diff --git a/srfi/srfi-13.c b/srfi/srfi-13.c
index 445178634..7de4fdfb7 100644
--- a/srfi/srfi-13.c
+++ b/srfi/srfi-13.c
@@ -61,7 +61,7 @@ SCM_DEFINE (scm_string_any, "string-any-c-code", 2, 2, 0,
 	    "successful application of @var{pred}.")
 #define FUNC_NAME s_scm_string_any
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
   SCM res;
 
@@ -108,7 +108,7 @@ SCM_DEFINE (scm_string_every, "string-every-c-code", 2, 2, 0,
 	    "application of @var{pred} to the last character of @var{s}.")
 #define FUNC_NAME s_scm_string_every
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
   SCM res;
 
@@ -189,7 +189,7 @@ SCM_DEFINE (scm_string_to_listS, "string->list", 1, 2, 0,
 	    "Convert the string @var{str} into a list of characters.")
 #define FUNC_NAME s_scm_string_to_listS
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
   SCM result = SCM_EOL;
 
@@ -645,7 +645,7 @@ SCM_DEFINE (scm_string_trim, "string-trim", 1, 3, 0,
 	    "trimmed.")
 #define FUNC_NAME s_scm_string_trim
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s, cstr,
@@ -720,7 +720,7 @@ SCM_DEFINE (scm_string_trim_right, "string-trim-right", 1, 3, 0,
 	    "trimmed.")
 #define FUNC_NAME s_scm_string_trim_right
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s, cstr,
@@ -795,7 +795,7 @@ SCM_DEFINE (scm_string_trim_both, "string-trim-both", 1, 3, 0,
 	    "trimmed.")
 #define FUNC_NAME s_scm_string_trim_both
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s, cstr,
@@ -906,7 +906,7 @@ SCM_DEFINE (scm_string_compare, "string-compare", 5, 4, 0,
 	    "@var{i} is the first position that does not match.")
 #define FUNC_NAME s_scm_string_compare
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -949,7 +949,7 @@ SCM_DEFINE (scm_string_compare_ci, "string-compare-ci", 5, 4, 0,
 	    "character comparison is done case-insensitively.")
 #define FUNC_NAME s_scm_string_compare_ci
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -987,7 +987,7 @@ SCM_DEFINE (scm_string_eq, "string=", 2, 4, 0,
 	    "value otherwise.")
 #define FUNC_NAME s_scm_string_eq
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1022,7 +1022,7 @@ SCM_DEFINE (scm_string_neq, "string<>", 2, 4, 0,
 	    "value otherwise.")
 #define FUNC_NAME s_scm_string_neq
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1057,7 +1057,7 @@ SCM_DEFINE (scm_string_lt, "string<", 2, 4, 0,
 	    "true value otherwise.")
 #define FUNC_NAME s_scm_string_lt
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1092,7 +1092,7 @@ SCM_DEFINE (scm_string_gt, "string>", 2, 4, 0,
 	    "true value otherwise.")
 #define FUNC_NAME s_scm_string_gt
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1127,7 +1127,7 @@ SCM_DEFINE (scm_string_le, "string<=", 2, 4, 0,
 	    "value otherwise.")
 #define FUNC_NAME s_scm_string_le
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1162,7 +1162,7 @@ SCM_DEFINE (scm_string_ge, "string>=", 2, 4, 0,
 	    "otherwise.")
 #define FUNC_NAME s_scm_string_ge
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1198,7 +1198,7 @@ SCM_DEFINE (scm_string_ci_eq, "string-ci=", 2, 4, 0,
 	    "case-insensitively.")
 #define FUNC_NAME s_scm_string_ci_eq
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1234,7 +1234,7 @@ SCM_DEFINE (scm_string_ci_neq, "string-ci<>", 2, 4, 0,
 	    "case-insensitively.")
 #define FUNC_NAME s_scm_string_ci_neq
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1270,7 +1270,7 @@ SCM_DEFINE (scm_string_ci_lt, "string-ci<", 2, 4, 0,
 	    "case-insensitively.")
 #define FUNC_NAME s_scm_string_ci_lt
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1306,7 +1306,7 @@ SCM_DEFINE (scm_string_ci_gt, "string-ci>", 2, 4, 0,
 	    "case-insensitively.")
 #define FUNC_NAME s_scm_string_ci_gt
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1342,7 +1342,7 @@ SCM_DEFINE (scm_string_ci_le, "string-ci<=", 2, 4, 0,
 	    "case-insensitively.")
 #define FUNC_NAME s_scm_string_ci_le
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1378,7 +1378,7 @@ SCM_DEFINE (scm_string_ci_ge, "string-ci>=", 2, 4, 0,
 	    "case-insensitively.")
 #define FUNC_NAME s_scm_string_ci_ge
 {
-  char * cstr1, * cstr2;
+  unsigned char * cstr1, * cstr2;
   int cstart1, cend1, cstart2, cend2;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s1, cstr1,
@@ -1658,7 +1658,7 @@ SCM_DEFINE (scm_string_indexS, "string-index", 2, 2, 0,
 	    "@end itemize")
 #define FUNC_NAME s_scm_string_indexS
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s, cstr,
@@ -1717,7 +1717,7 @@ SCM_DEFINE (scm_string_index_right, "string-index-right", 2, 2, 0,
 	    "@end itemize")
 #define FUNC_NAME s_scm_string_index_right
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s, cstr,
@@ -1777,7 +1777,7 @@ SCM_DEFINE (scm_string_skip, "string-skip", 2, 2, 0,
 	    "@end itemize")
 #define FUNC_NAME s_scm_string_skip
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s, cstr,
@@ -1837,7 +1837,7 @@ SCM_DEFINE (scm_string_skip_right, "string-skip-right", 2, 2, 0,
 	    "@end itemize")
 #define FUNC_NAME s_scm_string_skip_right
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
 
