diff --git a/xs.h b/xs.h
index 89f2ca9..45abfae 100644
--- a/xs.h
+++ b/xs.h
@@ -41,7 +41,7 @@ void _xs_destroy(char **var);
 #define xs_debug() raise(SIGTRAP)
 xstype xs_type(const char *data);
 int xs_size(const char *data);
-int xs_is_null(char *data);
+int xs_is_null(const char *data);
 d_char *xs_dup(const char *data);
 void *xs_realloc(void *ptr, size_t size);
 d_char *xs_expand(d_char *data, int offset, int size);
@@ -54,9 +54,9 @@ d_char *xs_str_new(const char *str);
 d_char *xs_replace_i(d_char *str, const char *sfrom, const char *sto);
 #define xs_replace(str, sfrom, sto) xs_replace_i(xs_dup(str), sfrom, sto)
 d_char *xs_fmt(const char *fmt, ...);
-int xs_str_in(char *haystack, char *needle);
-int xs_startswith(char *str, char *prefix);
-int xs_endswith(char *str, char *postfix);
+int xs_str_in(const char *haystack, const char *needle);
+int xs_startswith(const char *str, const char *prefix);
+int xs_endswith(const char *str, const char *postfix);
 d_char *xs_crop(d_char *str, int start, int end);
 d_char *xs_strip(d_char *str);
 d_char *xs_tolower(d_char *str);
@@ -79,8 +79,8 @@ d_char *xs_dict_del(d_char *dict, const char *key);
 d_char *xs_dict_set(d_char *dict, const char *key, const char *data);
 d_char *xs_val_new(xstype t);
 d_char *xs_number_new(double f);
-double xs_number_get(char *v);
-char *xs_number_str(char *v);
+double xs_number_get(const char *v);
+const char *xs_number_str(const char *v);
 
 extern int _xs_debug;
 
@@ -189,7 +189,7 @@ int xs_size(const char *data)
 }
 
 
-int xs_is_null(char *data)
+int xs_is_null(const char *data)
 /* checks for null */
 {
     return !!(data == NULL || xs_type(data) == XSTYPE_NULL);
@@ -323,7 +323,7 @@ d_char *xs_fmt(const char *fmt, ...)
 }
 
 
-int xs_str_in(char *haystack, char *needle)
+int xs_str_in(const char *haystack, const char *needle)
 /* finds needle in haystack and returns the offset or -1 */
 {
     char *s;
@@ -336,14 +336,14 @@ int xs_str_in(char *haystack, char *needle)
 }
 
 
-int xs_startswith(char *str, char *prefix)
+int xs_startswith(const char *str, const char *prefix)
 /* returns true if str starts with prefix */
 {
     return !!(xs_str_in(str, prefix) == 0);
 }
 
 
-int xs_endswith(char *str, char *postfix)
+int xs_endswith(const char *str, const char *postfix)
 /* returns true if str ends with postfix */
 {
     int ssz = strlen(str);
@@ -732,7 +732,7 @@ d_char *xs_number_new(double f)
 }
 
 
-double xs_number_get(char *v)
+double xs_number_get(const char *v)
 /* gets the number as a double */
 {
     double f = 0.0;
@@ -744,10 +744,10 @@ double xs_number_get(char *v)
 }
 
 
-char *xs_number_str(char *v)
+const char *xs_number_str(const char *v)
 /* gets the number as a string */
 {
-    char *p = NULL;
+    const char *p = NULL;
 
     if (v[0] == XSTYPE_NUMBER)
         p = &v[1];
diff --git a/xs_mime.h b/xs_mime.h
index 699ab39..072a98a 100644
--- a/xs_mime.h
+++ b/xs_mime.h
@@ -4,7 +4,7 @@
 
 #define _XS_MIME
 
-char *xs_mime_by_ext(char *file);
+char *xs_mime_by_ext(const char *file);
 
 #ifdef XS_IMPLEMENTATION
 
@@ -27,7 +27,7 @@ struct _mime_info {
 };
 
