diff --git a/src/gmio_stl/stl_io.c b/src/gmio_stl/stl_io.c
index e69db3d..42d9e87 100644
--- a/src/gmio_stl/stl_io.c
+++ b/src/gmio_stl/stl_io.c
@@ -84,7 +84,7 @@ int gmio_stl_read(gmio_transfer_t *trsf, gmio_stl_mesh_creator_t *creator)
 int gmio_stl_write_file(
         gmio_stl_format_t format,
         const char *filepath,
-        gmio_stl_mesh_t *mesh,
+        const gmio_stl_mesh_t *mesh,
         gmio_task_iface_t *task_iface)
 {
     int error = GMIO_ERROR_OK;
@@ -112,7 +112,7 @@ int gmio_stl_write_file(
 int gmio_stl_write(
         gmio_stl_format_t format,
         gmio_transfer_t *trsf,
-        gmio_stl_mesh_t *mesh)
+        const gmio_stl_mesh_t *mesh)
 {
     int error = GMIO_ERROR_OK;
 
diff --git a/src/gmio_stl/stl_io.h b/src/gmio_stl/stl_io.h
index cc3bc57..7196335 100644
--- a/src/gmio_stl/stl_io.h
+++ b/src/gmio_stl/stl_io.h
@@ -84,7 +84,7 @@ GMIO_LIBSTL_EXPORT
 int gmio_stl_write_file(
         gmio_stl_format_t format,
         const char* filepath,
-        gmio_stl_mesh_t* mesh,
+        const gmio_stl_mesh_t* mesh,
         gmio_task_iface_t* task_iface);
 
 /*! Writes STL mesh to stream
@@ -99,7 +99,7 @@ GMIO_LIBSTL_EXPORT
 int gmio_stl_write(
         gmio_stl_format_t format,
         gmio_transfer_t* trsf,
-        gmio_stl_mesh_t* mesh);
+        const gmio_stl_mesh_t* mesh);
 
 /* ========================================================================
  *  STL ascii