diff --git a/compress.c b/compress.c
index ea4dfbe..6e97626 100644
--- a/compress.c
+++ b/compress.c
@@ -29,7 +29,7 @@ int ZEXPORT compress2 (dest, destLen, source, sourceLen, level)
     z_stream stream;
     int err;
 
-    stream.next_in = (Bytef*)source;
+    stream.next_in = (z_const Bytef *)source;
     stream.avail_in = (uInt)sourceLen;
 #ifdef MAXSEG_64K
     /* Check for source > 64K on 16-bit machine: */
diff --git a/configure b/configure
index a69d7bf..bc39fd7 100755
--- a/configure
+++ b/configure
@@ -70,6 +70,7 @@ shared=1
 solo=0
 cover=0
 zprefix=0
+zconst=0
 build64=0
 gcc=0
 old_cc="$CC"
@@ -96,7 +97,7 @@ do
 case "$1" in
     -h* | --help)
       echo 'usage:' | tee -a configure.log
-      echo '  configure [--zprefix] [--prefix=PREFIX]  [--eprefix=EXPREFIX]' | tee -a configure.log
+      echo '  configure [--const] [--zprefix] [--prefix=PREFIX]  [--eprefix=EXPREFIX]' | tee -a configure.log
       echo '    [--static] [--64] [--libdir=LIBDIR] [--sharedlibdir=LIBDIR]' | tee -a configure.log
       echo '    [--includedir=INCLUDEDIR] [--archs="-arch i386 -arch x86_64"]' | tee -a configure.log
         exit 0 ;;
@@ -119,6 +120,7 @@ case "$1" in
     -a*=* | --archs=*) ARCHS=`echo $1 | sed 's/.*=//'`; shift ;;
     --sysconfdir=*) echo "ignored option: --sysconfdir" | tee -a configure.log; shift ;;
     --localstatedir=*) echo "ignored option: --localstatedir" | tee -a configure.log; shift ;;
+    -c* | --const) zconst=1; shift ;;
     *)
       echo "unknown option: $1" | tee -a configure.log
       echo "$0 --help for help" | tee -a configure.log
@@ -171,7 +173,11 @@ if test "$gcc" -eq 1 && ($cc -c $test.c) >> configure.log 2>&1; then
     SFLAGS="${SFLAGS} -m64"
   fi
   if test "${ZLIBGCCWARN}" = "YES"; then
-    CFLAGS="${CFLAGS} -Wall -Wextra -pedantic"
+    if test "$zconst" -eq 1; then
+      CFLAGS="${CFLAGS} -Wall -Wextra -Wcast-qual -pedantic -DZLIB_CONST"
+    else
+      CFLAGS="${CFLAGS} -Wall -Wextra -pedantic"
+    fi
   fi
   if test -z "$uname"; then
     uname=`(uname -s || echo unknown) 2>/dev/null`
diff --git a/contrib/infback9/infback9.c b/contrib/infback9/infback9.c
index 3691d4a..05fb3e3 100644
--- a/contrib/infback9/infback9.c
+++ b/contrib/infback9/infback9.c
@@ -222,7 +222,7 @@ out_func out;
 void FAR *out_desc;
 {
     struct inflate_state FAR *state;
-    unsigned char FAR *next;    /* next input */
+    z_const unsigned char FAR *next;    /* next input */
     unsigned char FAR *put;     /* next output */
     unsigned have;              /* available input */
     unsigned long left;         /* available output */
diff --git a/deflate.c b/deflate.c
index 4792345..4aa5afb 100644
--- a/deflate.c
+++ b/deflate.c
@@ -305,7 +305,7 @@ int ZEXPORT deflateInit2_(strm, level, method, windowBits, memLevel, strategy,
     if (s->window == Z_NULL || s->prev == Z_NULL || s->head == Z_NULL ||
         s->pending_buf == Z_NULL) {
         s->status = FINISH_STATE;
-        strm->msg = (char*)ERR_MSG(Z_MEM_ERROR);
+        strm->msg = ERR_MSG(Z_MEM_ERROR);
         deflateEnd (strm);
         return Z_MEM_ERROR;
     }
@@ -329,7 +329,7 @@ int ZEXPORT deflateSetDictionary (strm, dictionary, dictLength)
     uInt str, n;
     int wrap;
     unsigned avail;
-    unsigned char *next;
+    z_const unsigned char *next;
 
