diff --git a/include/os.h b/include/os.h
index 4be6b8010..c0f04c6af 100644
--- a/include/os.h
+++ b/include/os.h
@@ -115,7 +115,7 @@ extern void FlushIfCriticalOutputPending(void);
 
 extern void SetCriticalOutputPending(void);
 
-extern int WriteToClient(ClientPtr /*who*/, int /*count*/, char* /*buf*/);
+extern int WriteToClient(ClientPtr /*who*/, int /*count*/, const void* /*buf*/);
 
 extern void ResetOsBuffers(void);
 
@@ -448,7 +448,7 @@ typedef struct {
 extern CallbackListPtr ReplyCallback;
 typedef struct {
     ClientPtr client;
-    pointer replyData;
+    const void *replyData;
     unsigned long dataLenBytes;
     unsigned long bytesRemaining;
     Bool startOfReply;
diff --git a/os/io.c b/os/io.c
index e7ec60952..4f4a10903 100644
--- a/os/io.c
+++ b/os/io.c
@@ -730,11 +730,12 @@ SetCriticalOutputPending(void)
  *****************/
 
 _X_EXPORT int
-WriteToClient (ClientPtr who, int count, char *buf)
+WriteToClient (ClientPtr who, int count, const void *__buf)
 {
     OsCommPtr oc = (OsCommPtr)who->osPrivate;
     ConnectionOutputPtr oco = oc->output;
     int padBytes;
+    const char *buf = __buf;
 #ifdef DEBUG_COMMUNICATION
     Bool multicount = FALSE;
 #endif
@@ -871,13 +872,14 @@ WriteToClient (ClientPtr who, int count, char *buf)
  **********************/
 
 int
-FlushClient(ClientPtr who, OsCommPtr oc, char *extraBuf, int extraCount)
+FlushClient(ClientPtr who, OsCommPtr oc, const void *__extraBuf, int extraCount)
 {
     ConnectionOutputPtr oco = oc->output;
     int connection = oc->fd;
     XtransConnInfo trans_conn = oc->trans_conn;
     struct iovec iov[3];
     static char padBuffer[3];
+    const char *extraBuf = __extraBuf;
     long written;
     long padsize;
     long notWritten;
@@ -916,14 +918,14 @@ FlushClient(ClientPtr who, OsCommPtr oc, char *extraBuf, int extraCount)
 	    before = (-len); \
 	} else { \
 	    iov[i].iov_len = len; \
-	    iov[i].iov_base = (pointer) + before; \
+	    iov[i].iov_base = (pointer) + before;	\
 	    i++; \
 	    remain -= len; \
 	    before = 0; \
 	}
 
 	InsertIOV ((char *)oco->buf, oco->count)
-	InsertIOV (extraBuf, extraCount)
+	InsertIOV ((char *)extraBuf, extraCount)
 	InsertIOV (padBuffer, padsize)
 
 	errno = 0;
diff --git a/os/osdep.h b/os/osdep.h
index b6894c146..84f7177db 100644
--- a/os/osdep.h
+++ b/os/osdep.h
@@ -184,7 +184,7 @@ typedef struct _osComm {
 extern int FlushClient(
     ClientPtr /*who*/,
     OsCommPtr /*oc*/,
-    char* /*extraBuf*/,
+    const void * /*extraBuf*/,
     int /*extraCount*/
 );