Merge pull request #1600 in HDFFV/hdf5 from ~SONGYULU/hdf5_ray:HDFFV-10658-performance-drop-from-1-8 to develop

* commit 'b5ef82a1786605ae86502bc82086047720b7d4ca': (21 commits)
  Moving the handling of null prefix into H5_combine_path.
  Changing the prefix of external file and VDS from empty string to null for performance improvement.
  Improving the condition checking of empty string.
  Replacing string operation strdup with assignment for empty string.
  Adding back links_env.out which I accidentally removed in my previous commit.
  Taking out two unnecessary diff output files.
  Taking out unnecessary diff files for output.
  Adding some comments.
  Some coding style changes.
  Adding the standard output files for the external_env.c and vds_env.c tests.
  Small correction for my previous commit.
  Forgot to add external_common.c and external_common.h.
  Updated CMake for the splitting of external.c and vds.c.
  Minor fix: removal of unnecessary enum values.
  Minor fixes: updating the test vds_env.c according to the set up of vds.c.
  Left out this file in previous commit.
  HDFFV-10658 - setting and getting properties in API context: 1. switched to use the existing H5F_prefix_open_t for enum type; 2. put the common private function used by external.c and external_env.c into external_common.c
  This commit basically has the following changes: 1. restored the datatype, dataspace, and LCPL of the dataset for VOL connector back to the properties. 2. splitted external.c and vds.c because they called HDsetenv in the program, instead using shell scripts to set the environment variables. 3. changed H5CX_get_vds_prefix and H5CX_get_ext_file_prefix to use H5P_peek instead of H5P_get.
  HDFFV-10658: I left out this file in my previous commit.
  HDFFV-10658: 1. moving HDgetenv to dataset initialization stage to reduce the overhead; 2. restoring the retrieval of three vol properties to H5P_get instead of using API context to prepare for Quincey's upcoming refactoring work.
  ...

8 files changed, 220 insertions, 39 deletions

diff --git a/src/H5CX.c b/src/H5CX.c
index c57e65d..5474b63 100644
--- a/src/H5CX.c
+++ b/src/H5CX.c
@@ -189,6 +189,10 @@ typedef struct H5CX_t {
     hid_t dcpl_id;              /* DCPL ID for API operation */
     H5P_genplist_t *dcpl;       /* Dataset Creation Property List */
 
+    /* DAPL */
+    hid_t dapl_id;              /* DAPL ID for API operation */
+    H5P_genplist_t *dapl;       /* Dataset Access Property List */
+
     /* FAPL */
     hid_t fapl_id;              /* FAPL ID for API operation */
     H5P_genplist_t *fapl;       /* File Access Property List */
@@ -279,8 +283,14 @@ typedef struct H5CX_t {
     hbool_t nlinks_valid;       /* Whether number of soft / UD links to traverse is valid */
 
     /* Cached DCPL properties */
-    hbool_t do_min_dset_ohdr;   /* Whether to minimize dataset object header */
-    hbool_t do_min_dset_ohdr_valid;   /* Whether minimize dataset object header flag is valid */
+    hbool_t   do_min_dset_ohdr;   /* Whether to minimize dataset object header */
+    hbool_t   do_min_dset_ohdr_valid;   /* Whether minimize dataset object header flag is valid */
+
+    /* Cached DAPL properties */
+    char *extfile_prefix;               /* Prefix for external file                      */
+    hbool_t extfile_prefix_valid;       /* Whether the prefix for external file is valid */
+    char *vds_prefix;                   /* Prefix for VDS                                */
+    hbool_t vds_prefix_valid;           /* Whether the prefix for VDS is valid           */
 
     /* Cached FAPL properties */
     H5F_libver_t low_bound;     /* low_bound property for H5Pset_libver_bounds() */
@@ -348,6 +358,13 @@ typedef struct H5CX_dcpl_cache_t {
     hbool_t do_min_dset_ohdr;   /* Whether to minimize dataset object header */
 } H5CX_dcpl_cache_t;
 
