diff --git a/zconf.h b/zconf.h
index df7ff61..fd10c6a 100644
--- a/zconf.h
+++ b/zconf.h
@@ -214,6 +214,12 @@
 #  endif
 #endif
 
+#if defined(ZLIB_CONST) && !defined(z_const)
+#  define z_const const
+#else
+#  define z_const
+#endif
+
 /* Some Mac compilers merge all .h files incorrectly: */
 #if defined(__MWERKS__)||defined(applec)||defined(THINK_C)||defined(__SC__)
 #  define NO_DUMMY_DECL
diff --git a/zconf.h.cmakein b/zconf.h.cmakein
index 8450547..0b880ad 100644
--- a/zconf.h.cmakein
+++ b/zconf.h.cmakein
@@ -216,6 +216,12 @@
 #  endif
 #endif
 
+#if defined(ZLIB_CONST) && !defined(z_const)
+#  define z_const const
+#else
+#  define z_const
+#endif
+
 /* Some Mac compilers merge all .h files incorrectly: */
 #if defined(__MWERKS__)||defined(applec)||defined(THINK_C)||defined(__SC__)
 #  define NO_DUMMY_DECL
diff --git a/zconf.h.in b/zconf.h.in
index df7ff61..fd10c6a 100644
--- a/zconf.h.in
+++ b/zconf.h.in
@@ -214,6 +214,12 @@
 #  endif
 #endif
 
+#if defined(ZLIB_CONST) && !defined(z_const)
+#  define z_const const
+#else
+#  define z_const
+#endif
+
 /* Some Mac compilers merge all .h files incorrectly: */
 #if defined(__MWERKS__)||defined(applec)||defined(THINK_C)||defined(__SC__)
 #  define NO_DUMMY_DECL
diff --git a/zlib.h b/zlib.h
index 99cd058..14a925b 100644
--- a/zlib.h
+++ b/zlib.h
@@ -83,7 +83,7 @@ typedef void   (*free_func)  OF((voidpf opaque, voidpf address));
 struct internal_state;
 
 typedef struct z_stream_s {
-    Bytef    *next_in;  /* next input byte */
+    z_const Bytef *next_in;     /* next input byte */
     uInt     avail_in;  /* number of bytes available at next_in */
     uLong    total_in;  /* total number of input bytes read so far */
 
@@ -91,7 +91,7 @@ typedef struct z_stream_s {
     uInt     avail_out; /* remaining free space at next_out */
     uLong    total_out; /* total number of bytes output so far */
 
-    char     *msg;      /* last error message, NULL if no error */
+    z_const char *msg;  /* last error message, NULL if no error */
     struct internal_state FAR *state; /* not visible by applications */
 
     alloc_func zalloc;  /* used to allocate the internal state */