diff --git a/libisofs/filesrc.c b/libisofs/filesrc.c
index 239eb7a..79e73f9 100644
--- a/libisofs/filesrc.c
+++ b/libisofs/filesrc.c
@@ -286,6 +286,16 @@ int filesrc_writer_write_data(IsoImageWriter *writer)
                 }
             }
             continue;
+        } else if (res > 1) {
+            char *name = iso_stream_get_name(file->stream);
+            res = iso_msg_submit(t->image->id, ISO_FILE_CANT_WRITE, 0, 
+                      "Size of file \"%s\" has changed. It will be %s", name,
+                      (res == 2 ? "truncated" : "padded with 0's"));
+            free(name);
+            if (res < 0) {
+                filesrc_close(file);
+                return res; /* aborted due to error severity */
+            }
         }
 
         /* write file contents to image */
diff --git a/libisofs/stream.c b/libisofs/stream.c
index 9d315c6..c264065 100644
--- a/libisofs/stream.c
+++ b/libisofs/stream.c
@@ -30,12 +30,28 @@ typedef struct
 static
 int fsrc_open(IsoStream *stream)
 {
+    int ret;
+    struct stat info;
+    off_t esize;
     IsoFileSource *src;
     if (stream == NULL) {
         return ISO_NULL_POINTER;
     }
     src = ((FSrcStreamData*)stream->data)->src;
-    return iso_file_source_open(src);
+    ret = iso_file_source_stat(src, &info);
+    if (ret < 0) {
+        return ret;
+    }
+    ret = iso_file_source_open(src);
+    if (ret < 0) {
+        return ret;
+    }
+    esize = ((FSrcStreamData*)stream->data)->size;
+    if (info.st_size == esize) {
+        return ISO_SUCCESS;
+    } else {
+        return (esize > info.st_size) ? 3 : 2;
+    }
 }
 
 static
diff --git a/libisofs/stream.h b/libisofs/stream.h
index 0a81a03..1261b37 100644
--- a/libisofs/stream.h
+++ b/libisofs/stream.h
@@ -32,10 +32,9 @@ typedef struct IsoStream_Iface
     /**
      * Opens the stream.
      * 
-     * TODO it would be great to return a different success code if the
-     * underlying source size has changed.
      * @return 
-     *     1 on success, < 0 on error
+     *     1 on success, 2 file greater than expected, 3 file smaller than
+     *     expected, < 0 on error
      */
     int (*open)(IsoStream *stream);