diff --git a/src/tf.c b/src/tf.c
index ddacf70a3b..3d695019ce 100644
--- a/src/tf.c
+++ b/src/tf.c
@@ -1487,7 +1487,7 @@ tf_func_strchr(ddb_tf_context_t *ctx, int argc, const uint16_t *arglens, const c
     TF_EVAL_CHECK(len, ctx, args, arglens[0], out, outlen, fail_on_undef);
 
     int32_t charpos;
-    char *pos = u8_strchr(out, needle, &charpos);
+    const char *pos = u8_strchr(out, needle, &charpos);
 
     if (!pos) {
         // 0 is used to indicate not found
@@ -1517,7 +1517,7 @@ tf_func_strrchr(ddb_tf_context_t *ctx, int argc, const uint16_t *arglens, const
     uint32_t needle = u8_nextchar(str, &dummy);
 
     int32_t acc = 0, charpos;
-    char *pos = out;
+    const char *pos = out;
     do {
         pos = u8_strchr(pos, needle, &charpos);
         if (pos) {
diff --git a/src/utf8.c b/src/utf8.c
index 2657ad29f8..ca4ada3ed1 100644
--- a/src/utf8.c
+++ b/src/utf8.c
@@ -117,7 +117,7 @@ int u8_toucs(uint32_t *dest, int32_t sz, const char *src, int32_t srcsz)
    the NUL as well.
    the destination string will never be bigger than the source string.
 */
-int u8_toutf8(char *dest, int32_t sz, uint32_t *src, int32_t srcsz)
+int u8_toutf8(char *dest, int32_t sz, const uint32_t *src, int32_t srcsz)
 {
     uint32_t ch;
     int32_t i = 0;
@@ -199,7 +199,7 @@ int u8_offset(const char *str, int32_t charnum)
 }
 
 /* byte offset => charnum */
-int u8_charnum(char *s, int32_t offset)
+int u8_charnum(const char *s, int32_t offset)
 {
     int32_t charnum = 0, offs=0;
 
@@ -212,7 +212,7 @@ int u8_charnum(char *s, int32_t offset)
 }
 
 /* number of characters, not including a null terminator */
-int u8_strlen(char *s)
+int u8_strlen(const char *s)
 {
     int32_t count = 0;
     int32_t i = 0;
@@ -448,7 +448,7 @@ int u8_escape(char *buf, int32_t sz, const char *src, int32_t escape_quotes)
     return c;
 }
 
-char *u8_strchr(char *s, uint32_t ch, int32_t *charn)
+const char *u8_strchr(const char *s, uint32_t ch, int32_t *charn)
 {
     int32_t i = 0, lasti=0;
     uint32_t c;
@@ -465,7 +465,7 @@ char *u8_strchr(char *s, uint32_t ch, int32_t *charn)
     return NULL;
 }
 
-char *u8_memchr(char *s, uint32_t ch, size_t sz, int32_t *charn)
+const char *u8_memchr(const char *s, uint32_t ch, size_t sz, int32_t *charn)
 {
     int32_t i = 0, lasti=0;
     uint32_t c;
@@ -490,7 +490,7 @@ char *u8_memchr(char *s, uint32_t ch, size_t sz, int32_t *charn)
     return NULL;
 }
 
-int u8_is_locale_utf8(char *locale)
+int u8_is_locale_utf8(const char *locale)
 {
     /* this code based on libutf8 */
     const char* cp = locale;
@@ -509,7 +509,7 @@ int u8_is_locale_utf8(char *locale)
     return 0;
 }
 
-int u8_vprintf(char *fmt, va_list ap)
+int u8_vprintf(const char *fmt, va_list ap)
 {
     int32_t cnt, sz=0;
     char *buf;
@@ -530,7 +530,7 @@ int u8_vprintf(char *fmt, va_list ap)
     return cnt;
 }
 
-int u8_printf(char *fmt, ...)
+int u8_printf(const char *fmt, ...)
 {
     int32_t cnt;
     va_list args;
diff --git a/src/utf8.h b/src/utf8.h
index 984a766711..23d9d223e4 100644
--- a/src/utf8.h
+++ b/src/utf8.h
@@ -44,7 +44,7 @@
 int u8_toucs(uint32_t *dest, int32_t sz, const char *src, int32_t srcsz);
 
 /* the opposite conversion */
-int u8_toutf8(char *dest, int32_t sz, uint32_t *src, int32_t srcsz);
+int u8_toutf8(char *dest, int32_t sz, const uint32_t *src, int32_t srcsz);
 
 /* single character to UTF-8 */
 int u8_wc_toutf8(char *dest, uint32_t ch);
@@ -53,7 +53,7 @@ int u8_wc_toutf8(char *dest, uint32_t ch);
 int u8_offset(const char *str, int32_t charnum);
 
 /* byte offset to character number */
-int u8_charnum(char *s, int32_t offset);
+int u8_charnum(const char *s, int32_t offset);
 
 /* return next character, updating an index variable */
 uint32_t u8_nextchar(const char *s, int32_t *i);
@@ -100,22 +100,22 @@ int hex_digit(char c);
 
 /* return a pointer to the first occurrence of ch in s, or NULL if not
    found. character index of found character returned in *charn. */
-char *u8_strchr(char *s, uint32_t ch, int32_t *charn);
+const char *u8_strchr(const char *s, uint32_t ch, int32_t *charn);
 
 /* same as the above, but searches a buffer of a given size instead of
    a NUL-terminated string. */
-char *u8_memchr(char *s, uint32_t ch, size_t sz, int32_t *charn);
+const char *u8_memchr(const char *s, uint32_t ch, size_t sz, int32_t *charn);
 
 /* count the number of characters in a UTF-8 string */
-int u8_strlen(char *s);
+int u8_strlen(const char *s);
 
-int u8_is_locale_utf8(char *locale);
+int u8_is_locale_utf8(const char *locale);
 
 /* printf where the format string and arguments may be in UTF-8.
    you can avoid this function and just use ordinary printf() if the current
    locale is UTF-8. */
-int u8_vprintf(char *fmt, va_list ap);
-int u8_printf(char *fmt, ...);
+int u8_vprintf(const char *fmt, va_list ap);
+int u8_printf(const char *fmt, ...);
 
 // validate utf8 string
 // returns 1 if valid, 0 otherwise