+/* Typedef for cached default dataset access property list information */
+/* (Same as the cached DXPL struct, above, except for the default DXPL) */
+typedef struct H5CX_dapl_cache_t {
+    char *extfile_prefix;       /* Prefix for external file */
+    char *vds_prefix;           /* Prefix for VDS           */
+} H5CX_dapl_cache_t;
+
 /* Typedef for cached default file access property list information */
 /* (Same as the cached DXPL struct, above, except for the default DCPL) */
 typedef struct H5CX_fapl_cache_t {
@@ -355,7 +372,6 @@ typedef struct H5CX_fapl_cache_t {
     H5F_libver_t high_bound;    /* high_bound property for H5Pset_libver_bounds */
 } H5CX_fapl_cache_t;
 
-
 /********************/
 /* Local Prototypes */
 /********************/
@@ -391,6 +407,9 @@ static H5CX_lapl_cache_t H5CX_def_lapl_cache;
 /* Define a "default" dataset creation property list cache structure to use for default DCPLs */
 static H5CX_dcpl_cache_t H5CX_def_dcpl_cache;
 
+/* Define a "default" dataset access property list cache structure to use for default DAPLs */
+static H5CX_dapl_cache_t H5CX_def_dapl_cache;
+
 /* Define a "default" file access property list cache structure to use for default FAPLs */
 static H5CX_fapl_cache_t H5CX_def_fapl_cache;
 
@@ -418,6 +437,7 @@ H5CX__init_package(void)
     H5P_genplist_t *dx_plist;           /* Data transfer property list */
     H5P_genplist_t *la_plist;           /* Link access property list */
     H5P_genplist_t *dc_plist;           /* Dataset creation property list */
+    H5P_genplist_t *da_plist;           /* Dataset access property list */
     H5P_genplist_t *fa_plist;           /* File access property list */
     herr_t ret_value = SUCCEED;         /* Return value */
 
@@ -532,6 +552,23 @@ H5CX__init_package(void)
     if(H5P_get(dc_plist, H5D_CRT_MIN_DSET_HDR_SIZE_NAME, &H5CX_def_dcpl_cache.do_min_dset_ohdr) < 0)
         HGOTO_ERROR(H5E_CONTEXT, H5E_CANTGET, FAIL, "Can't retrieve dataset minimize flag")
 
+    /* Reset the "default DAPL cache" information */
+    HDmemset(&H5CX_def_dapl_cache, 0, sizeof(H5CX_dapl_cache_t));
+
+    /* Get the default DAPL cache information */
+
+    /* Get the default dataset access property list */
+    if(NULL == (da_plist = (H5P_genplist_t *)H5I_object(H5P_DATASET_ACCESS_DEFAULT)))
+        HGOTO_ERROR(H5E_CONTEXT, H5E_BADTYPE, FAIL, "not a dataset create property list")
+
+    /* Get the prefix for the external file */
+    if(H5P_peek(da_plist, H5D_ACS_EFILE_PREFIX_NAME, &H5CX_def_dapl_cache.extfile_prefix) < 0)
+        HGOTO_ERROR(H5E_CONTEXT, H5E_CANTGET, FAIL, "Can't retrieve prefix for external file")
+
+    /* Get the prefix for the VDS file */
+    if(H5P_peek(da_plist, H5D_ACS_VDS_PREFIX_NAME, &H5CX_def_dapl_cache.vds_prefix) < 0)
+        HGOTO_ERROR(H5E_CONTEXT, H5E_CANTGET, FAIL, "Can't retrieve prefix for VDS")
+
     /* Reset the "default FAPL cache" information */
     HDmemset(&H5CX_def_fapl_cache, 0, sizeof(H5CX_fapl_cache_t));
 
@@ -548,7 +585,6 @@ H5CX__init_package(void)
     if(H5P_get(fa_plist, H5F_ACS_LIBVER_HIGH_BOUND_NAME, &H5CX_def_fapl_cache.high_bound) < 0)
         HGOTO_ERROR(H5E_CONTEXT, H5E_CANTGET, FAIL, "Can't retrieve dataset minimize flag")
 
-
 done:
     FUNC_LEAVE_NOAPI(ret_value)
 } /* end H5CX__init_package() */
@@ -672,6 +708,8 @@ H5CX__push_common(H5CX_node_t *cnode)
 
