7 +404,7 @@ H5MM_calloc(size_t size)
 void *
 H5MM_realloc(void *mem, size_t size)
 {
-    void *ret_value;
+    void *ret_value = NULL;
 
     /* Use FUNC_ENTER_NOAPI_NOINIT_NOERR here to avoid performance issues */
     FUNC_ENTER_NOAPI_NOINIT_NOERR
@@ -458,7 +459,6 @@ H5MM_realloc(void *mem, size_t size)
  *              Failure:    abort()
  *
  * Programmer:  Robb Matzke
- *              matzke@llnl.gov
  *              Jul 10 1997
  *-------------------------------------------------------------------------
  */
@@ -490,17 +490,16 @@ done:
  *              an error will be raised.
  *
  * Return:      Success:    Pointer to a new string
- *              Failure:    abort()
+ *              Failure:    NULL
  *
  * Programmer:  Robb Matzke
- *              matzke@llnl.gov
  *              Jul 10 1997
  *-------------------------------------------------------------------------
  */
 char *
 H5MM_strdup(const char *s)
 {
-    char *ret_value;
+    char *ret_value = NULL;
 
     FUNC_ENTER_NOAPI(NULL)
 
@@ -528,7 +527,6 @@ done:
  *              Failure:    never fails
  *
  * Programmer:  Robb Matzke
- *              matzke@llnl.gov
  *              Jul 10 1997
  *
  *-------------------------------------------------------------------------
diff --git a/src/H5Pdapl.c b/src/H5Pdapl.c
index eab9337..6407d4c 100644
--- a/src/H5Pdapl.c
+++ b/src/H5Pdapl.c
@@ -16,8 +16,8 @@
 /*-------------------------------------------------------------------------
  *
  * Created:		H5Pdapl.c
- *			October 27, 2008
- *			Neil Fortner <nfortne2@hdfgroup.org>
+ *              October 27, 2008
+ *              Neil Fortner <nfortne2@hdfgroup.org>
  *
  * Purpose:		Dataset access property list class routines
  *
@@ -39,7 +39,8 @@
 #include "H5Eprivate.h"		/* Error handling */
 #include "H5Fprivate.h"		/* Files */
 #include "H5Iprivate.h"		/* IDs */
-#include "H5Ppkg.h"		/* Property lists */
+#include "H5Ppkg.h"         /* Property lists */
+#include "H5MMprivate.h"	/* Memory management */
 
 
 /****************/
@@ -72,6 +73,17 @@
 #define H5D_ACS_VDS_PRINTF_GAP_DEF              (hsize_t)0
 #define H5D_ACS_VDS_PRINTF_GAP_ENC              H5P__encode_hsize_t
 #define H5D_ACS_VDS_PRINTF_GAP_DEC              H5P__decode_hsize_t
+/* Definitions for external file prefix */
+#define H5D_ACS_EFILE_PREFIX_SIZE               sizeof(char *)
+#define H5D_ACS_EFILE_PREFIX_DEF                NULL /*default is no prefix */
+#define H5D_ACS_EFILE_PREFIX_SET                H5P__dapl_efile_pref_set
+#define H5D_ACS_EFILE_PREFIX_GET                H5P__dapl_efile_pref_get
+#define H5D_ACS_EFILE_PREFIX_ENC                H5P__dapl_efile_pref_enc
+#define H5D_ACS_EFILE_PREFIX_DEC                H5P__dapl_efile_pref_dec
+#define H5D_ACS_EFILE_PREFIX_DEL                H5P__dapl_efile_pref_del
+#define H5D_ACS_EFILE_PREFIX_COPY               H5P__dapl_efile_pref_copy
+#define H5D_ACS_EFILE_PREFIX_CMP                H5P__dapl_efile_pref_cmp
+#define H5D_ACS_EFILE_PREFIX_CLOSE              H5P__dapl_efile_pref_close
 
