diff --git a/gzwrite.c b/gzwrite.c
index e8defc6..2cc14f8 100644
--- a/gzwrite.c
+++ b/gzwrite.c
@@ -191,7 +191,7 @@ int ZEXPORT gzwrite(file, buf, len)
             n = state->size - strm->avail_in;
             if (n > len)
                 n = len;
-            memcpy(strm->next_in + strm->avail_in, buf, n);
+            memcpy(state->in + strm->avail_in, buf, n);
             strm->avail_in += n;
             state->pos += n;
             buf = (char *)buf + n;
@@ -207,7 +207,7 @@ int ZEXPORT gzwrite(file, buf, len)
 
         /* directly compress user buffer to file */
         strm->avail_in = len;
-        strm->next_in = (voidp)buf;
+        strm->next_in = buf;
         state->pos += len;
         if (gz_comp(state, Z_NO_FLUSH) == -1)
             return 0;
@@ -248,7 +248,7 @@ int ZEXPORT gzputc(file, c)
     if (strm->avail_in < state->size) {
         if (strm->avail_in == 0)
             strm->next_in = state->in;
-        strm->next_in[strm->avail_in++] = c;
+        state->in[strm->avail_in++] = c;
         state->pos++;
         return c;
     }
diff --git a/infback.c b/infback.c
index af3a8c9..4beca98 100644
--- a/infback.c
+++ b/infback.c
@@ -246,7 +246,7 @@ out_func out;
 void FAR *out_desc;
 {
     struct inflate_state FAR *state;
-    unsigned char FAR *next;    /* next input */
+    const unsigned char FAR *next; /* next input */
     unsigned char FAR *put;     /* next output */
     unsigned have, left;        /* available input and output */
     unsigned long hold;         /* bit buffer */
diff --git a/inffast.c b/inffast.c
index 2f1d60b..7acd24a 100644
--- a/inffast.c
+++ b/inffast.c
@@ -69,8 +69,8 @@ z_streamp strm;
 unsigned start;         /* inflate()'s starting value for strm->avail_out */
 {
     struct inflate_state FAR *state;
-    unsigned char FAR *in;      /* local strm->next_in */
-    unsigned char FAR *last;    /* while in < last, enough input available */
+    const unsigned char FAR *in; /* local strm->next_in */
+    const unsigned char FAR *last; /* while in < last, enough input available */
     unsigned char FAR *out;     /* local strm->next_out */
     unsigned char FAR *beg;     /* inflate()'s initial strm->next_out */
     unsigned char FAR *end;     /* while out < end, enough space available */
diff --git a/inflate.c b/inflate.c
index a8431ab..7ce9eaf 100644
--- a/inflate.c
+++ b/inflate.c
@@ -97,7 +97,7 @@ local int updatewindow OF((z_streamp strm, unsigned out));
 #ifdef BUILDFIXED
    void makefixed OF((void));
 #endif
-local unsigned syncsearch OF((unsigned FAR *have, unsigned char FAR *buf,
+local unsigned syncsearch OF((unsigned FAR *have, const unsigned char FAR *buf,
                               unsigned len));
 
 int ZEXPORT inflateReset(strm)
@@ -591,7 +591,7 @@ z_streamp strm;
 int flush;
 {
     struct inflate_state FAR *state;
-    unsigned char FAR *next;    /* next input */
+    const unsigned char FAR *next; /* next input */
     unsigned char FAR *put;     /* next output */
     unsigned have, left;        /* available input and output */
     unsigned long hold;         /* bit buffer */
@@ -1321,7 +1321,7 @@ gz_headerp head;
  */
 local unsigned syncsearch(have, buf, len)
 unsigned FAR *have;
-unsigned char FAR *buf;
+const unsigned char FAR *buf;
 unsigned len;
 {
     unsigned got;
diff --git a/zlib.h b/zlib.h
index bfbba83..361149c 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 */
+    const Bytef *next_in; /* next input byte */
     uInt     avail_in;  /* number of bytes available at next_in */
     uLong    total_in;  /* total nb of input bytes read so far */
 
@@ -967,7 +967,7 @@ ZEXTERN int ZEXPORT inflateBackInit OF((z_streamp strm, int windowBits,
    the version of the header file.
 */
 
-typedef unsigned (*in_func) OF((void FAR *, unsigned char FAR * FAR *));
+typedef unsigned (*in_func) OF((void FAR *, const unsigned char FAR * FAR *));
 typedef int (*out_func) OF((void FAR *, unsigned char FAR *, unsigned));
 
 ZEXTERN int ZEXPORT inflateBack OF((z_streamp strm,