     /* Set non-zero context info */
     cnode->ctx.dxpl_id = H5P_DATASET_XFER_DEFAULT;
+    cnode->ctx.dcpl_id = H5P_DATASET_CREATE_DEFAULT;
+    cnode->ctx.dapl_id = H5P_DATASET_ACCESS_DEFAULT;
     cnode->ctx.lapl_id = H5P_LINK_ACCESS_DEFAULT;
     cnode->ctx.fapl_id = H5P_FILE_ACCESS_DEFAULT;
     cnode->ctx.tag = H5AC__INVALID_TAG;
@@ -1158,6 +1196,7 @@ H5CX_set_apl(hid_t *acspl_id, const H5P_libclass_t *libclass,
         *acspl_id = *libclass->def_plist_id;
     else {
         htri_t is_lapl;         /* Whether the access property list is (or is derived from) a link access property list */
+        htri_t is_dapl;         /* Whether the access property list is (or is derived from) a dataset access property list */
         htri_t is_fapl;         /* Whether the access property list is (or is derived from) a file access property list */
 
 #ifdef H5CX_DEBUG
@@ -1172,6 +1211,12 @@ H5CX_set_apl(hid_t *acspl_id, const H5P_libclass_t *libclass,
         else if(is_lapl)
             (*head)->ctx.lapl_id = *acspl_id;
 
+        /* Check for dataset access property and set API context if so */
+        if((is_dapl = H5P_class_isa(*libclass->pclass, *H5P_CLS_DACC->pclass)) < 0)
+            HGOTO_ERROR(H5E_CONTEXT, H5E_CANTGET, FAIL, "can't check for dataset access class")
+        else if(is_dapl)
+            (*head)->ctx.dapl_id = *acspl_id;
+
         /* Check for file access property and set API context if so */
         if((is_fapl = H5P_class_isa(*libclass->pclass, *H5P_CLS_FACC->pclass)) < 0)
             HGOTO_ERROR(H5E_CONTEXT, H5E_CANTGET, FAIL, "can't check for file access class")
@@ -2443,6 +2488,120 @@ done:
 