 /******************/
 /* Local Typedefs */
@@ -100,6 +112,16 @@ static herr_t H5P__decode_chunk_cache_nbytes(const void **_pp, void *_value);
 static herr_t H5P__dacc_vds_view_enc(const void *value, void **pp, size_t *size);
 static herr_t H5P__dacc_vds_view_dec(const void **pp, void *value);
 
+/* Property list callbacks */
+static herr_t H5P__dapl_efile_pref_set(hid_t prop_id, const char* name, size_t size, void* value);
+static herr_t H5P__dapl_efile_pref_get(hid_t prop_id, const char* name, size_t size, void* value);
+static herr_t H5P__dapl_efile_pref_enc(const void *value, void **_pp, size_t *size);
+static herr_t H5P__dapl_efile_pref_dec(const void **_pp, void *value);
+static herr_t H5P__dapl_efile_pref_del(hid_t prop_id, const char* name, size_t size, void* value);
+static herr_t H5P__dapl_efile_pref_copy(const char* name, size_t size, void* value);
+static int H5P__dapl_efile_pref_cmp(const void *value1, const void *value2, size_t size);
+static herr_t H5P__dapl_efile_pref_close(const char* name, size_t size, void* value);
+
 
 /*********************/
 /* Package Variables */
@@ -134,6 +156,9 @@ const H5P_libclass_t H5P_CLS_DACC[1] = {{
 /* Local Variables */
 /*******************/
 
+/* Property value defaults */
+static const char *H5D_def_efile_prefix_g = H5D_ACS_EFILE_PREFIX_DEF; /* Default external file prefix string */
+
 
 
 /*-------------------------------------------------------------------------
@@ -187,12 +212,274 @@ H5P__dacc_reg_prop(H5P_genclass_t *pclass)
             NULL, NULL, NULL, NULL) < 0)
         HGOTO_ERROR(H5E_PLIST, H5E_CANTINSERT, FAIL, "can't insert property into class")
 
+    /* Register property for external file prefix */
+    if(H5P_register_real(pclass, H5D_ACS_EFILE_PREFIX_NAME, H5D_ACS_EFILE_PREFIX_SIZE, &H5D_def_efile_prefix_g, 
+            NULL, H5D_ACS_EFILE_PREFIX_SET, H5D_ACS_EFILE_PREFIX_GET, H5D_ACS_EFILE_PREFIX_ENC, H5D_ACS_EFILE_PREFIX_DEC,
+            H5D_ACS_EFILE_PREFIX_DEL, H5D_ACS_EFILE_PREFIX_COPY, H5D_ACS_EFILE_PREFIX_CMP, H5D_ACS_EFILE_PREFIX_CLOSE) < 0)
+         HGOTO_ERROR(H5E_PLIST, H5E_CANTINSERT, FAIL, "can't insert property into class")
+
 done:
     FUNC_LEAVE_NOAPI(ret_value)
 } /* end H5P__dacc_reg_prop() */
 
