diff --git a/gzlib.c b/gzlib.c
index 71785cc..a52c55b 100644
--- a/gzlib.c
+++ b/gzlib.c
@@ -615,7 +615,6 @@ void ZLIB_INTERNAL gz_error(state, err, msg)
     strcat(state->msg, ": ");
     strcat(state->msg, msg);
 #endif
-    return;
 }
 
 #ifndef INT_MAX
diff --git a/gzwrite.c b/gzwrite.c
index b03097c..2905cce 100644
--- a/gzwrite.c
+++ b/gzwrite.c
@@ -11,7 +11,8 @@ local int gz_comp OF((gz_statep, int));
 local int gz_zero OF((gz_statep, z_off64_t));
 
 /* Initialize state for writing a gzip file.  Mark initialization by setting
-   state->size to non-zero.  Return -1 on failure or 0 on success. */
+   state->size to non-zero.  Return -1 on a memory allocation failure, or 0 on
+   success. */
 local int gz_init(state)
     gz_statep state;
 {
@@ -63,11 +64,11 @@ local int gz_init(state)
 }
 
 /* Compress whatever is at avail_in and next_in and write to the output file.
-   Return -1 if there is an error writing to the output file, otherwise 0.
-   flush is assumed to be a valid deflate() flush value.  If flush is Z_FINISH,
-   then the deflate() state is reset to start a new gzip stream.  If gz->direct
-   is true, then simply write to the output file without compressing, and
-   ignore flush. */
+   Return -1 if there is an error writing to the output file or if gz_init()
+   fails to allocate memory, otherwise 0.  flush is assumed to be a valid
+   deflate() flush value.  If flush is Z_FINISH, then the deflate() state is
+   reset to start a new gzip stream.  If gz->direct is true, then simply write
+   to the output file without compressing, and ignore flush. */
 local int gz_comp(state, flush)
     gz_statep state;
     int flush;
@@ -136,7 +137,8 @@ local int gz_comp(state, flush)
     return 0;
 }
 
-/* Compress len zeros to output.  Return -1 on error, 0 on success. */
+/* Compress len zeros to output.  Return -1 on a write error or memory
+   allocation failure by gz_comp(), or 0 on success. */
 local int gz_zero(state, len)
     gz_statep state;
     z_off64_t len;
@@ -324,23 +326,23 @@ int ZEXPORTVA gzvprintf(gzFile file, const char *format, va_list va)
 
     /* get internal structure */
     if (file == NULL)
-        return -1;
+        return Z_STREAM_ERROR;
     state = (gz_statep)file;
     strm = &(state->strm);
 
     /* check that we're writing and that there's no error */
     if (state->mode != GZ_WRITE || state->err != Z_OK)
-        return 0;
+        return Z_STREAM_ERROR;
 
     /* make sure we have some buffer space */
     if (state->size == 0 && gz_init(state) == -1)
-        return 0;
+        return state->err;
 
     /* check for seek request */
     if (state->seek) {
         state->seek = 0;
         if (gz_zero(state, state->skip) == -1)
-            return 0;
+            return state->err;
     }
 
     /* do the printf() into the input buffer, put length in len -- the input
@@ -378,7 +380,7 @@ int ZEXPORTVA gzvprintf(gzFile file, const char *format, va_list va)
         left = strm->avail_in - state->size;
         strm->avail_in = state->size;
         if (gz_comp(state, Z_NO_FLUSH) == -1)
-            return 0;
+            return state->err;
         memcpy(state->in, state->in + state->size, left);
         strm->next_in = state->in;
         strm->avail_in = left;
@@ -414,27 +416,27 @@ int ZEXPORTVA gzprintf (file, format, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10,
 
     /* get internal structure */
     if (file == NULL)
-        return -1;
+        return Z_STREAM_ERROR;
     state = (gz_statep)file;
     strm = &(state->strm);
 
     /* check that can really pass pointer in ints */
     if (sizeof(int) != sizeof(void *))
-        return 0;
+        return Z_STREAM_ERROR;
 
     /* check that we're writing and that there's no error */
     if (state->mode != GZ_WRITE || state->err != Z_OK)
-        return 0;
+        return Z_STREAM_ERROR;
 
