Nanopb: API reference
=====================
+.. include :: menu.rst
+
.. contents ::
+Compilation options
+===================
+The following options can be specified using -D switch given to the C compiler:
+
+============================ ==============================================================================================
+__BIG_ENDIAN__ Set this if your platform stores integers and floats in big-endian format.
+ Mixed-endian systems (different layout for ints and floats) are currently not supported.
+NANOPB_INTERNALS Set this to expose the field encoder functions that are hidden since nanopb-0.1.3.
+PB_MAX_REQUIRED_FIELDS Maximum number of required fields to check for presence. Default value is 64.
+PB_MANY_FIELDS Add support for tag numbers > 255 and fields larger than 255 bytes or 255 array entries.
+ Increases code size 9 bytes per each field. Compiler error will tell if you need this.
+============================ ==============================================================================================
+
pb.h
====
PB_HTYPE_OPTIONAL 0x10 Use separate *has_<field>* boolean to specify
whether the field is present.
PB_HTYPE_ARRAY 0x20 A repeated field with preallocated array.
+ Separate *<field>_count* for number of items.
PB_HTYPE_CALLBACK 0x30 A field with dynamic storage size, data is
actually a pointer to a structure containing a
callback function.
pb_field_t
----------
-Describes a single structure field with memory position in relation to others. ::
+Describes a single structure field with memory position in relation to others. The descriptions are usually autogenerated. ::
typedef struct _pb_field_t pb_field_t;
struct _pb_field_t {
:type: LTYPE and HTYPE of the field.
:data_offset: Offset of field data, relative to the end of the previous field.
:size_offset: Offset of *bool* flag for optional fields or *size_t* count for arrays, relative to field data.
-:data_size: Size of a single data entry, in bytes.
+:data_size: Size of a single data entry, in bytes. For PB_LTYPE_BYTES, the size of the byte array inside the containing structure. For PB_HTYPE_CALLBACK, size of the C data type if known.
:array_size: Maximum number of entries in an array, if it is an array type.
:ptr: Pointer to default value for optional fields, or to submessage description for PB_LTYPE_SUBMESSAGE.
+The *uint8_t* datatypes limit the maximum size of a single item to 255 bytes and arrays to 255 items. Compiler will give error if the values are too large. The types can be changed to larger ones by defining *PB_MANY_FIELDS*.
+
+pb_bytes_array_t
+----------------
+An byte array with a field for storing the length::
+
+ typedef struct {
+ size_t size;
+ uint8_t bytes[1];
+ } pb_bytes_array_t;
+
+In an actual array, the length of *bytes* may be different.
+
+pb_callback_t
+-------------
+Part of a message structure, for fields with type PB_HTYPE_CALLBACK::
+
+ typedef struct _pb_callback_t pb_callback_t;
+ struct _pb_callback_t {
+ union {
+ bool (*decode)(pb_istream_t *stream, const pb_field_t *field, void *arg);
+ bool (*encode)(pb_ostream_t *stream, const pb_field_t *field, const void *arg);
+ } funcs;
+
+ void *arg;
+ };
+
+The *arg* is passed to the callback when calling. It can be used to store any information that the callback might need.
+
+When calling `pb_encode`_, *funcs.encode* is used, and similarly when calling `pb_decode`_, *funcs.decode* is used. The function pointers are stored in the same memory location but are of incompatible types. You can set the function pointer to NULL to skip the field.
+
+pb_wire_type_t
+--------------
+Protocol Buffers wire types. These are used with `pb_encode_tag`_. ::
+
+ typedef enum {
+ PB_WT_VARINT = 0,
+ PB_WT_64BIT = 1,
+ PB_WT_STRING = 2,
+ PB_WT_32BIT = 5
+ } pb_wire_type_t;
+
pb_encode.h
===========
Normally pb_encode simply walks through the fields description array and serializes each field in turn. However, submessages must be serialized twice: first to calculate their size and then to actually write them to output. This causes some constraints for callback fields, which must return the same data on every call.
-pb_encode_varint
-----------------
-Encodes an unsigned integer in the varint_ format. ::
+.. sidebar:: Encoding fields manually
- bool pb_encode_varint(pb_ostream_t *stream, uint64_t value);
+ The functions with names *pb_encode_\** are used when dealing with callback fields. The typical reason for using callbacks is to have an array of unlimited size. In that case, `pb_encode`_ will call your callback function, which in turn will call *pb_encode_\** functions repeatedly to write out values.