 
 /*-------------------------------------------------------------------------
+ * Function:    H5CX_get_ext_file_prefix
+ *
+ * Purpose:     Retrieves the prefix for external file
+ *
+ * Return:      Non-negative on success / Negative on failure
+ *
+ * Programmer:  Raymond Lu
+ *              March 6, 2019
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5CX_get_ext_file_prefix(char **extfile_prefix)
+{
+    H5CX_node_t **head = H5CX_get_my_context();  /* Get the pointer to the head of the API context, for this thread */
+    herr_t ret_value = SUCCEED;         /* Return value */
+
+    FUNC_ENTER_NOAPI(FAIL)
+
+    /* Sanity check */
+    HDassert(extfile_prefix);
+    HDassert(head && *head);
+    HDassert(H5P_DEFAULT != (*head)->ctx.dapl_id);
+
+    /* Check if the value has been retrieved already */
+    if(!(*head)->ctx.extfile_prefix_valid) {
+        /* Check for default DAPL */
+        if((*head)->ctx.dapl_id == H5P_DATASET_ACCESS_DEFAULT)
+            (*head)->ctx.extfile_prefix = H5CX_def_dapl_cache.extfile_prefix;
+        else {
+            /* Check if the property list is already available */
+            if(NULL == (*head)->ctx.dapl)
+                /* Get the dataset access property list pointer */
+                if(NULL == ((*head)->ctx.dapl = (H5P_genplist_t *)H5I_object((*head)->ctx.dapl_id)))
+                    HGOTO_ERROR(H5E_CONTEXT, H5E_BADTYPE, FAIL, "can't get default dataset access property list")
+
+            /* Get the prefix for the external file */
+            /* (Note: 'peek', not 'get' - if this turns out to be a problem, we may need
+             *          to copy it and free this in the H5CX pop routine. -QAK)
+             */
+            if(H5P_peek((*head)->ctx.dapl, H5D_ACS_EFILE_PREFIX_NAME, &(*head)->ctx.extfile_prefix) < 0)
+                HGOTO_ERROR(H5E_CONTEXT, H5E_CANTGET, FAIL, "Can't retrieve external file prefix")
+        } /* end else */
+
+        /* Mark the value as valid */
+        (*head)->ctx.extfile_prefix_valid = TRUE;
+    } /* end if */
+
+    /* Get the value */
+    *extfile_prefix = (*head)->ctx.extfile_prefix;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5CX_get_ext_file_prefix() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5CX_get_vds_prefix
+ *
+ * Purpose:     Retrieves the prefix for VDS
+ *
+ * Return:      Non-negative on success / Negative on failure
+ *
+ * Programmer:  Raymond Lu
+ *              March 6, 2019
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5CX_get_vds_prefix(char **vds_prefix)
+{
+    H5CX_node_t **head = H5CX_get_my_context();  /* Get the pointer to the head of the API context, for this thread */
+    herr_t ret_value = SUCCEED;         /* Return value */
+
+    FUNC_ENTER_NOAPI(FAIL)
+
+    /* Sanity check */
+    HDassert(vds_prefix);
+    HDassert(head && *head);
+    HDassert(H5P_DEFAULT != (*head)->ctx.dapl_id);
+
+    /* Check if the value has been retrieved already */
+    if(!(*head)->ctx.vds_prefix_valid) {
+        /* Check for default DAPL */
+        if((*head)->ctx.dapl_id == H5P_DATASET_ACCESS_DEFAULT)
+            (*head)->ctx.vds_prefix = H5CX_def_dapl_cache.vds_prefix;
+        else {
+            /* Check if the property list is already available */
+            if(NULL == (*head)->ctx.dapl)
+                /* Get the dataset access property list pointer */
+                if(NULL == ((*head)->ctx.dapl = (H5P_genplist_t *)H5I_object((*head)->ctx.dapl_id)))
+                    HGOTO_ERROR(H5E_CONTEXT, H5E_BADTYPE, FAIL, "can't get default dataset access property list")
+
+            /* Get the prefix for the VDS */
+            /* (Note: 'peek', not 'get' - if this turns out to be a problem, we may need
+             *          to copy it and free this in the H5CX pop routine. -QAK)
+             */
+            if(H5P_peek((*head)->ctx.dapl, H5D_ACS_VDS_PREFIX_NAME, &(*head)->ctx.vds_prefix) < 0)
+                HGOTO_ERROR(H5E_CONTEXT, H5E_CANTGET, FAIL, "Can't retrieve VDS prefix")
+        } /* end else */
+
+        /* Mark the value as valid */
+        (*head)->ctx.vds_prefix_valid = TRUE;
+    } /* end if */
+
+    /* Get the value */
+    *vds_prefix = (*head)->ctx.vds_prefix;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5CX_get_vds_prefix() */
+
+
+/*-------------------------------------------------------------------------
  * Function:    H5CX_set_tag
  *
  * Purpose:     Sets the object tag for the current API call context.
diff --git a/src/H5CXprivate.h b/src/H5CXprivate.h
index 4270155..638c32a 100644
--- a/src/H5CXprivate.h
+++ b/src/H5CXprivate.h
@@ -128,6 +128,10 @@ H5_DLL herr_t H5CX_get_nlinks(size_t *nlinks);
 /* "Getter" routines for DCPL properties cached in API context */
 H5_DLL herr_t H5CX_get_dset_min_ohdr_flag(hbool_t *dset_min_ohdr_flag);
 
+/* "Getter" routines for DAPL properties cached in API context */
+H5_DLL herr_t H5CX_get_ext_file_prefix(char **prefix_extfile);
+H5_DLL herr_t H5CX_get_vds_prefix(char **prefix_vds);
+
 /* "Getter" routines for FAPL properties cached in API context */
 H5_DLL herr_t H5CX_get_libver_bounds(H5F_libver_t *low_bound, H5F_libver_t *high_bound);
 
diff --git a/src/H5D.c b/src/H5D.c
index fc350f2..413be37 100644
--- a/src/H5D.c
+++ b/src/H5D.c
@@ -135,6 +135,9 @@ H5Dcreate2(hid_t loc_id, const char *name, hid_t type_id, hid_t space_id,
         if(TRUE != H5P_isa_class(dcpl_id, H5P_DATASET_CREATE))
             HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, H5I_INVALID_HID, "dcpl_id is not a dataset create property list ID")
 
