Issue #25923: Added more const qualifiers to signatures of static and private functions.
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index dbd604a..8bf3922 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -540,7 +540,7 @@
 }
 
 static PyObject *
-err_mode(char *action)
+err_mode(const char *action)
 {
     _PyIO_State *state = IO_STATE();
     if (state != NULL)
@@ -1043,7 +1043,7 @@
 }
 #endif /* HAVE_FTRUNCATE */
 
-static char *
+static const char *
 mode_string(fileio *self)
 {
     if (self->created) {