     /* make sure we have some buffer space */
     if (state->size == 0 && gz_init(state) == -1)
-        return 0;
+        return state->error;
 
     /* check for seek request */
     if (state->seek) {
         state->seek = 0;
         if (gz_zero(state, state->skip) == -1)
-            return 0;
+            return state->error;
     }
 
     /* do the printf() into the input buffer, put length in len -- the input
@@ -477,7 +479,7 @@ int ZEXPORTVA gzprintf (file, format, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10,
         left = strm->avail_in - state->size;
         strm->avail_in = state->size;
         if (gz_comp(state, Z_NO_FLUSH) == -1)
-            return 0;
+            return state->err;
         memcpy(state->in, state->in + state->size, left);
         strm->next_in = state->in;
         strm->avail_in = left;
@@ -496,7 +498,7 @@ int ZEXPORT gzflush(file, flush)
 
     /* get internal structure */
     if (file == NULL)
-        return -1;
+        return Z_STREAM_ERROR;
     state = (gz_statep)file;
 
     /* check that we're writing and that there's no error */
@@ -511,7 +513,7 @@ int ZEXPORT gzflush(file, flush)
     if (state->seek) {
         state->seek = 0;
         if (gz_zero(state, state->skip) == -1)
-            return -1;
+            return state->err;
     }
 
     /* compress remaining data with requested flush */
@@ -546,7 +548,7 @@ int ZEXPORT gzsetparams(file, level, strategy)
     if (state->seek) {
         state->seek = 0;
         if (gz_zero(state, state->skip) == -1)
-            return -1;
+            return state->err;
     }
 
     /* change compression parameters for subsequent input */
diff --git a/zlib.h b/zlib.h
index aa130a8..b396709 100644
--- a/zlib.h
+++ b/zlib.h
@@ -1355,10 +1355,12 @@ ZEXTERN int ZEXPORT gzbuffer OF((gzFile file, unsigned size));
 ZEXTERN int ZEXPORT gzsetparams OF((gzFile file, int level, int strategy));
 /*
      Dynamically update the compression level or strategy.  See the description
-   of deflateInit2 for the meaning of these parameters.
+   of deflateInit2 for the meaning of these parameters.  Previously provided
+   data is flushed before the parameter change.
 
-     gzsetparams returns Z_OK if success, or Z_STREAM_ERROR if the file was not
-   opened for writing.
+     gzsetparams returns Z_OK if success, Z_STREAM_ERROR if the file was not
+   opened for writing, Z_ERRNO if there is an error writing the flushed data,
+   or Z_MEM_ERROR if there is a memory allocation error.
 */
 
 ZEXTERN int ZEXPORT gzread OF((gzFile file, voidp buf, unsigned len));
@@ -1401,15 +1403,15 @@ ZEXTERN int ZEXPORTVA gzprintf Z_ARG((gzFile file, const char *format, ...));
 /*
      Converts, formats, and writes the arguments to the compressed file under
    control of the format string, as in fprintf.  gzprintf returns the number of
-   uncompressed bytes actually written, or 0 in case of error.  The number of
-   uncompressed bytes written is limited to 8191, or one less than the buffer
-   size given to gzbuffer().  The caller should assure that this limit is not
-   exceeded.  If it is exceeded, then gzprintf() will return an error (0) with
-   nothing written.  In this case, there may also be a buffer overflow with
-   unpredictable consequences, which is possible only if zlib was compiled with
-   the insecure functions sprintf() or vsprintf() because the secure snprintf()
-   or vsnprintf() functions were not available.  This can be determined using
-   zlibCompileFlags().
+   uncompressed bytes actually written, or a negative zlib error code in case
+   of error.  The number of uncompressed bytes written is limited to 8191, or
+   one less than the buffer size given to gzbuffer().  The caller should assure
+   that this limit is not exceeded.  If it is exceeded, then gzprintf() will
+   return an error (0) with nothing written.  In this case, there may also be a
+   buffer overflow with unpredictable consequences, which is possible only if
+   zlib was compiled with the insecure functions sprintf() or vsprintf()
+   because the secure snprintf() or vsnprintf() functions were not available.
+   This can be determined using zlibCompileFlags().
 */
 
 ZEXTERN int ZEXPORT gzputs OF((gzFile file, const char *s));