     if (strm == Z_NULL || strm->state == Z_NULL || dictionary == Z_NULL)
         return Z_STREAM_ERROR;
@@ -359,7 +359,7 @@ int ZEXPORT deflateSetDictionary (strm, dictionary, dictLength)
     avail = strm->avail_in;
     next = strm->next_in;
     strm->avail_in = dictLength;
-    strm->next_in = (Bytef *)dictionary;
+    strm->next_in = (z_const Bytef *)dictionary;
     fill_window(s);
     while (s->lookahead >= MIN_MATCH) {
         str = s->strstart;
diff --git a/gzlib.c b/gzlib.c
index c10fabb..e4ca576 100644
--- a/gzlib.c
+++ b/gzlib.c
@@ -541,7 +541,8 @@ const char * ZEXPORT gzerror(file, errnum)
     /* return error information */
     if (errnum != NULL)
         *errnum = state->err;
-    return state->msg == NULL ? "" : state->msg;
+    return state->err == Z_MEM_ERROR ? "out of memory" :
+                                       (state->msg == NULL ? "" : state->msg);
 }
 
 /* -- see zlib.h -- */
@@ -592,16 +593,13 @@ void ZLIB_INTERNAL gz_error(state, err, msg)
     if (msg == NULL)
         return;
 
-    /* for an out of memory error, save as static string */
-    if (err == Z_MEM_ERROR) {
-        state->msg = (char *)msg;
+    /* for an out of memory error, return literal string when requested */
+    if (err == Z_MEM_ERROR)
         return;
-    }
 
     /* construct error message with path */
     if ((state->msg = malloc(strlen(state->path) + strlen(msg) + 3)) == NULL) {
         state->err = Z_MEM_ERROR;
-        state->msg = (char *)"out of memory";
         return;
     }
 #if !defined(NO_snprintf) && !defined(NO_vsnprintf)
diff --git a/gzread.c b/gzread.c
index 8ace830..52985c9 100644
--- a/gzread.c
+++ b/gzread.c
@@ -58,7 +58,8 @@ local int gz_avail(state)
         return -1;
     if (state->eof == 0) {
         if (strm->avail_in) {       /* copy what's there to the start */
-            unsigned char *p = state->in, *q = strm->next_in;
+            unsigned char *p = state->in;
+            unsigned const char *q = strm->next_in;
             unsigned n = strm->avail_in;
             do {
                 *p++ = *q++;
diff --git a/gzwrite.c b/gzwrite.c
index 27cb342..bf57913 100644
--- a/gzwrite.c
+++ b/gzwrite.c
@@ -168,7 +168,6 @@ int ZEXPORT gzwrite(file, buf, len)
     unsigned len;
 {
     unsigned put = len;
-    unsigned n;
     gz_statep state;
     z_streamp strm;
 
@@ -208,16 +207,19 @@ int ZEXPORT gzwrite(file, buf, len)
     if (len < state->size) {
         /* copy to input buffer, compress when full */
         do {
+            unsigned have, copy;
+
             if (strm->avail_in == 0)
                 strm->next_in = state->in;
-            n = state->size - strm->avail_in;
-            if (n > len)
-                n = len;
-            memcpy(strm->next_in + strm->avail_in, buf, n);
-            strm->avail_in += n;
-            state->x.pos += n;
-            buf = (char *)buf + n;
-            len -= n;
+            have = strm->next_in + strm->avail_in - state->in;
+            copy = state->size - have;
+            if (copy > len)
+                copy = len;
+            memcpy(state->in + have, buf, copy);
+            strm->avail_in += copy;
+            state->x.pos += copy;
+            buf = (const char *)buf + copy;
+            len -= copy;
             if (len && gz_comp(state, Z_NO_FLUSH) == -1)
                 return 0;
         } while (len);
@@ -229,7 +231,7 @@ int ZEXPORT gzwrite(file, buf, len)
 