 
 /*-------------------------------------------------------------------------
+ * Function:    H5P__dapl_efile_pref_set
+ *
+ * Purpose:     Copies an external file prefix property when it's set
+ *              for a property list
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5P__dapl_efile_pref_set(hid_t H5_ATTR_UNUSED prop_id, const char H5_ATTR_UNUSED *name,
+    size_t H5_ATTR_UNUSED size, void *value)
+{
+    FUNC_ENTER_STATIC_NOERR
+
+    /* Sanity check */
+    HDassert(value);
+
+    /* Copy the prefix */
+    *(char **)value = H5MM_xstrdup(*(const char **)value);
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5P__dapl_efile_pref_set() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5P__dapl_efile_pref_get
+ *
+ * Purpose:     Copies an external file prefix property when it's retrieved
+ *              from a property list
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5P__dapl_efile_pref_get(hid_t H5_ATTR_UNUSED prop_id, const char H5_ATTR_UNUSED *name,
+    size_t H5_ATTR_UNUSED size, void *value)
+{
+    FUNC_ENTER_STATIC_NOERR
+
+    /* Sanity check */
+    HDassert(value);
+
+    /* Copy the prefix */
+    *(char **)value = H5MM_xstrdup(*(const char **)value);
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5P__dapl_efile_pref_get() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5P__dapl_efile_pref_enc
+ *
+ * Purpose:     Callback routine which is called whenever the efile flags
+ *              property in the dataset access property list is
+ *              encoded.
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5P__dapl_efile_pref_enc(const void *value, void **_pp, size_t *size)
+{
+    const char *efile_pref = *(const char * const *)value;
+    uint8_t **pp = (uint8_t **)_pp;
+    size_t len = 0;
+    uint64_t enc_value;
+    unsigned enc_size;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    HDcompile_assert(sizeof(size_t) <= sizeof(uint64_t));
+
+    /* calculate prefix length */
+    if(NULL != efile_pref)
+        len = HDstrlen(efile_pref);
+
+    enc_value = (uint64_t)len;
+    enc_size = H5VM_limit_enc_size(enc_value);
+    HDassert(enc_size < 256);
+
+    if(NULL != *pp) {
+        /* encode the length of the prefix */
+        *(*pp)++ = (uint8_t)enc_size;
+        UINT64ENCODE_VAR(*pp, enc_value, enc_size);
+
+        /* encode the prefix */
+        if(NULL != efile_pref) {
+            HDmemcpy(*(char **)pp, efile_pref, len);
+            *pp += len;
+        } /* end if */
+    } /* end if */
+
+    *size += (1 + enc_size);
+    if(NULL != efile_pref)
+        *size += len;
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5P__dapl_efile_pref_enc() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5P__dapl_efile_pref_dec
+ *
+ * Purpose:     Callback routine which is called whenever the efile prefix
+ *              property in the dataset access property list is
+ *              decoded.
+ *
+ * Return:	    SUCCEED/FAIL
+ *
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5P__dapl_efile_pref_dec(const void **_pp, void *_value)
+{
+    char **efile_pref = (char **)_value;
+    const uint8_t **pp = (const uint8_t **)_pp;
+    size_t len;
+    uint64_t enc_value;                 /* Decoded property value */
+    unsigned enc_size;                  /* Size of encoded property */
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    HDassert(pp);
+    HDassert(*pp);
+    HDassert(efile_pref);
+    HDcompile_assert(sizeof(size_t) <= sizeof(uint64_t));
+
+    /* Decode the size */
+    enc_size = *(*pp)++;
+    HDassert(enc_size < 256);
+
+    /* Decode the value */
+    UINT64DECODE_VAR(*pp, enc_value, enc_size);
+    len = (size_t)enc_value;
+
+    if(0 != len) {
+        /* Make a copy of the user's prefix string */
+        if(NULL == (*efile_pref = (char *)H5MM_malloc(len + 1)))
+            HGOTO_ERROR(H5E_RESOURCE, H5E_CANTINIT, FAIL, "memory allocation failed for prefix")
+        HDstrncpy(*efile_pref, *(const char **)pp, len);
+        (*efile_pref)[len] = '\0';
+
+        *pp += len;
+    } /* end if */
+    else
+        *efile_pref = NULL;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5P__dapl_efile_pref_dec() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5P__dapl_efile_pref_del
+ *
+ * Purpose:     Frees memory used to store the external file prefix string
+ *
+ * Return:      SUCCEED (Can't fail)
+ *
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5P__dapl_efile_pref_del(hid_t H5_ATTR_UNUSED prop_id, const char H5_ATTR_UNUSED *name,
+    size_t H5_ATTR_UNUSED size, void *value)
+{
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    HDassert(value);
+
+    H5MM_xfree(*(void **)value);
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5P__dapl_efile_pref_del() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5P__dapl_efile_pref_copy
+ *
+ * Purpose:     Creates a copy of the external file prefix string
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5P__dapl_efile_pref_copy(const char H5_ATTR_UNUSED *name, size_t H5_ATTR_UNUSED size, void *value)
+{
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    HDassert(value);
+
+    *(char **)value = H5MM_xstrdup(*(const char **)value);
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5P__dapl_efile_pref_copy() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:       H5P__dapl_efile_pref_cmp
+ *
+ * Purpose:        Callback routine which is called whenever the efile prefix
+ *                 property in the dataset creation property list is
+ *                 compared.
+ *
+ * Return:         zero if VALUE1 and VALUE2 are equal, non zero otherwise.
+ *
+ *-------------------------------------------------------------------------
+ */
+static int
+H5P__dapl_efile_pref_cmp(const void *value1, const void *value2, size_t H5_ATTR_UNUSED size)
+{
+    const char *pref1 = *(const char * const *)value1;
+    const char *pref2 = *(const char * const *)value2;
+    int ret_value = 0;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    if(NULL == pref1 && NULL != pref2)
+        HGOTO_DONE(1);
+    if(NULL != pref1 && NULL == pref2)
+        HGOTO_DONE(-1);
+    if(NULL != pref1 && NULL != pref2)
+        ret_value = HDstrcmp(pref1, pref2);
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5P__dapl_efile_pref_cmp() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5P__dapl_efile_pref_close
+ *
+ * Purpose:     Frees memory used to store the external file prefix string
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5P__dapl_efile_pref_close(const char H5_ATTR_UNUSED *name, size_t H5_ATTR_UNUSED size, void *value)
+{
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    HDassert(value);
+
+    H5MM_xfree(*(void **)value);
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5P__dapl_efile_pref_close() */
+
+
+/*-------------------------------------------------------------------------
  * Function:	H5Pset_chunk_cache
  *
  * Purpose:	Set the number of objects in the meta data cache and the
@@ -787,3 +1074,94 @@ done:
     FUNC_LEAVE_API(ret_value)
 } /* end H5Pget_virtual_printf_gap() */
 