-:stream: Output stream to write to. 1-10 bytes will be written.
-:value: Value to encode.
-:returns: True on success, false on IO error.
+ The tag of a field must be encoded separately with `pb_encode_tag_for_field`_. After that, you can call exactly one of the content-writing functions to encode the payload of the field. For repeated fields, you can repeat this process multiple times.
-.. _varint: http://code.google.com/apis/protocolbuffers/docs/encoding.html#varints
+ Writing packed arrays is a little bit more involved: you need to use `pb_encode_tag` and specify `PB_WT_STRING` as the wire type. Then you need to know exactly how much data you are going to write, and use `pb_encode_varint`_ to write out the number of bytes before writing the actual data. Substreams can be used to determine the number of bytes beforehand; see `pb_encode_submessage`_ source code for an example.
pb_encode_tag
-------------
:stream: Output stream to write to. 1-5 bytes will be written.
:wiretype: PB_WT_VARINT, PB_WT_64BIT, PB_WT_STRING or PB_WT_32BIT
-:field_number: Identifier for the field, defined in the .proto file.
+:field_number: Identifier for the field, defined in the .proto file. You can get it from field->tag.
:returns: True on success, false on IO error.
pb_encode_tag_for_field
This function only considers the LTYPE of the field. You can use it from your field callbacks, because the source generator writes correct LTYPE also for callback type fields.
-pb_encode_string
+Wire type mapping is as follows:
+
+========================= ============
+LTYPEs Wire type
+========================= ============
+VARINT, SVARINT PB_WT_VARINT
+FIXED64 PB_WT_64BIT
+STRING, BYTES, SUBMESSAGE PB_WT_STRING
+FIXED32 PB_WT_32BIT
+========================= ============
+
+pb_encode_varint
----------------
-Writes the length of a string as varint and then contents of the string. Used for writing fields with wire type PB_WT_STRING. ::
+Encodes a signed or unsigned integer in the varint_ format. Works for fields of type `bool`, `enum`, `int32`, `int64`, `uint32` and `uint64`::
- bool pb_encode_string(pb_ostream_t *stream, const uint8_t *buffer, size_t size);
+ bool pb_encode_varint(pb_ostream_t *stream, uint64_t value);
-:stream: Output stream to write to.
-:buffer: Pointer to string data.
-:size: Number of bytes in the string.
+:stream: Output stream to write to. 1-10 bytes will be written.
+:value: Value to encode. Just cast e.g. int32_t directly to uint64_t.
:returns: True on success, false on IO error.
-.. sidebar:: Field encoders
-
- The functions with names beginning with *pb_enc_* are called field encoders. Each PB_LTYPE has an own field encoder, which handles translating from C data into Protocol Buffers data.
+.. _varint: http://code.google.com/apis/protocolbuffers/docs/encoding.html#varints
- By using the *data_size* in the field description and by taking advantage of C casting rules, it has been possible to combine many data types to a single LTYPE. For example, *int32*, *uint32*, *int64*, *uint64*, *bool* and *enum* are all handled by *pb_enc_varint*.
+pb_encode_svarint
+-----------------
+Encodes a signed integer in the 'zig-zagged' format. Works for fields of type `sint32` and `sint64`::
- Each field encoder only encodes the contents of the field. The tag must be encoded separately with `pb_encode_tag_for_field`_.
+ bool pb_encode_svarint(pb_ostream_t *stream, int64_t value);
- You can use the field encoders from your callbacks.
+(parameters are the same as for `pb_encode_varint`_
-pb_enc_varint
--------------
-Field encoder for PB_LTYPE_VARINT. Takes the first *field->data_size* bytes from src, casts them as *uint64_t* and calls `pb_encode_varint`_. ::
+pb_encode_string
+----------------
+Writes the length of a string as varint and then contents of the string. Works for fields of type `bytes` and `string`::
- bool pb_enc_varint(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+ bool pb_encode_string(pb_ostream_t *stream, const uint8_t *buffer, size_t size);
:stream: Output stream to write to.
-:field: Field description structure. Only *data_size* matters.
-:src: Pointer to start of the field data.
+:buffer: Pointer to string data.
+:size: Number of bytes in the string. Pass `strlen(s)` for strings.
:returns: True on success, false on IO error.
