* fns.c (Flength): Don't overflow int when computing a list length.

Use EMACS_INT, not int, to avoid unwanted truncation on 64-bit hosts.
Check for QUIT every 1024 entries rather than every other entry;
that's faster and is responsive enough.  Report an error instead of
overflowing an integer.

2 files changed, 16 insertions, 10 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index dd61843bc85..5d70c56cc5c 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,11 @@
 2011-06-14  Paul Eggert  <eggert@cs.ucla.edu>
 
+        * fns.c (Flength): Don't overflow int when computing a list length.
+        Use EMACS_INT, not int, to avoid unwanted truncation on 64-bit hosts.
+        Check for QUIT every 1024 entries rather than every other entry;
+        that's faster and is responsive enough.  Report an error instead of
+        overflowing an integer.
+
         * alloc.c: Check that resized vectors' lengths fit in fixnums.
         (header_size, word_size): New constants.
         (allocate_vectorlike): Don't check size overflow here.
diff --git a/src/fns.c b/src/fns.c
index 333a75ac2b2..7d5d1bd5d99 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -110,7 +110,6 @@ To get the number of bytes, use `string-bytes'.  */)
   (register Lisp_Object sequence)
 {
   register Lisp_Object val;
-  register int i;
 
   if (STRINGP (sequence))
     XSETFASTINT (val, SCHARS (sequence));
@@ -124,19 +123,20 @@ To get the number of bytes, use `string-bytes'.  */)
     XSETFASTINT (val, ASIZE (sequence) & PSEUDOVECTOR_SIZE_MASK);
   else if (CONSP (sequence))
     {
-      i = 0;
-      while (CONSP (sequence))
+      EMACS_INT i = 0;
+
+      do
         {
-          sequence = XCDR (sequence);
           ++i;
-
-          if (!CONSP (sequence))
-            break;
-
+          if ((i & ((1 << 10) - 1)) == 0)
+            {
+              if (MOST_POSITIVE_FIXNUM < i)
+                error ("List too long");
+              QUIT;
+            }
           sequence = XCDR (sequence);
-          ++i;
-          QUIT;
         }
+      while (CONSP (sequence));
 
       CHECK_LIST_END (sequence, sequence);