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Pset_efile_prefix
+ *
+ * Purpose:     Set a prefix to be used for any external files.
+ *
+ *              If the prefix starts with ${ORIGIN}, this will be replaced by
+ *              the absolute path of the directory of the HDF5 file containing
+ *              the dataset.
+ *
+ *              If the prefix is ".", no prefix will be applied.
+ *
+ *              This property can be overwritten by the environment variable
+ *              HDF5_EXTFILE_PREFIX.
+ *
+ * Return:	Non-negative on success/Negative on failure
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5Pset_efile_prefix(hid_t plist_id, const char *prefix)
+{
+    H5P_genplist_t *plist;              /* Property list pointer */
+    herr_t ret_value = SUCCEED;         /* Return value */
+
+    FUNC_ENTER_API(FAIL)
+    H5TRACE2("e", "i*s", plist_id, prefix);
+
+    /* Get the plist structure */
+    if(NULL == (plist = H5P_object_verify(plist_id, H5P_DATASET_ACCESS)))
+        HGOTO_ERROR(H5E_ATOM, H5E_BADATOM, FAIL, "can't find object for ID")
+
+    /* Set prefix */
+    if(H5P_set(plist, H5D_ACS_EFILE_PREFIX_NAME, &prefix) < 0)
+        HGOTO_ERROR(H5E_PLIST, H5E_CANTSET, FAIL, "can't set prefix info")
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Pset_efile_prefix() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Pget_efile_prefix
+ *
+ * Purpose:	Gets the prefix to be used for any external files.
+ *
+ *              If the pointer is not NULL, it points to a user-allocated
+ *              buffer.
+ *
+ * Return:	Non-negative on success/Negative on failure
+ *
+ *-------------------------------------------------------------------------
+ */
+ssize_t
+H5Pget_efile_prefix(hid_t plist_id, char *prefix, size_t size)
+{
+    H5P_genplist_t *plist;              /* Property list pointer */
+    char *my_prefix;                    /* Library's copy of the prefix */
+    size_t	len;                    /* Length of prefix string */
+    ssize_t 	ret_value;              /* Return value */
+
+    FUNC_ENTER_API(FAIL)
+    H5TRACE3("Zs", "i*sz", plist_id, prefix, size);
+
+    /* Get the plist structure */
+    if(NULL == (plist = H5P_object_verify(plist_id, H5P_DATASET_ACCESS)))
+        HGOTO_ERROR(H5E_ATOM, H5E_BADATOM, FAIL, "can't find object for ID")
+
+    /* Get the current prefix */
+    if(H5P_peek(plist, H5D_ACS_EFILE_PREFIX_NAME, &my_prefix) < 0)
+        HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, FAIL, "can't get external file prefix")
+
+    /* Check for prefix being set */
+    if(my_prefix) {
+        /* Copy to user's buffer, if given */
+        len = HDstrlen(my_prefix);
+        if(prefix) {
+            HDstrncpy(prefix, my_prefix, MIN(len + 1, size));
+            if(len >= size)
+                prefix[size - 1] = '\0';
+        } /* end if */
+    } /* end if */
+    else
+        len = 0;
+
+    /* Set return value */
+    ret_value = (ssize_t)len;
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Pget_efile_prefix() */
diff --git a/src/H5Plapl.c b/src/H5Plapl.c
index b7c682c..239f2a5 100644
--- a/src/H5Plapl.c
+++ b/src/H5Plapl.c
@@ -783,7 +783,8 @@ done:
  *-------------------------------------------------------------------------
  */
 static herr_t
-H5P__lacc_elink_pref_del(hid_t H5_ATTR_UNUSED prop_id, const char H5_ATTR_UNUSED *name, size_t H5_ATTR_UNUSED size, void *value)
+H5P__lacc_elink_pref_del(hid_t H5_ATTR_UNUSED prop_id, const char H5_ATTR_UNUSED *name,
+    size_t H5_ATTR_UNUSED size, void *value)
 {
     FUNC_ENTER_STATIC_NOERR
 