         /* directly compress user buffer to file */
         strm->avail_in = len;
-        strm->next_in = (voidp)buf;
+        strm->next_in = (z_const Bytef *)buf;
         state->x.pos += len;
         if (gz_comp(state, Z_NO_FLUSH) == -1)
             return 0;
@@ -244,6 +246,7 @@ int ZEXPORT gzputc(file, c)
     gzFile file;
     int c;
 {
+    unsigned have;
     unsigned char buf[1];
     gz_statep state;
     z_streamp strm;
@@ -267,10 +270,12 @@ int ZEXPORT gzputc(file, c)
 
     /* try writing to input buffer for speed (state->size == 0 if buffer not
        initialized) */
-    if (strm->avail_in < state->size) {
-        if (strm->avail_in == 0)
-            strm->next_in = state->in;
-        strm->next_in[strm->avail_in++] = c;
+    if (strm->avail_in == 0)
+        strm->next_in = state->in;
+    have = strm->next_in + strm->avail_in - state->in;
+    if (have < state->size) {
+        state->in[have] = c;
+        strm->avail_in++;
         state->x.pos++;
         return c & 0xff;
     }
diff --git a/infback.c b/infback.c
index 981aff1..f3833c2 100644
--- a/infback.c
+++ b/infback.c
@@ -255,7 +255,7 @@ out_func out;
 void FAR *out_desc;
 {
     struct inflate_state FAR *state;
-    unsigned char FAR *next;    /* next input */
+    z_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..0d78f23 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 */
+    z_const unsigned char FAR *in;      /* local strm->next_in */
+    z_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 d1efdb8..870f89b 100644
--- a/inflate.c
+++ b/inflate.c
@@ -93,11 +93,12 @@
 
 /* function prototypes */
 local void fixedtables OF((struct inflate_state FAR *state));
-local int updatewindow OF((z_streamp strm, unsigned out));
+local int updatewindow OF((z_streamp strm, const unsigned char FAR *end,
+                           unsigned copy));
 #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 inflateResetKeep(strm)
@@ -375,12 +376,13 @@ void makefixed()
    output will fall in the output data, making match copies simpler and faster.
    The advantage may be dependent on the size of the processor's data caches.
  */
-local int updatewindow(strm, out)
+local int updatewindow(strm, end, copy)
 z_streamp strm;
-unsigned out;
+const Bytef *end;
+unsigned copy;
 {
     struct inflate_state FAR *state;
-    unsigned copy, dist;
+    unsigned dist;
 
     state = (struct inflate_state FAR *)strm->state;
 
@@ -400,19 +402,18 @@ unsigned out;
     }
 
     /* copy state->wsize or less output bytes into the circular window */
-    copy = out - strm->avail_out;
     if (copy >= state->wsize) {
-        zmemcpy(state->window, strm->next_out - state->wsize, state->wsize);
+        zmemcpy(state->window, end - state->wsize, state->wsize);
         state->wnext = 0;
         state->whave = state->wsize;
     }
     else {
         dist = state->wsize - state->wnext;
         if (dist > copy) dist = copy;
-        zmemcpy(state->window + state->wnext, strm->next_out - copy, dist);
+        zmemcpy(state->window + state->wnext, end - copy, dist);
         copy -= dist;
         if (copy) {
-            zmemcpy(state->window, strm->next_out - copy, copy);
+            zmemcpy(state->window, end - copy, copy);
             state->wnext = copy;
             state->whave = state->wsize;
         }
@@ -606,7 +607,7 @@ z_streamp strm;
 int flush;
 {
     struct inflate_state FAR *state;
-    unsigned char FAR *next;    /* next input */
+    z_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 */
@@ -920,7 +921,7 @@ int flush;
             while (state->have < 19)
                 state->lens[order[state->have++]] = 0;
             state->next = state->codes;
-            state->lencode = (code const FAR *)(state->next);
+            state->lencode = (const code FAR *)(state->next);
             state->lenbits = 7;
             ret = inflate_table(CODES, state->lens, 19, &(state->next),
                                 &(state->lenbits), state->work);
@@ -994,7 +995,7 @@ int flush;
                values here (9 and 6) without reading the comments in inftrees.h
                concerning the ENOUGH constants, which depend on those values */
             state->next = state->codes;
-            state->lencode = (code const FAR *)(state->next);
+            state->lencode = (const code FAR *)(state->next);
             state->lenbits = 9;
             ret = inflate_table(LENS, state->lens, state->nlen, &(state->next),
                                 &(state->lenbits), state->work);
@@ -1003,7 +1004,7 @@ int flush;
                 state->mode = BAD;
                 break;
             }
-            state->distcode = (code const FAR *)(state->next);
+            state->distcode = (const code FAR *)(state->next);
             state->distbits = 6;
             ret = inflate_table(DISTS, state->lens + state->nlen, state->ndist,
                             &(state->next), &(state->distbits), state->work);
@@ -1230,7 +1231,7 @@ int flush;
     RESTORE();
     if (state->wsize || (out != strm->avail_out && state->mode < BAD &&
             (state->mode < CHECK || flush != Z_FINISH)))
-        if (updatewindow(strm, out)) {
+        if (updatewindow(strm, strm->next_out, out - strm->avail_out)) {
             state->mode = MEM;
             return Z_MEM_ERROR;
         }
@@ -1294,8 +1295,6 @@ uInt dictLength;
 {
     struct inflate_state FAR *state;
     unsigned long dictid;
-    unsigned char *next;
-    unsigned avail;
     int ret;
 
