tools/xenstore: Document failure for xs_{read,directory,read_watch}

Those functions can return NULL on failure, document it in the public
header.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/xenstore/include/xenstore.h

@@ -103,6 +103,7 @@ void xs_daemon_destroy_postfork(struct xs_handle *);
 /* Get contents of a directory.
  * Returns a malloced array: call free() on it after use.
  * Num indicates size.
+ * Returns NULL on failure.
  */
 char **xs_directory(struct xs_handle *h, xs_transaction_t t,
 		    const char *path, unsigned int *num);
@@ -110,6 +111,7 @@ char **xs_directory(struct xs_handle *h, xs_transaction_t t,
 /* Get the value of a single file, nul terminated.
  * Returns a malloced value: call free() on it after use.
  * len indicates length in bytes, not including terminator.
+ * Returns NULL on failure.
  */
 void *xs_read(struct xs_handle *h, xs_transaction_t t,
 	      const char *path, unsigned int *len);
@@ -196,8 +198,9 @@ int xs_fileno(struct xs_handle *h);
 char **xs_check_watch(struct xs_handle *h);
 
 /* Find out what node change was on (will block if nothing pending).
- * Returns array containing the path and token. Use XS_WATCH_* to access these
- * elements. Call free() after use.
+ * Returns array containing the path and token, or NULL.
+ * Use XS_WATCH_* to access these elements.
+ * Call free() after use.
  */
 char **xs_read_watch(struct xs_handle *h, unsigned int *num);
 

tools/xenstore/xs.c

@@ -680,6 +680,7 @@ char **xs_directory(struct xs_handle *h, xs_transaction_t t,
 /* Get the value of a single file, nul terminated.
  * Returns a malloced value: call free() on it after use.
  * len indicates length in bytes, not including the nul.
+ * Returns NULL on failure.
  */
 void *xs_read(struct xs_handle *h, xs_transaction_t t,
 	      const char *path, unsigned int *len)