diff --git a/src/H5Ppublic.h b/src/H5Ppublic.h
index 6a97918..e102776 100644
--- a/src/H5Ppublic.h
+++ b/src/H5Ppublic.h
@@ -400,6 +400,8 @@ H5_DLL herr_t H5Pset_virtual_view(hid_t plist_id, H5D_vds_view_t view);
 H5_DLL herr_t H5Pget_virtual_view(hid_t plist_id, H5D_vds_view_t *view);
 H5_DLL herr_t H5Pset_virtual_printf_gap(hid_t plist_id, hsize_t gap_size);
 H5_DLL herr_t H5Pget_virtual_printf_gap(hid_t plist_id, hsize_t *gap_size);
+H5_DLL herr_t H5Pset_efile_prefix(hid_t dapl_id, const char* prefix);
+H5_DLL ssize_t H5Pget_efile_prefix(hid_t dapl_id, char* prefix /*out*/, size_t size);
 
 /* Dataset xfer property list (DXPL) routines */
 H5_DLL herr_t H5Pset_data_transform(hid_t plist_id, const char* expression);
diff --git a/src/H5private.h b/src/H5private.h
index 74763a5..0331e2c 100644
--- a/src/H5private.h
+++ b/src/H5private.h
@@ -1189,6 +1189,9 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDsetbuf
     #define HDsetbuf(F,S)    setbuf(F,S)
 #endif /* HDsetbuf */
+#ifndef HDsetenv
+    #define HDsetenv(N,V,O)    setenv(N,V,O)
+#endif /* HDsetenv */
 #ifndef HDsetgid
     #define HDsetgid(G)    setgid(G)
 #endif /* HDsetgid */
@@ -2557,7 +2560,8 @@ H5_DLL uint32_t H5_hash_string(const char *str);
 H5_DLL time_t H5_make_time(struct tm *tm);
 
 /* Functions for building paths, etc. */
-H5_DLL herr_t   H5_build_extpath(const char *, char ** /*out*/ );
+H5_DLL herr_t   H5_build_extpath(const char *name, char **extpath /*out*/);
+H5_DLL herr_t   H5_combine_path(const char *path1, const char *path2, char **full_name /*out*/);
 