     /* check state */
@@ -1314,13 +1313,7 @@ uInt dictLength;
 
     /* copy dictionary to window using updatewindow(), which will amend the
        existing dictionary if appropriate */
-    next = strm->next_out;
-    avail = strm->avail_out;
-    strm->next_out = (Bytef *)dictionary + dictLength;
-    strm->avail_out = 0;
-    ret = updatewindow(strm, dictLength);
-    strm->avail_out = avail;
-    strm->next_out = next;
+    ret = updatewindow(strm, dictionary + dictLength, dictLength);
     if (ret) {
         state->mode = MEM;
         return Z_MEM_ERROR;
@@ -1360,7 +1353,7 @@ 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/test/example.c b/test/example.c
index f515a48..138a699 100644
--- a/test/example.c
+++ b/test/example.c
@@ -26,7 +26,7 @@
     } \
 }
 
-const char hello[] = "hello, hello!";
+z_const char hello[] = "hello, hello!";
 /* "hello world" would be more standard, but the repeated "hello"
  * stresses the compression code better, sorry...
  */
@@ -212,7 +212,7 @@ void test_deflate(compr, comprLen)
     err = deflateInit(&c_stream, Z_DEFAULT_COMPRESSION);
     CHECK_ERR(err, "deflateInit");
 
-    c_stream.next_in  = (Bytef*)hello;
+    c_stream.next_in  = (z_const unsigned char *)hello;
     c_stream.next_out = compr;
 
     while (c_stream.total_in != len && c_stream.total_out < comprLen) {
@@ -387,7 +387,7 @@ void test_flush(compr, comprLen)
     err = deflateInit(&c_stream, Z_DEFAULT_COMPRESSION);
     CHECK_ERR(err, "deflateInit");
 
-    c_stream.next_in  = (Bytef*)hello;
+    c_stream.next_in  = (z_const unsigned char *)hello;
     c_stream.next_out = compr;
     c_stream.avail_in = 3;
     c_stream.avail_out = (uInt)*comprLen;
@@ -476,7 +476,7 @@ void test_dict_deflate(compr, comprLen)
     c_stream.next_out = compr;
     c_stream.avail_out = (uInt)comprLen;
 
-    c_stream.next_in = (Bytef*)hello;
+    c_stream.next_in = (z_const unsigned char *)hello;
     c_stream.avail_in = (uInt)strlen(hello)+1;
 
