Index: src/OFHTTPClient.m
==================================================================
--- src/OFHTTPClient.m
+++ src/OFHTTPClient.m
@@ -518,12 +518,12 @@
 				forRequest: _request];
 		return false;
 	}
 }
 
--    (bool)socket: (OFTCPSocket *)socket
-  didWriteRequest: (const void *)request
+-  (size_t)socket: (OFTCPSocket *)socket
+  didWriteRequest: (const void **)request
 	   length: (size_t)length
 	  context: (id)context
 	exception: (id)exception
 {
 	OFData *body;
@@ -548,30 +548,30 @@
 				  length: [body count] * [body itemSize]
 				  target: self
 				selector: @selector(socket:didWriteBody:length:
 					      context:exception:)
 				 context: nil];
-		return false;
+		return 0;
 	} else
 		return [self socket: socket
 		       didWriteBody: NULL
 			     length: 0
 			    context: nil
 			  exception: nil];
 }
 
-- (bool)socket: (OFTCPSocket *)socket
-  didWriteBody: (const void *)body
-	length: (size_t)length
-       context: (id)context
-     exception: (id)exception
+- (size_t)socket: (OFTCPSocket *)socket
+    didWriteBody: (const void **)body
+	  length: (size_t)length
+	 context: (id)context
+       exception: (id)exception
 {
 	[socket asyncReadLineWithTarget: self
 			       selector: @selector(socket:didReadLine:context:
 					      exception:)
 				context: nil];
-	return false;
+	return 0;
 }
 
 - (void)handleSocket: (OFTCPSocket *)socket
 {
 	/*

Index: src/OFRunLoop.m
==================================================================
--- src/OFRunLoop.m
+++ src/OFRunLoop.m
@@ -285,11 +285,13 @@
 	if (_writtenLength != _length && exception == nil)
 		return true;
 
 # ifdef OF_HAVE_BLOCKS
 	if (_block != NULL) {
-		if (!_block(object, _buffer, _writtenLength, exception))
+		_length = _block(object, &_buffer, _writtenLength, exception);
+
+		if (_length == 0)
 			return false;
 
 		_writtenLength = 0;
 		return true;
 	} else {
@@ -296,12 +298,14 @@
 # endif
 		bool (*func)(id, SEL, OFStream *, const void *, size_t, id,
 		    id) = (bool (*)(id, SEL, OFStream *, const void *, size_t,
 		    id, id))[_target methodForSelector: _selector];
 
-		if (!func(_target, _selector, object, _buffer, _writtenLength,
-		    _context, exception))
+		_length = func(_target, _selector, object, &_buffer,
+		    _writtenLength, _context, exception);
+
+		if (_length == 0)
 			return false;
 
 		_writtenLength = 0;
 		return true;
 # ifdef OF_HAVE_BLOCKS

Index: src/OFStream.h
==================================================================
--- src/OFStream.h
+++ src/OFStream.h
@@ -66,18 +66,22 @@
 /*!
  * @brief A block which is called when data was written to the stream.
  *
  * @param stream The stream to which data was written
  * @param buffer The buffer which was written to the stream
- * @param length The length of the data that bas been written
+ * @param bytesWritten The number of bytes which have been written. This
+ *		       matches the length specified on the asynchronous write
+ *		       if no exception was encountered.
  * @param exception An exception which occurred while reading or `nil` on
  *		    success
- * @return A bool whether another write with the same buffer, length and
- *	   callback block should be performed
+ * @return The length to repeat the write with or 0 if it should not repeat.
+ *	   The buffer may be changed, so that every time a new buffer and length
+ *	   can be specified while the callback stays the same.
  */
-typedef bool (^of_stream_async_write_block_t)(OFStream *stream,
-    const void *buffer, size_t length, id _Nullable exception);
+typedef size_t (^of_stream_async_write_block_t)(OFStream *stream,
+    const void *_Nonnull *_Nonnull buffer, size_t bytesWritten,
+    id _Nullable exception);
 #endif
 
 /*!
  * @class OFStream OFStream.h ObjFW/OFStream.h
  *
@@ -805,16 +809,18 @@
  *
  * @param buffer The buffer from which the data is written into the stream. The
  *		 buffer needs to be valid until the write request is completed!
  * @param length The length of the data that should be written
  * @param target The target on which the selector should be called when the
- *		 data has been received. If the method returns true, the same
- *		 buffer and length will be written again and same method will
- *		 be called again.
+ *		 data has been written. The method should return the length for
+ *		 the next write with the same callback or 0 if it should not
+ *		 repeat. The buffer may be changed, so that every time a new
+ *		 buffer and length can be specified while the callback stays
+ *		 the same.
  * @param selector The selector to call on the target. The signature must be
- *		   `bool (OFStream *stream, const void *buffer, size_t length,
- *		   id context, id exception)`.
+ *		   `bool (OFStream *stream, const void *buffer,
+ *		   size_t bytesWritten, id context, id exception)`.
  */
 - (void)asyncWriteBuffer: (const void *)buffer
 		  length: (size_t)length
 		  target: (id)target
 		selector: (SEL)selector
@@ -828,11 +834,15 @@
  *	 valid file descriptor in order for this to work!
  *
  * @param buffer The buffer from which the data is written into the stream. The
  *		 buffer needs to be valid until the write request is completed!
  * @param length The length of the data that should be written
- * @param block The block to call when the data has been written
+ * @param block The block to call when the data has been written. It should
+ *		return the length for the next write with the same callback or
+ *		0 if it should not repeat. The buffer may be changed, so that
+ *		every time a new buffer and length can be specified while the
+ *		callback stays the same.
  */
 - (void)asyncWriteBuffer: (const void *)buffer
 		  length: (size_t)length
 		   block: (of_stream_async_write_block_t)block;
 # endif