 /* Functions for debugging */
 H5_DLL herr_t H5_buffer_dump(FILE *stream, int indent, const uint8_t *buf,
diff --git a/src/H5system.c b/src/H5system.c
index 4baebc5..420ec85 100644
--- a/src/H5system.c
+++ b/src/H5system.c
@@ -757,7 +757,7 @@ done:
  */
 int
 Wgettimeofday(struct timeval *tv, struct timezone *tz)
- {
+{
   union {
     unsigned long long ns100; /*time since 1 Jan 1601 in 100ns units */
     FILETIME ft;
@@ -783,7 +783,42 @@ Wgettimeofday(struct timeval *tv, struct timezone *tz)
   /* Always return 0 as per Open Group Base Specifications Issue 6.
      Do not set errno on error.  */
   return 0;
-}
+} /* end Wgettimeofday() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    Wsetenv
+ *
+ * Purpose:     Wrapper function for setenv on Windows systems.
+ *              Interestingly, getenv *is* available in the Windows
+ *              POSIX layer, just not setenv.
+ *
+ * Return:      Success:    0
+ *              Failure:    non-zero error code
+ *
+ * Programmer:  Dana Robinson
+ *              February 2016
+ *
+ *-------------------------------------------------------------------------
+ */
+int
+Wsetenv(const char *name, const char *value, int overwrite)
+{
+    size_t bufsize;
+    errno_t err;
+
+    /* If we're not overwriting, check if the environment variable exists.
+     * If it does (i.e.: the required buffer size to store the variable's
+     * value is non-zero), then return an error code.
+     */
+    if(!overwrite) {
+        err = getenv_s(&bufsize, NULL, 0, name);
+        if(err || bufsize)
+            return (int)err
+    } /* end if */
+
+    return (int)_putenv_s(name, value);
+} /* end Wsetenv() */
 
 #ifdef H5_HAVE_WINSOCK2_H
 #pragma comment(lib, "advapi32.lib")
@@ -881,26 +916,25 @@ Wflock(int fd, int operation) {
 
 
 /*-------------------------------------------------------------------------
- * Function: H5_build_extpath
+ * Function:    H5_build_extpath
  *
- * Purpose:  To build the path for later searching of target file for external
- *    link.  This path can be either:
+ * Purpose:     To build the path for later searching of target file for external
+ *              links and external files.  This path can be either:
  *                  1. The absolute path of NAME
  *                      or
  *                  2. The current working directory + relative path of NAME
  *
- * Return:  Success:        0
- *    Failure:  -1
+ * Return:      SUCCEED/FAIL
  *
  * Programmer:  Vailin Choi
- *    April 2, 2008
+ *              April 2, 2008
  *
  *-------------------------------------------------------------------------
  */
 #define MAX_PATH_LEN     1024
 
 herr_t
-H5_build_extpath(const char *name, char **extpath/*out*/)
+H5_build_extpath(const char *name, char **extpath /*out*/)
 {
     char        *full_path = NULL;      /* Pointer to the full path, as built or passed in */
     char        *cwdpath = NULL;        /* Pointer to the current working directory path */
@@ -998,5 +1032,80 @@ done:
         H5MM_xfree(new_name);
 
