(scm_substring_read_only,

scm_c_substring_read_only, scm_i_substring_read_only): New.
(RO_STRING_TAG, IS_RO_STRING): New.
(scm_i_string_writable_chars): Bail on read-only strings.

libguile/strings.c

@@ -152,6 +152,12 @@ SCM_MUTEX (stringbuf_write_mutex);
 
 #define IS_STRING(str)        (SCM_NIMP(str) && SCM_TYP7(str) == STRING_TAG)
 
+/* Read-only strings.
+ */
+
+#define RO_STRING_TAG         (scm_tc7_string + 0x200)
+#define IS_RO_STRING(str)     (SCM_CELL_TYPE(str)==RO_STRING_TAG)
+
 /* Mutation-sharing substrings
  */
 
@@ -211,6 +217,20 @@ scm_i_substring (SCM str, size_t start, size_t end)
 			  (scm_t_bits) end - start);
 }
 
+SCM
+scm_i_substring_read_only (SCM str, size_t start, size_t end)
+{
+  SCM buf;
+  size_t str_start;
+  get_str_buf_start (&str, &buf, &str_start);
+  scm_i_plugin_mutex_lock (&stringbuf_write_mutex);
+  SET_STRINGBUF_SHARED (buf);
+  scm_i_plugin_mutex_unlock (&stringbuf_write_mutex);
+  return scm_double_cell (RO_STRING_TAG, SCM_UNPACK(buf),
+			  (scm_t_bits)str_start + start,
+			  (scm_t_bits) end - start);
+}
+
 SCM
 scm_i_substring_copy (SCM str, size_t start, size_t end)
 {
@@ -251,6 +271,13 @@ scm_c_substring (SCM str, size_t start, size_t end)
   return scm_i_substring (str, start, end);
 }
 
+SCM
+scm_c_substring_read_only (SCM str, size_t start, size_t end)
+{
+  validate_substring_args (str, start, end);
+  return scm_i_substring_read_only (str, start, end);
+}
+
 SCM
 scm_c_substring_copy (SCM str, size_t start, size_t end)
 {
@@ -298,11 +325,15 @@ scm_i_string_chars (SCM str)
 }
 
 char *
-scm_i_string_writable_chars (SCM str)
+scm_i_string_writable_chars (SCM orig_str)
 {
-  SCM buf;
+  SCM buf, str = orig_str;
   size_t start;
+
   get_str_buf_start (&str, &buf, &start);
+  if (IS_RO_STRING (str))
+    scm_misc_error (NULL, "string is read-only: ~s", scm_list_1 (orig_str));
+
   scm_i_plugin_mutex_lock (&stringbuf_write_mutex);
   if (STRINGBUF_SHARED (buf))
     {
@@ -664,6 +695,32 @@ SCM_DEFINE (scm_substring, "substring", 2, 1, 0,
 }
 #undef FUNC_NAME
 
+SCM_DEFINE (scm_substring_read_only, "substring/read-only", 2, 1, 0,
+	    (SCM str, SCM start, SCM end),
+	    "Return a newly allocated string formed from the characters\n"
+            "of @var{str} beginning with index @var{start} (inclusive) and\n"
+	    "ending with index @var{end} (exclusive).\n"
+            "@var{str} must be a string, @var{start} and @var{end} must be\n"
+	    "exact integers satisfying:\n"
+	    "\n"
+            "0 <= @var{start} <= @var{end} <= (string-length @var{str}).\n"
+	    "\n"
+	    "The returned string is read-only.\n")
+#define FUNC_NAME s_scm_substring_read_only
+{
+  size_t len, from, to;
+
+  SCM_VALIDATE_STRING (1, str);
+  len = scm_i_string_length (str);
+  from = scm_to_unsigned_integer (start, 0, len);
+  if (SCM_UNBNDP (end))
+    to = len;
+  else
+    to = scm_to_unsigned_integer (end, from, len);
+  return scm_i_substring_read_only (str, from, to);
+}
+#undef FUNC_NAME
+
 SCM_DEFINE (scm_substring_copy, "substring/copy", 2, 1, 0,
 	    (SCM str, SCM start, SCM end),
 	    "Return a newly allocated string formed from the characters\n"

libguile/strings.h

@@ -83,6 +83,7 @@ SCM_API SCM scm_string_length (SCM str);
 SCM_API SCM scm_string_ref (SCM str, SCM k);
 SCM_API SCM scm_string_set_x (SCM str, SCM k, SCM chr);
 SCM_API SCM scm_substring (SCM str, SCM start, SCM end);
+SCM_API SCM scm_substring_read_only (SCM str, SCM start, SCM end);
 SCM_API SCM scm_substring_shared (SCM str, SCM start, SCM end);
 SCM_API SCM scm_substring_copy (SCM str, SCM start, SCM end);
 SCM_API SCM scm_string_append (SCM args);
@@ -92,6 +93,7 @@ SCM_API size_t scm_c_string_length (SCM str);
 SCM_API SCM scm_c_string_ref (SCM str, size_t pos);
 SCM_API void scm_c_string_set_x (SCM str, size_t pos, SCM chr);
 SCM_API SCM scm_c_substring (SCM str, size_t start, size_t end);
+SCM_API SCM scm_c_substring_read_only (SCM str, size_t start, size_t end);
 SCM_API SCM scm_c_substring_shared (SCM str, size_t start, size_t end);
 SCM_API SCM scm_c_substring_copy (SCM str, size_t start, size_t end);
 
@@ -110,6 +112,7 @@ SCM_API SCM scm_makfromstrs (int argc, char **argv);
 
 SCM_API SCM scm_i_make_string (size_t len, char **datap);
 SCM_API SCM scm_i_substring (SCM str, size_t start, size_t end);
+SCM_API SCM scm_i_substring_read_only (SCM str, size_t start, size_t end);
 SCM_API SCM scm_i_substring_shared (SCM str, size_t start, size_t end);
 SCM_API SCM scm_i_substring_copy (SCM str, size_t start, size_t end);
 SCM_API size_t scm_i_string_length (SCM str);