Fixed issue 1 reported by Erik Rosen:

[apps/agl-service-can-low-level.git] / docs / concepts.rst

diff --git a/docs/concepts.rst b/docs/concepts.rst
index e607640..1d0fe08 100644 (file)
--- a/docs/concepts.rst
+++ b/docs/concepts.rst
@@ -4,7 +4,7 @@ Nanopb: Basic concepts
 
 .. include :: menu.rst
 
-The things outlined here are common to both the encoder and the decoder part.
+The things outlined here are the underlying concepts of the nanopb design.
 
 .. contents::
 
@@ -158,7 +158,10 @@ required bytes data = 1 [(nanopb).max_size = 40];
                                                                                 | Person_data_t data;
 =============================================================================== =======================
 
-The maximum lengths are checked in runtime. If string/bytes/array exceeds the allocated length, *pb_decode* will return false. 
+The maximum lengths are checked in runtime. If string/bytes/array exceeds the allocated length, *pb_decode* will return false.
+
+Note: for the *bytes* datatype, the field length checking may not be exact.
+The compiler may add some padding to the *pb_bytes_t* structure, and the nanopb runtime doesn't know how much of the structure size is padding. Therefore it uses the whole length of the structure for storing data, which is not very smart but shouldn't cause problems. In practise, this means that if you specify *(nanopb).max_size=5* on a *bytes* field, you may be able to store 6 bytes there. For the *string* field type, the length limit is exact.
 
 Field callbacks
 ===============
@@ -201,7 +204,7 @@ Decoding callbacks
 
     bool (*decode)(pb_istream_t *stream, const pb_field_t *field, void *arg);
 
-When decoding, the callback receives a length-limited substring that reads the contents of a single field. The field tag has already been read.
+When decoding, the callback receives a length-limited substring that reads the contents of a single field. The field tag has already been read. For *string* and *bytes*, the length value has already been parsed, and is available at *stream->bytes_left*.
 
 The callback will be called multiple times for repeated fields. For packed fields, you can either read multiple values until the stream ends, or leave it to `pb_decode`_ to call your function over and over until all values have been read.