Fix breakage of SRFI-4 C accessors

* libguile/srfi-4.c (DEFINE_SRFI_4_C_FUNCS): Fix bad assumption that
  width was a byte width.  Thanks very much to Barry Fishman for the
  report, and to Daniel Llorens for tracking it down.

* test-suite/standalone/Makefile.am (test_srfi_4_CFLAGS):
* test-suite/standalone/test-srfi-4.c: Add test.

libguile/srfi-4.c

@@ -137,12 +137,13 @@
                                               scm_t_array_handle *h,    \
                                               size_t *lenp, ssize_t *incp) \
   {                                                                     \
+    size_t byte_width = width * sizeof (ctype);                         \
     if (!scm_is_bytevector (uvec)                                       \
-        || (scm_c_bytevector_length (uvec) % width))                    \
+        || (scm_c_bytevector_length (uvec) % byte_width))               \
       scm_wrong_type_arg_msg (NULL, 0, uvec, #tag "vector");            \
     scm_array_get_handle (uvec, h);                                     \
     if (lenp)                                                           \
-      *lenp = scm_c_bytevector_length (uvec) / width;                   \
+      *lenp = scm_c_bytevector_length (uvec) / byte_width;              \
     if (incp)                                                           \
       *incp = 1;                                                        \
     return ((ctype *)h->writable_elements);                             \

test-suite/standalone/Makefile.am

@@ -211,6 +211,13 @@ test_scm_c_bind_keyword_arguments_LDADD = $(LIBGUILE_LDADD)
 check_PROGRAMS += test-scm-c-bind-keyword-arguments
 TESTS += test-scm-c-bind-keyword-arguments
 
+# test-srfi-4
+test_srfi_4_SOURCES = test-srfi-4.c
+test_srfi_4_CFLAGS = ${test_cflags}
+test_srfi_4_LDADD = $(LIBGUILE_LDADD)
+check_PROGRAMS += test-srfi-4
+TESTS += test-srfi-4
+
 if HAVE_SHARED_LIBRARIES
 
 # test-extensions

test-suite/standalone/test-srfi-4.c

@@ -0,0 +1,87 @@
+/* Copyright (C) 2014 Free Software Foundation, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public License
+ * as published by the Free Software Foundation; either version 3 of
+ * the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301 USA
+ */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <libguile.h>
+
+#include <stdio.h>
+#include <assert.h>
+
+static void
+test_writable_elements ()
+{
+  SCM elts = scm_list_4 (scm_from_int (1), scm_from_int (2),
+                         scm_from_int (3), scm_from_int (4));
+
+  {
+    SCM v = scm_u32vector (elts);
+    size_t len;
+    ssize_t inc;
+    scm_t_array_handle h;
+    scm_t_uint32 *elts = scm_u32vector_writable_elements (v, &h, &len, &inc);
+    assert (len == 4);
+    assert (inc == 1);
+    assert (elts[0] == 1);
+    assert (elts[3] == 4);
+    scm_array_handle_release (&h);
+  }
+
+  {
+    SCM v = scm_f32vector (elts);
+    size_t len;
+    ssize_t inc;
+    scm_t_array_handle h;
+    float *elts = scm_f32vector_writable_elements (v, &h, &len, &inc);
+    assert (len == 4);
+    assert (inc == 1);
+    assert (elts[0] == 1.0);
+    assert (elts[3] == 4.0);
+    scm_array_handle_release (&h);
+  }
+
+  {
+    SCM v = scm_c32vector (elts);
+    size_t len;
+    ssize_t inc;
+    scm_t_array_handle h;
+    float *elts = scm_c32vector_writable_elements (v, &h, &len, &inc);
+    assert (len == 4);
+    assert (inc == 1);
+    assert (elts[0] == 1.0);
+    assert (elts[1] == 0.0);
+    assert (elts[6] == 4.0);
+    assert (elts[7] == 0.0);
+    scm_array_handle_release (&h);
+  }
+}
+
+static void
+tests (void *data, int argc, char **argv)
+{
+  test_writable_elements ();
+}
+
+int
+main (int argc, char *argv[])
+{
+  scm_boot_guile (argc, argv, tests, NULL);
+  return 0;
+}