diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index da09f5be700..f26f233f0b1 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2015-10-26  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* guile/scm-ports.c (ioscm_make_gdb_stdio_port): Pass non-const
+	char pointer to scm_mode_bits.
+
 2015-10-26  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* symtab.c (default_make_symbol_completion_list_break_on_1): Add
diff --git a/gdb/guile/scm-ports.c b/gdb/guile/scm-ports.c
index 10d7ee2659e..5d529b35fbb 100644
--- a/gdb/guile/scm-ports.c
+++ b/gdb/guile/scm-ports.c
@@ -357,29 +357,36 @@ ioscm_init_stdio_buffers (SCM port, long mode_bits)
 static SCM
 ioscm_make_gdb_stdio_port (int fd)
 {
-  int is_a_tty = isatty (fd);
   const char *name;
   long mode_bits;
   SCM port;
+  char buf[3];
+
+  memset (buf, 0, sizeof (buf));
 
   switch (fd)
     {
     case 0:
       name = input_port_name;
-      mode_bits = scm_mode_bits (is_a_tty ? "r0" : "r");
+      buf[0] = 'r';
       break;
     case 1:
       name = output_port_name;
-      mode_bits = scm_mode_bits (is_a_tty ? "w0" : "w");
+      buf[0] = 'w';
       break;
     case 2:
       name = error_port_name;
-      mode_bits = scm_mode_bits (is_a_tty ? "w0" : "w");
+      buf[0] = 'w';
       break;
     default:
       gdb_assert_not_reached ("bad stdio file descriptor");
     }
 
+  if (isatty (fd))
+    buf[1] = '0';
+
+  mode_bits = scm_mode_bits (buf);
+
   port = ioscm_open_port (stdio_port_desc, mode_bits);
 
   scm_set_port_filename_x (port, gdbscm_scm_from_c_string (name));