+ bool (*encode)(pb_ostream_t *stream, const pb_field_t *field, const void *arg);
+
+When encoding, the callback should write out complete fields, including the wire type and field number tag. It can write as many or as few fields as it likes. For example, if you want to write out an array as *repeated* field, you should do it all in a single call.
+
+Usually you can use `pb_encode_tag_for_field`_ to encode the wire type and tag number of the field. However, if you want to encode a repeated field as a packed array, you must call `pb_encode_tag`_ instead to specify a wire type of *PB_WT_STRING*.
+
+If the callback is used in a submessage, it will be called multiple times during a single call to `pb_encode`_. In this case, it must produce the same amount of data every time. If the callback is directly in the main message, it is called only once.
+
+.. _`pb_encode`: reference.html#pb-encode
+.. _`pb_encode_tag_for_field`: reference.html#pb-encode-tag-for-field
+.. _`pb_encode_tag`: reference.html#pb-encode-tag
+
+This callback writes out a dynamically sized string::
+
+ bool write_string(pb_ostream_t *stream, const pb_field_t *field, const void *arg)
+ {
+ char *str = get_string_from_somewhere();
+ if (!pb_encode_tag_for_field(stream, field))
+ return false;
+
+ return pb_encode_string(stream, (uint8_t*)str, strlen(str));
+ }
+
+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. 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.
+
+.. _`pb_decode`: reference.html#pb-decode
+
+This callback reads multiple integers and prints them::
+
+ bool read_ints(pb_istream_t *stream, const pb_field_t *field, void *arg)
+ {
+ while (stream->bytes_left)
+ {
+ uint64_t value;
+ if (!pb_decode_varint(stream, &value))
+ return false;
+ printf("%lld\n", value);
+ }
+ return true;
+ }
+
+Field description array
+=======================
+
+For using the *pb_encode* and *pb_decode* functions, you need an array of pb_field_t constants describing the structure you wish to encode. This description is usually autogenerated from .proto file.
+
+For example this submessage in the Person.proto file::
+
+ message Person {
+ message PhoneNumber {
+ required string number = 1 [(nanopb).max_size = 40];
+ optional PhoneType type = 2 [default = HOME];
+ }
+ }
+
+generates this field description array for the structure *Person_PhoneNumber*::
+
+ const pb_field_t Person_PhoneNumber_fields[3] = {
+ {1, PB_HTYPE_REQUIRED | PB_LTYPE_STRING,
+ offsetof(Person_PhoneNumber, number), 0,
+ pb_membersize(Person_PhoneNumber, number), 0, 0},
+
+ {2, PB_HTYPE_OPTIONAL | PB_LTYPE_VARINT,
+ pb_delta(Person_PhoneNumber, type, number),
+ pb_delta(Person_PhoneNumber, has_type, type),
+ pb_membersize(Person_PhoneNumber, type), 0,
+ &Person_PhoneNumber_type_default},
+
+ PB_LAST_FIELD
+ };
+
+
+Return values and error handling
+================================
+
+Most functions in nanopb return bool: *true* means success, *false* means failure. There is also some support for error messages for debugging purposes: the error messages go in *stream->errmsg*.
+
+The error messages help in guessing what is the underlying cause of the error. The most common error conditions are:
+
+1) Running out of memory. Because everything is allocated from the stack, nanopb can't detect this itself. Encoding or decoding the same type of a message always takes the same amount of stack space. Therefore, if it works once, it works always.
+2) Invalid field description. These are usually stored as constants, so if it works under the debugger, it always does.
+3) IO errors in your own stream callbacks.
+4) Errors that happen in your callback functions.
+5) Exceeding the max_size or bytes_left of a stream.
+6) Exceeding the max_size of a string or array field
+7) Invalid protocol buffers binary message.