   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, s, cstr,
@@ -1896,7 +1896,7 @@ SCM_DEFINE (scm_string_count, "string-count", 2, 2, 0,
 	    "@end itemize")
 #define FUNC_NAME s_scm_string_count
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
   int count = 0;
 
@@ -2442,7 +2442,7 @@ SCM_DEFINE (scm_string_map, "string-map", 2, 2, 0,
 	    "string elements is not specified.")
 #define FUNC_NAME s_scm_string_map
 {
-  char * cstr, *p;
+  unsigned char * cstr, *p;
   int cstart, cend;
   SCM result;
 
@@ -2473,7 +2473,7 @@ SCM_DEFINE (scm_string_map_x, "string-map!", 2, 2, 0,
 	    "modified in-place, the return value is not specified.")
 #define FUNC_NAME s_scm_string_map_x
 {
-  char * cstr, *p;
+  unsigned char * cstr, *p;
   int cstart, cend;
 
   SCM_VALIDATE_PROC (1, proc);
@@ -2502,7 +2502,7 @@ SCM_DEFINE (scm_string_fold, "string-fold", 3, 2, 0,
 	    "result of @var{kons}' application.")
 #define FUNC_NAME s_scm_string_fold
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
   SCM result;
 
@@ -2529,7 +2529,7 @@ SCM_DEFINE (scm_string_fold_right, "string-fold-right", 3, 2, 0,
 	    "result of @var{kons}' application.")
 #define FUNC_NAME s_scm_string_fold_right
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
   SCM result;
 
@@ -2678,7 +2678,7 @@ SCM_DEFINE (scm_string_for_each, "string-for-each", 2, 2, 0,
 	    "return value is not specified.")
 #define FUNC_NAME s_scm_string_for_each
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
 
   SCM_VALIDATE_PROC (1, proc);
@@ -2914,7 +2914,7 @@ SCM_DEFINE (scm_string_filter, "string-filter", 2, 2, 0,
 	    "character set, it is tested for membership.")
 #define FUNC_NAME s_scm_string_filter
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
   SCM result;
   int idx;
@@ -2980,7 +2980,7 @@ SCM_DEFINE (scm_string_delete, "string-delete", 2, 2, 0,
 	    "character set, it is tested for membership.")
 #define FUNC_NAME s_scm_string_delete
 {
-  char * cstr;
+  unsigned char * cstr;
   int cstart, cend;
   SCM result;
   int idx;