diff --git a/src/data.h b/src/data.h
index 481a245e..90e0a883 100644
--- a/src/data.h
+++ b/src/data.h
@@ -1,170 +1,174 @@
 /* data.h - Internal data object abstraction interface.
    Copyright (C) 2002, 2004, 2005 g10 Code GmbH
 
    This file is part of GPGME.
 
    GPGME is free software; you can redistribute it and/or modify it
    under the terms of the GNU Lesser General Public License as
    published by the Free Software Foundation; either version 2.1 of
    the License, or (at your option) any later version.
 
    GPGME is distributed in the hope that it will be useful, but
    WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Lesser General Public License for more details.
 
    You should have received a copy of the GNU Lesser General Public
    License along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
    02111-1307, USA.  */
 
 #ifndef DATA_H
 #define DATA_H
 
 #if HAVE_CONFIG_H
 #include <config.h>
 #endif
 
 #ifdef HAVE_SYS_TYPES_H
 # include <sys/types.h>
 #endif
 #include <limits.h>
 #include <stdint.h>
 
 #include "gpgme.h"
 
 
 /* Read up to SIZE bytes into buffer BUFFER from the data object with
    the handle DH.  Return the number of characters read, 0 on EOF and
    -1 on error.  If an error occurs, errno is set.  */
 typedef gpgme_ssize_t (*gpgme_data_read_cb) (gpgme_data_t dh,
                                              void *buffer,
                                              size_t size);
 
 /* Write up to SIZE bytes from buffer BUFFER to the data object with
    the handle DH.  Return the number of characters written, or -1 on
    error.  If an error occurs, errno is set.  */
 typedef gpgme_ssize_t (*gpgme_data_write_cb) (gpgme_data_t dh,
                                               const void *buffer,
                                               size_t size);
 
 /* Set the current position from where the next read or write starts
    in the data object with the handle DH to OFFSET, relative to
    WHENCE.  */
 typedef gpgme_off_t (*gpgme_data_seek_cb) (gpgme_data_t dh,
                                             gpgme_off_t offset,
                                             int whence);
 
 /* Release the data object with the handle DH.  */
 typedef void (*gpgme_data_release_cb) (gpgme_data_t dh);
 
 /* Get the FD associated with the handle DH, or -1.  */
 typedef int (*gpgme_data_get_fd_cb) (gpgme_data_t dh);
 
 struct _gpgme_data_cbs
 {
   gpgme_data_read_cb read;
   gpgme_data_write_cb write;
   gpgme_data_seek_cb seek;
   gpgme_data_release_cb release;
   gpgme_data_get_fd_cb get_fd;
 };
 
 struct gpgme_data
 {
   struct _gpgme_data_cbs *cbs;
   gpgme_data_encoding_t encoding;
   unsigned int propidx;  /* Index into the property table.  */
 
 #ifdef PIPE_BUF
 #define BUFFER_SIZE PIPE_BUF
 #else
 #ifdef _POSIX_PIPE_BUF
 #define BUFFER_SIZE _POSIX_PIPE_BUF
 #else
+#ifdef HAVE_W32_SYSTEM
+#define BUFFER_SIZE 4096
+#else
 #define BUFFER_SIZE 512
 #endif
+#endif
 #endif
   char pending[BUFFER_SIZE];
   int pending_len;
 
   /* File name of the data object.  */
   char *file_name;
 
   /* Hint on the to be expected total size of the data.  */
   gpgme_off_t size_hint;
 
   union
   {
     /* For gpgme_data_new_from_fd.  */
     int fd;
 
     /* For gpgme_data_new_from_stream.  */
     FILE *stream;
 
     /* For gpgme_data_new_from_estream.  */
     gpgrt_stream_t e_stream;
 
     /* For gpgme_data_new_from_cbs.  */
     struct
     {
       gpgme_data_cbs_t cbs;
       void *handle;
     } user;
 
     /* For gpgme_data_new_from_mem.  */
     struct
     {
       char *buffer;
       const char *orig_buffer;
       /* Allocated size of BUFFER.  */
       size_t size;
       size_t length;
       gpgme_off_t offset;
     } mem;
 
     /* For gpgme_data_new_from_read_cb.  */
     struct
     {
       int (*cb) (void *, char *, size_t, size_t *);
       void *handle;
     } old_user;
   } data;
 };
 
 
 /* The data property types.  */
 typedef enum
   {
     DATA_PROP_NONE = 0,   /* Dummy property. */
     DATA_PROP_BLANKOUT    /* Do not return the held data.  */
   } data_prop_t;
 
 
 
 /* Return the data object's serial number for handle DH.  */
 uint64_t _gpgme_data_get_dserial (gpgme_data_t dh);
 
 /* Set an internal property of a data object.  */
 gpg_error_t _gpgme_data_set_prop (gpgme_data_t dh, uint64_t dserial,
                                   data_prop_t name, int value);
 
 /* Get an internal property of a data object.  */
 gpg_error_t _gpgme_data_get_prop (gpgme_data_t dh, uint64_t dserial,
                                   data_prop_t name, int *r_value);
 
 /* Create a new data object.  */
 gpgme_error_t _gpgme_data_new (gpgme_data_t *r_dh,
 			       struct _gpgme_data_cbs *cbs);
 
 void _gpgme_data_release (gpgme_data_t dh);
 
 /* Get the file descriptor associated with DH, if possible.  Otherwise
    return -1.  */
 int _gpgme_data_get_fd (gpgme_data_t dh);
 
 /* Get the size-hint value for DH or 0 if not available.  */
 gpgme_off_t _gpgme_data_get_size_hint (gpgme_data_t dh);
 
 
 #endif	/* DATA_H */