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Re: Guile: What's wrong with this?

From: Mark H Weaver
Subject: Re: Guile: What's wrong with this?
Date: Wed, 04 Jan 2012 18:19:35 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.92 (gnu/linux)

Bruce Korb <address@hidden> writes:

>> ERROR: In procedure string-upcase:
>> ERROR: string is read-only: ""
>> Scheme evaluation error.  AutoGen ABEND-ing in template
>>      confmacs.tlib on line 209
>> Failing Guile command:  = = = = =
>> (set! tmp-text (get "act-text"))
>>        (set! TMP-text (string-upcase tmp-text))
> What in heck is string-upcase doing trying to write to its input string?
> Why was the string returned by ag_scm_get() (the function bound to "get")
> an immutable string anyway?

Good questions indeed.  I spent a bunch of time investigating this, and
found some bugs that might have caused this problem, although I'm not

Bruce: Can you please see if the patch below fixes this problem?

Mike: Would you be willing to review this (very small) patch to see if
it makes sense to you?  I'd like a second opinion from someone familiar
with that subsystem before I commit it.


>From a8da72937ff4d04e8d39531773cc05e676b2be1c Mon Sep 17 00:00:00 2001
From: Mark H Weaver <address@hidden>
Date: Wed, 4 Jan 2012 17:59:27 -0500
Subject: [PATCH] Fix bugs related to mutation-sharing substrings

* libguile/strings.c (scm_i_is_narrow_string, scm_i_try_narrow_string,
  scm_i_string_set_x): Check to see if the provided string is a
  mutation-sharing substring, and do the right thing in that case.
  Previously, if such a string was passed to these functions, they would
  behave very badly: while trying to fetch and/or mutate the cell
  containing the stringbuf, they were actually fetching or mutating the
  cell containing original shared string.  That's because
  mutation-sharing substring store the original string in CELL_1,
  whereas all other strings store the stringbuf there.
 libguile/strings.c |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/libguile/strings.c b/libguile/strings.c
index 666a951..1628aee 100644
--- a/libguile/strings.c
+++ b/libguile/strings.c
@@ -436,6 +436,9 @@ scm_i_string_length (SCM str)
 scm_i_is_narrow_string (SCM str)
+  if (IS_SH_STRING (str))
+    str = SH_STRING_STRING (str);
@@ -446,6 +449,9 @@ scm_i_is_narrow_string (SCM str)
 scm_i_try_narrow_string (SCM str)
+  if (IS_SH_STRING (str))
+    str = SH_STRING_STRING (str);
   SET_STRING_STRINGBUF (str, narrow_stringbuf (STRING_STRINGBUF (str)));
   return scm_i_is_narrow_string (str);
@@ -664,6 +670,12 @@ scm_i_string_strcmp (SCM sstr, size_t start_x, const char 
 scm_i_string_set_x (SCM str, size_t p, scm_t_wchar chr)
+  if (IS_SH_STRING (str))
+    {
+      p += STRING_START (str);
+      str = SH_STRING_STRING (str);
+    }
   if (chr > 0xFF && scm_i_is_narrow_string (str))
     SET_STRING_STRINGBUF (str, wide_stringbuf (STRING_STRINGBUF (str)));

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