     err = deflate(&c_stream, Z_FINISH);
diff --git a/trees.c b/trees.c
index 8c32b21..1fd7759 100644
--- a/trees.c
+++ b/trees.c
@@ -146,8 +146,8 @@ local void send_tree      OF((deflate_state *s, ct_data *tree, int max_code));
 local int  build_bl_tree  OF((deflate_state *s));
 local void send_all_trees OF((deflate_state *s, int lcodes, int dcodes,
                               int blcodes));
-local void compress_block OF((deflate_state *s, ct_data *ltree,
-                              ct_data *dtree));
+local void compress_block OF((deflate_state *s, const ct_data *ltree,
+                              const ct_data *dtree));
 local int  detect_data_type OF((deflate_state *s));
 local unsigned bi_reverse OF((unsigned value, int length));
 local void bi_windup      OF((deflate_state *s));
@@ -972,7 +972,8 @@ void ZLIB_INTERNAL _tr_flush_block(s, buf, stored_len, last)
     } else if (s->strategy == Z_FIXED || static_lenb == opt_lenb) {
 #endif
         send_bits(s, (STATIC_TREES<<1)+last, 3);
-        compress_block(s, (ct_data *)static_ltree, (ct_data *)static_dtree);
+        compress_block(s, (const ct_data *)static_ltree,
+                       (const ct_data *)static_dtree);
 #ifdef DEBUG
         s->compressed_len += 3 + s->static_len;
 #endif
@@ -980,7 +981,8 @@ void ZLIB_INTERNAL _tr_flush_block(s, buf, stored_len, last)
         send_bits(s, (DYN_TREES<<1)+last, 3);
         send_all_trees(s, s->l_desc.max_code+1, s->d_desc.max_code+1,
                        max_blindex+1);
-        compress_block(s, (ct_data *)s->dyn_ltree, (ct_data *)s->dyn_dtree);
+        compress_block(s, (const ct_data *)s->dyn_ltree,
+                       (const ct_data *)s->dyn_dtree);
 #ifdef DEBUG
         s->compressed_len += 3 + s->opt_len;
 #endif
@@ -1057,8 +1059,8 @@ int ZLIB_INTERNAL _tr_tally (s, dist, lc)
  */
 local void compress_block(s, ltree, dtree)
     deflate_state *s;
-    ct_data *ltree; /* literal tree */
-    ct_data *dtree; /* distance tree */
+    const ct_data *ltree; /* literal tree */
+    const ct_data *dtree; /* distance tree */
 {
     unsigned dist;      /* distance of matched string */
     int lc;             /* match length or unmatched char (if dist == 0) */
diff --git a/uncompr.c b/uncompr.c
index ad98be3..242e949 100644
--- a/uncompr.c
+++ b/uncompr.c
@@ -30,7 +30,7 @@ int ZEXPORT uncompress (dest, destLen, source, sourceLen)
     z_stream stream;
     int err;
 
-    stream.next_in = (Bytef*)source;
+    stream.next_in = (z_const Bytef *)source;
     stream.avail_in = (uInt)sourceLen;
     /* Check for source > 64K on 16-bit machine: */
     if ((uLong)stream.avail_in != sourceLen) return Z_BUF_ERROR;
diff --git a/zlib.h b/zlib.h
index cd47df5..55e3c2c 100644
--- a/zlib.h
+++ b/zlib.h
@@ -1022,7 +1022,8 @@ 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 *,
+                                z_const unsigned char FAR * FAR *));
 typedef int (*out_func) OF((void FAR *, unsigned char FAR *, unsigned));
 
 ZEXTERN int ZEXPORT inflateBack OF((z_streamp strm,
diff --git a/zutil.c b/zutil.c
index 65e0d3b..23d2ebe 100644
--- a/zutil.c
+++ b/zutil.c
@@ -14,7 +14,7 @@
 struct internal_state      {int dummy;}; /* for buggy compilers */
 #endif
 
-const char * const z_errmsg[10] = {
+z_const char * const z_errmsg[10] = {
 "need dictionary",     /* Z_NEED_DICT       2  */
 "stream end",          /* Z_STREAM_END      1  */
 "",                    /* Z_OK              0  */
diff --git a/zutil.h b/zutil.h
index 4e3dcc6..0f02cbe 100644
--- a/zutil.h
+++ b/zutil.h
@@ -44,13 +44,13 @@ typedef unsigned short ush;
 typedef ush FAR ushf;
 typedef unsigned long  ulg;
 
-extern const char * const z_errmsg[10]; /* indexed by 2-zlib_error */
+extern z_const char * const z_errmsg[10]; /* indexed by 2-zlib_error */
 /* (size given to avoid silly warnings with Visual C++) */
 
 #define ERR_MSG(err) z_errmsg[Z_NEED_DICT-(err)]
 
 #define ERR_RETURN(strm,err) \
-  return (strm->msg = (char*)ERR_MSG(err), (err))
+  return (strm->msg = ERR_MSG(err), (err))
 /* To be used only when the state is known to be valid */
 
         /* common constants */