Index: src/OFRunLoop.m
==================================================================
--- src/OFRunLoop.m
+++ src/OFRunLoop.m
@@ -284,25 +284,31 @@
 
 	if (_writtenLength != _length && exception == nil)
 		return true;
 
 # ifdef OF_HAVE_BLOCKS
-	if (_block != NULL)
-		_block(object, _buffer, _writtenLength, exception);
-	else {
+	if (_block != NULL) {
+		if (!_block(object, _buffer, _writtenLength, exception))
+			return false;
+
+		_writtenLength = 0;
+		return true;
+	} else {
 # endif
-		void (*func)(id, SEL, OFStream *, const void *, size_t, id,
-		    id) = (void (*)(id, SEL, OFStream *, const void *, size_t,
+		bool (*func)(id, SEL, OFStream *, const void *, size_t, id,
+		    id) = (bool (*)(id, SEL, OFStream *, const void *, size_t,
 		    id, id))[_target methodForSelector: _selector];
 
-		func(_target, _selector, object, _buffer, _writtenLength,
-		    _context, exception);
+		if (!func(_target, _selector, object, _buffer, _writtenLength,
+		    _context, exception))
+			return false;
+
+		_writtenLength = 0;
+		return true;
 # ifdef OF_HAVE_BLOCKS
 	}
 # endif
-
-	return false;
 }
 
 - (void)dealloc
 {
 	[_block release];

Index: src/OFStream.h
==================================================================
--- src/OFStream.h
+++ src/OFStream.h
@@ -69,12 +69,14 @@
  * @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 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
  */
-typedef void (^of_stream_async_write_block_t)(OFStream *stream,
+typedef bool (^of_stream_async_write_block_t)(OFStream *stream,
     const void *buffer, size_t length, id _Nullable exception);
 #endif
 
 /*!
  * @class OFStream OFStream.h ObjFW/OFStream.h
@@ -802,14 +804,16 @@
  *	 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 target The target on which the selector should be called when the data
- *		 has been 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.
  * @param selector The selector to call on the target. The signature must be
- *		   `void (OFStream *stream, const void *buffer, size_t length,
+ *		   `bool (OFStream *stream, const void *buffer, size_t length,
  *		   id context, id exception)`.
  */
 - (void)asyncWriteBuffer: (const void *)buffer
 		  length: (size_t)length
 		  target: (id)target