+    /* Set the DCPL for the API context */
+    H5CX_set_dcpl(dcpl_id);
+
     /* Verify access property list and set up collective metadata if appropriate */
     if(H5CX_set_apl(&dapl_id, H5P_CLS_DACC, loc_id, TRUE) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_CANTSET, H5I_INVALID_HID, "can't set access property list info")
@@ -228,6 +231,9 @@ H5Dcreate_anon(hid_t loc_id, hid_t type_id, hid_t space_id, hid_t dcpl_id,
         if(TRUE != H5P_isa_class(dcpl_id, H5P_DATASET_CREATE))
             HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, H5I_INVALID_HID, "not dataset create property list ID")
 
+    /* Set the DCPL for the API context */
+    H5CX_set_dcpl(dcpl_id);
+
     /* Verify access property list and set up collective metadata if appropriate */
     if(H5CX_set_apl(&dapl_id, H5P_CLS_DACC, loc_id, TRUE) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_CANTSET, H5I_INVALID_HID, "can't set access property list info")
diff --git a/src/H5Ddeprec.c b/src/H5Ddeprec.c
index 76827b4..85371ae 100644
--- a/src/H5Ddeprec.c
+++ b/src/H5Ddeprec.c
@@ -136,6 +136,9 @@ H5Dcreate1(hid_t loc_id, const char *name, hid_t type_id, hid_t space_id,
         if(TRUE != H5P_isa_class(dcpl_id, H5P_DATASET_CREATE))
             HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, H5I_INVALID_HID, "not dataset create property list ID")
 
+    /* Set the DCPL for the API context */
+    H5CX_set_dcpl(dcpl_id);
+
     /* Verify access property list and set up collective metadata if appropriate */
     if(H5CX_set_apl(&dapl_id, H5P_CLS_DACC, loc_id, TRUE) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_CANTSET, H5I_INVALID_HID, "can't set access property list info")
diff --git a/src/H5Defl.c b/src/H5Defl.c
index 19f3a00..91caa61 100644
--- a/src/H5Defl.c
+++ b/src/H5Defl.c
@@ -477,7 +477,6 @@ H5D__efl_readvv(const H5D_io_info_t *io_info,
     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);
@@ -561,7 +560,6 @@ H5D__efl_writevv(const H5D_io_info_t *io_info,
     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);
diff --git a/src/H5Dint.c b/src/H5Dint.c
index 280d07e..220f026 100644
--- a/src/H5Dint.c
+++ b/src/H5Dint.c
@@ -54,7 +54,7 @@ static herr_t H5D__init_type(H5F_t *file, const H5D_t *dset, hid_t type_id, cons
 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, H5D_t *dset, hid_t dapl_id);
-static herr_t H5D__build_file_prefix(const H5D_t *dset, hid_t dapl_id, const char *prefix_type, char **file_prefix);
+static herr_t H5D__build_file_prefix(const H5D_t *dset, H5F_prefix_open_t prefix_type, char **file_prefix);
 static herr_t H5D__open_oid(H5D_t *dataset, hid_t dapl_id);
 static herr_t H5D__init_storage(const H5D_io_info_t *io_info, hbool_t full_overwrite,
         hsize_t old_dim[]);
@@ -112,6 +112,10 @@ static const H5I_class_t H5I_DATASET_CLS[1] = {{
 /* Flag indicating "top" of interface has been initialized */
 static hbool_t H5D_top_package_initialize_s = FALSE;
 
+/* Prefixes of VDS and external file from the environment variables
+ * HDF5_EXTFILE_PREFIX and HDF5_VDS_PREFIX */ 
+const static char *H5D_prefix_ext_env = NULL;
+const static char *H5D_prefix_vds_env = NULL;
 