 
-char *xs_mime_by_ext(char *file)
+char *xs_mime_by_ext(const char *file)
 /* returns the MIME type by file extension */
 {
     struct _mime_info *mi = mime_info;
diff --git a/xs_openssl.h b/xs_openssl.h
index 9b0f71e..0652637 100644
--- a/xs_openssl.h
+++ b/xs_openssl.h
@@ -9,10 +9,10 @@ d_char *xs_sha1_hex(const void *input, int size);
 d_char *xs_sha256_hex(const void *input, int size);
 d_char *xs_sha256_base64(const void *input, int size);
 d_char *xs_rsa_genkey(int bits);
-d_char *xs_rsa_sign(char *secret, char *mem, int size);
-int xs_rsa_verify(char *pubkey, char *mem, int size, char *b64sig);
-d_char *xs_evp_sign(char *secret, char *mem, int size);
-int xs_evp_verify(char *pubkey, char *mem, int size, char *b64sig);
+d_char *xs_rsa_sign(const char *secret, const char *mem, int size);
+int xs_rsa_verify(const char *pubkey, const char *mem, int size, const char *b64sig);
+d_char *xs_evp_sign(const char *secret, const char *mem, int size);
+int xs_evp_verify(const char *pubkey, const char *mem, int size, const char *b64sig);
 
 
 #ifdef XS_IMPLEMENTATION
@@ -119,7 +119,7 @@ d_char *xs_rsa_genkey(int bits)
 }
 
 
-d_char *xs_rsa_sign(char *secret, char *mem, int size)
+d_char *xs_rsa_sign(const char *secret, const char *mem, int size)
 /* signs a memory block (secret is in PEM format) */
 {
     d_char *signature = NULL;
@@ -146,7 +146,7 @@ d_char *xs_rsa_sign(char *secret, char *mem, int size)
 }
 
 
-int xs_rsa_verify(char *pubkey, char *mem, int size, char *b64sig)
+int xs_rsa_verify(const char *pubkey, const char *mem, int size, const char *b64sig)
 /* verifies a base64 block, returns non-zero on ok */
 {
     int r = 0;
@@ -176,7 +176,7 @@ int xs_rsa_verify(char *pubkey, char *mem, int size, char *b64sig)
 }
 
 
-d_char *xs_evp_sign(char *secret, char *mem, int size)
+d_char *xs_evp_sign(const char *secret, const char *mem, int size)
 /* signs a memory block (secret is in PEM format) */
 {
     d_char *signature = NULL;
@@ -217,7 +217,7 @@ d_char *xs_evp_sign(char *secret, char *mem, int size)
 }
 
 
-int xs_evp_verify(char *pubkey, char *mem, int size, char *b64sig)
+int xs_evp_verify(const char *pubkey, const char *mem, int size, const char *b64sig)
 /* verifies a base64 block, returns non-zero on ok */
 {
     int r = 0;
diff --git a/xs_set.h b/xs_set.h
index 7d7a762..2beb454 100644
--- a/xs_set.h
+++ b/xs_set.h
@@ -13,7 +13,7 @@ typedef struct _xs_set {
 
 xs_set *xs_set_new(int elems);
 void xs_set_free(xs_set *s);
-int xs_set_add(xs_set *s, char *data);
+int xs_set_add(xs_set *s, const char *data);
 
 
 #ifdef XS_IMPLEMENTATION
@@ -40,7 +40,7 @@ void xs_set_free(xs_set *s)
 }
 
 
-unsigned int _xs_set_hash(char *data, int size)
+unsigned int _xs_set_hash(const char *data, int size)
 {
     unsigned int hash = 0x666;
     int n;
@@ -54,7 +54,7 @@ unsigned int _xs_set_hash(char *data, int size)
 }
 
 
-int xs_set_add(xs_set *s, char *data)
+int xs_set_add(xs_set *s, const char *data)
 /* adds the data to the set */
 /* returns: 1 if added, 0 if already there, -1 if it's full */
 {
diff --git a/xs_socket.h b/xs_socket.h
index a3fe6a4..d2853a2 100644
--- a/xs_socket.h
+++ b/xs_socket.h
@@ -5,7 +5,7 @@
 #define _XS_SOCKET_H
 
 int xs_socket_timeout(int s, double rto, double sto);
-int xs_socket_server(char *addr, int port);
+int xs_socket_server(const char *addr, int port);
 FILE *xs_socket_accept(int rs);
 
 
@@ -40,7 +40,7 @@ int xs_socket_timeout(int s, double rto, double sto)
 }
 
 
-int xs_socket_server(char *addr, int port)
+int xs_socket_server(const char *addr, int port)
 /* opens a server socket */
 {
     int rs = -1;
diff --git a/xs_version.h b/xs_version.h
new file mode 100644
index 0000000..ca721ea
--- /dev/null
+++ b/xs_version.h
@@ -0,0 +1 @@
+/* b6512569814e4b409191a24193abe54d00427df2 */