     FUNC_LEAVE_NOAPI(ret_value)
-} /* H5_build_extpath() */
+} /* end H5_build_extpath() */
+
+
+/*--------------------------------------------------------------------------
+ * Function:    H5_combine_path
+ *
+ * Purpose:     If path2 is relative, interpret path2 as relative to path1
+ *              and store the result in full_name. Otherwise store path2
+ *              in full_name.
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ * Programmer:  Steffen Kiess
+ *              June 22, 2015
+ *--------------------------------------------------------------------------
+ */
+herr_t
+H5_combine_path(const char* path1, const char* path2, char **full_name /*out*/)
+{
+    size_t      path1_len;            /* length of path1 */
+    size_t      path2_len;            /* length of path2 */
+    herr_t      ret_value = SUCCEED;  /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    HDassert(path1);
+    HDassert(path2);
+
+    path1_len = HDstrlen(path1);
+    path2_len = HDstrlen(path2);
+
+    if(*path1 == '\0' || H5_CHECK_ABSOLUTE(path2)) {
+
+        /* If path1 is empty or path2 is absolute, simply use path2 */
+        if(NULL == (*full_name = (char *)H5MM_strdup(path2)))
+            HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "memory allocation failed")
+
+    } /* end if */ 
+    else if(H5_CHECK_ABS_PATH(path2)) {
+
+        /* On windows path2 is a path absolute name */
+        if (H5_CHECK_ABSOLUTE(path1) || H5_CHECK_ABS_DRIVE(path1)) {
+            /* path1 is absolute or drive absolute and path2 is path absolute.
+             * Use the drive letter of path1 + path2
+             */
+            if(NULL == (*full_name = (char *)H5MM_malloc(path2_len + 3)))
+                HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "unable to allocate path2 buffer")
+            HDsnprintf(*full_name, (path2_len + 3), "%c:%s", path1[0], path2);
+        } /* end if */
+        else {
+            /* On windows path2 is path absolute name ("\foo\bar"),
+             * path1 does not have a drive letter (i.e. is "a\b" or "\a\b").
+             * Use path2.
+             */
+            if(NULL == (*full_name = (char *)H5MM_strdup(path2)))
+                HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "memory allocation failed")
+        } /* end else */
+
+    } /* end else if */
+    else {
+
+        /* Relative path2:
+         * Allocate a buffer to hold path1 + path2 + possibly the delimiter
+         *      + terminating null byte
+         */
+        if(NULL == (*full_name = (char *)H5MM_malloc(path1_len + path2_len + 2)))
+            HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "unable to allocate filename buffer")
+
+        /* Compose the full file name */
+        HDsnprintf(*full_name, (path1_len + path2_len + 2), "%s%s%s", path1,
+                   (H5_CHECK_DELIMITER(path1[path1_len - 1]) ? "" : H5_DIR_SEPS), path2);
+    } /* end else */
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5_combine_name() */
 
diff --git a/src/H5win32defs.h b/src/H5win32defs.h
index e84def9..2a0f22d 100644
--- a/src/H5win32defs.h
+++ b/src/H5win32defs.h
@@ -71,21 +71,24 @@ struct timezone {
 };
 
 #ifdef __cplusplus
-        extern "C" {
+extern "C" {
 #endif /* __cplusplus */
-        H5_DLL int Wgettimeofday(struct timeval *tv, struct timezone *tz);
-        H5_DLL int Wflock(int fd, int operation);
-        H5_DLL char* Wgetlogin(void);
-        H5_DLL int c99_snprintf(char* str, size_t size, const char* format, ...);
-        H5_DLL int c99_vsnprintf(char* str, size_t size, const char* format, va_list ap);
+    H5_DLL int Wgettimeofday(struct timeval *tv, struct timezone *tz);
+    H5_DLL int Wsetenv(const char *name, const char *value, int overwrite);
+    H5_DLL int Wflock(int fd, int operation);
+    H5_DLL char* Wgetlogin(void);
+    H5_DLL int c99_snprintf(char* str, size_t size, const char* format, ...);
+    H5_DLL int c99_vsnprintf(char* str, size_t size, const char* format, va_list ap);
 #ifdef __cplusplus
-        }
+}
 #endif /* __cplusplus */
+
 #define HDgettimeofday(V,Z) Wgettimeofday(V,Z)
+#define HDsetenv(N,V,O)     Wsetenv(N,V,O)
 #define HDflock(F,L)        Wflock(F,L)
 #define HDgetlogin()        Wgetlogin()
 #define HDsnprintf          c99_snprintf /*varargs*/
-#define HDvsnprintf         c99_vsnprintf
+#define HDvsnprintf         c99_vsnprintf /*varargs*/
 
 #endif /* H5_HAVE_VISUAL_STUDIO */
 
@@ -98,5 +101,7 @@ struct timezone {
 #ifndef H5_HAVE_MINGW
 #define HDftruncate(F,L)    _chsize_s(F,L)
 #define HDfseek(F,O,W)      _fseeki64(F,O,W)
-#endif
+#endif /* H5_HAVE_MINGW */
+
 #endif /* H5_HAVE_WIN32_API */
+