 
 /*-------------------------------------------------------------------------
@@ -188,6 +192,10 @@ H5D__init_package(void)
     /* Mark "top" of interface as initialized, too */
     H5D_top_package_initialize_s = TRUE;
 
+    /* Retrieve the prefixes of VDS and external file from the environment variable */
+    H5D_prefix_vds_env = HDgetenv("HDF5_VDS_PREFIX");
+    H5D_prefix_ext_env = HDgetenv("HDF5_EXTFILE_PREFIX");
+
 done:
     FUNC_LEAVE_NOAPI(ret_value)
 } /* end H5D__init_package() */
@@ -488,9 +496,6 @@ H5D__new(hid_t dcpl_id, hbool_t creating, hbool_t vl_type)
         new_dset->dcpl_id = H5P_copy_plist(plist, FALSE);
     } /* end else */
 
-    /* Set the DCPL for the API context */
-    H5CX_set_dcpl(new_dset->dcpl_id);
-
     /* Set return value */
     ret_value = new_dset;
 
@@ -1083,8 +1088,7 @@ done:
  *--------------------------------------------------------------------------
  */
 static herr_t
-H5D__build_file_prefix(const H5D_t *dset, hid_t dapl_id, const char *prefix_type,
-    char **file_prefix /*out*/)
+H5D__build_file_prefix(const H5D_t *dset, H5F_prefix_open_t prefix_type, char **file_prefix /*out*/)
 {
     char            *prefix = NULL;       /* prefix used to look for the file               */
     char            *filepath = NULL;     /* absolute path of directory the HDF5 file is in */
@@ -1105,20 +1109,22 @@ H5D__build_file_prefix(const H5D_t *dset, hid_t dapl_id, const char *prefix_type
     /* XXX: Future thread-safety note - getenv is not required
      *      to be reentrant.
      */
-    if(HDstrcmp(prefix_type, H5D_ACS_VDS_PREFIX_NAME) == 0)
-        prefix = HDgetenv("HDF5_VDS_PREFIX");
-    else if(HDstrcmp(prefix_type, H5D_ACS_EFILE_PREFIX_NAME) == 0)
-        prefix = HDgetenv("HDF5_EXTFILE_PREFIX");
-    else
-        HGOTO_ERROR(H5E_PLIST, H5E_BADTYPE, FAIL, "prefix name is not sensible")
-
-    if(prefix == NULL || *prefix == '\0') {
-        /* Set prefix to value of prefix_type 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, prefix_type, &prefix) < 0)
-            HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, FAIL, "can't get file prefix")
-    } /* end if */
+    if(H5F_PREFIX_VDS == prefix_type) {
+        prefix = (char *)H5D_prefix_vds_env;
+
+        if(prefix == NULL || *prefix == '\0') {
+            if(H5CX_get_vds_prefix(&prefix) < 0)
+                HGOTO_ERROR(H5E_DATASET, H5E_CANTGET, FAIL, "can't get the prefix for vds file")
+        }
+    } else if(H5F_PREFIX_EFILE == prefix_type) {
+        prefix = (char *)H5D_prefix_ext_env;
+
+        if(prefix == NULL || *prefix == '\0') {
+            if(H5CX_get_ext_file_prefix(&prefix) < 0)
+                HGOTO_ERROR(H5E_DATASET, H5E_CANTGET, FAIL, "can't get the prefix for the external file")
+        }
+    } else
+        HGOTO_ERROR(H5E_DATASET, H5E_BADTYPE, FAIL, "prefix name is not sensible")
 
     /* Prefix has to be checked for NULL / empty string again because the
      * code above might have updated it.
@@ -1127,8 +1133,7 @@ H5D__build_file_prefix(const H5D_t *dset, hid_t dapl_id, const char *prefix_type
         /* filename is interpreted as relative to the current directory,
          * does not need to be expanded
          */
-        if(NULL == (*file_prefix = (char *)H5MM_strdup("")))
-            HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "memory allocation failed")
+        *file_prefix = NULL;
     } /* end if */
     else {
         if(HDstrncmp(prefix, "${ORIGIN}", HDstrlen("${ORIGIN}")) == 0) {
@@ -1326,11 +1331,11 @@ H5D__create(H5F_t *file, hid_t type_id, const H5S_t *space, hid_t dcpl_id,
         HGOTO_ERROR(H5E_DATASET, H5E_CANTINIT, NULL, "unable to set up flush append property")
 
     /* Set the external file prefix */
-    if(H5D__build_file_prefix(new_dset, dapl_id, H5D_ACS_EFILE_PREFIX_NAME, &new_dset->shared->extfile_prefix) < 0)
+    if(H5D__build_file_prefix(new_dset, H5F_PREFIX_EFILE, &new_dset->shared->extfile_prefix) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_CANTINIT, NULL, "unable to initialize external file prefix")
 
     /* Set the VDS file prefix */
-    if(H5D__build_file_prefix(new_dset, dapl_id, H5D_ACS_VDS_PREFIX_NAME, &new_dset->shared->vds_prefix) < 0)
+    if(H5D__build_file_prefix(new_dset, H5F_PREFIX_VDS, &new_dset->shared->vds_prefix) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_CANTINIT, NULL, "unable to initialize VDS prefix")
 
     /* Add the dataset to the list of opened objects in the file */
@@ -1485,11 +1490,11 @@ H5D_open(const H5G_loc_t *loc, hid_t dapl_id)
         HGOTO_ERROR(H5E_DATASET, H5E_CANTCOPY, NULL, "can't copy path")
 
     /* Get the external file prefix */
-    if(H5D__build_file_prefix(dataset, dapl_id, H5D_ACS_EFILE_PREFIX_NAME, &extfile_prefix) < 0)
+    if(H5D__build_file_prefix(dataset, H5F_PREFIX_EFILE, &extfile_prefix) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_CANTINIT, NULL, "unable to initialize external file prefix")
 
     /* Get the VDS prefix */
-    if(H5D__build_file_prefix(dataset, dapl_id, H5D_ACS_VDS_PREFIX_NAME, &vds_prefix) < 0)
+    if(H5D__build_file_prefix(dataset, H5F_PREFIX_VDS, &vds_prefix) < 0)
         HGOTO_ERROR(H5E_DATASET, H5E_CANTINIT, NULL, "unable to initialize VDS prefix")
 
     /* Check if dataset was already open */
@@ -1533,8 +1538,13 @@ H5D_open(const H5G_loc_t *loc, hid_t dapl_id)
         /* 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")
+        if(extfile_prefix && dataset->shared->extfile_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")
+        } else {
+            if(extfile_prefix || dataset->shared->extfile_prefix)
+                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) {
diff --git a/src/H5Fprivate.h b/src/H5Fprivate.h
index e3860a0..0fa2214 100644
--- a/src/H5Fprivate.h
+++ b/src/H5Fprivate.h
@@ -702,8 +702,9 @@ typedef enum H5F_mem_page_t {
 
 /* Type of prefix for opening prefixed files */
 typedef enum H5F_prefix_open_t {
-    H5F_PREFIX_VDS,             /* Virtual dataset prefix */
-    H5F_PREFIX_ELINK            /* External link prefix */
+    H5F_PREFIX_VDS   = 0,           /* Virtual dataset prefix */
+    H5F_PREFIX_ELINK = 1,           /* External link prefix   */
+    H5F_PREFIX_EFILE = 2            /* External file prefix   */
 } H5F_prefix_open_t;
 
 
diff --git a/src/H5system.c b/src/H5system.c
index 2ddc29a..35123db 100644
--- a/src/H5system.c
+++ b/src/H5system.c
@@ -1256,13 +1256,13 @@ H5_combine_path(const char* path1, const char* path2, char **full_name /*out*/)
 
     FUNC_ENTER_NOAPI_NOINIT
 
-    HDassert(path1);
     HDassert(path2);
 
-    path1_len = HDstrlen(path1);
+    if(path1)
+        path1_len = HDstrlen(path1);
     path2_len = HDstrlen(path2);
 
-    if(*path1 == '\0' || H5_CHECK_ABSOLUTE(path2)) {
+    if(path1 == NULL || *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)))