-pb_enc_svarint
---------------
-Field encoder for PB_LTYPE_SVARINT. Similar to `pb_enc_varint`_, except first zig-zag encodes the value for more efficient negative number encoding. ::
-
- bool pb_enc_svarint(pb_ostream_t *stream, const pb_field_t *field, const void *src);
-
-(parameters are the same as for `pb_enc_varint`_)
-
-The number is considered negative if the high-order bit of the value is set. On big endian computers, it is the highest bit of *\*src*. On little endian computers, it is the highest bit of *\*(src + field->data_size - 1)*.
-
-pb_enc_fixed
-------------
-Field encoder for PB_LTYPE_FIXED. Writes the data in little endian order. On big endian computers, reverses the order of bytes. ::
-
- bool pb_enc_fixed(pb_ostream_t *stream, const pb_field_t *field, const void *src);
-
-(parameters are the same as for `pb_enc_varint`_)
-
-The same function is used for both integers, floats and doubles. This break encoding of double values on architectures where they are mixed endian (primarily some arm processors with hardware FPU).
-
-pb_enc_bytes
-------------
-Field encoder for PB_LTYPE_BYTES. Just calls `pb_encode_string`_. ::
-
- bool pb_enc_bytes(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+pb_encode_fixed32
+-----------------
+Writes 4 bytes to stream and swaps bytes on big-endian architectures. Works for fields of type `fixed32`, `sfixed32` and `float`::
-:stream: Output stream to write to.
-:field: Not used.
-:src: Pointer to a structure similar to pb_bytes_array_t.
-:returns: True on success, false on IO error.
+ bool pb_encode_fixed32(pb_ostream_t *stream, const void *value);
-This function expects a pointer to a structure with a *size_t* field at start, and a variable sized byte array after it. The platform-specific field offset is inferred from *pb_bytes_array_t*, which has a byte array of size 1.
+:stream: Output stream to write to.
+:value: Pointer to a 4-bytes large C variable, for example `uint32_t foo;`.
+:returns: True on success, false on IO error.
-pb_enc_string
--------------
-Field encoder for PB_LTYPE_STRING. Determines size of string with strlen() and then calls `pb_encode_string`_. ::
+pb_encode_fixed64
+-----------------
+Writes 8 bytes to stream and swaps bytes on big-endian architecture. Works for fields of type `fixed64`, `sfixed64` and `double`::
- bool pb_enc_string(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+ bool pb_encode_fixed64(pb_ostream_t *stream, const void *value);
-:stream: Output stream to write to.
-:field: Not used.
-:src: Pointer to a null-terminated string.
-:returns: True on success, false on IO error.
+:stream: Output stream to write to.
+:value: Pointer to a 8-bytes large C variable, for example `uint64_t foo;`.
+:returns: True on success, false on IO error.
-pb_enc_submessage
------------------
-Field encoder for PB_LTYPE_SUBMESSAGE. Calls `pb_encode`_ to perform the actual encoding. ::
+pb_encode_submessage
+--------------------
+Encodes a submessage field, including the size header for it. Works for fields of any message type::
- bool pb_enc_submessage(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+ bool pb_encode_submessage(pb_ostream_t *stream, const pb_field_t fields[], const void *src_struct);
:stream: Output stream to write to.
-:field: Field description structure. The *ptr* field must be a pointer to a field description array for the submessage.
+:fields: Pointer to the autogenerated field description array for the submessage type, e.g. `MyMessage_fields`.
:src: Pointer to the structure where submessage data is.
:returns: True on success, false on IO errors, pb_encode errors or if submessage size changes between calls.
In Protocol Buffers format, the submessage size must be written before the submessage contents. Therefore, this function has to encode the submessage twice in order to know the size beforehand.
-If the submessage contains callback fields, the callback function might misbehave and write out a different amount of data on the second call. This situation is recognized and *false* is returned, but it is up to the caller to ensure that the receiver of the message does not interpret it as valid data.
+If the submessage contains callback fields, the callback function might misbehave and write out a different amount of data on the second call. This situation is recognized and *false* is returned, but garbage will be written to the output before the problem is detected.
pb_decode.h
===========
End of file is signalled by *stream->bytes_left* being zero after pb_read returns false.
+pb_decode
+---------
+Read and decode all fields of a structure. Reads until EOF on input stream. ::
+
+ bool pb_decode(pb_istream_t *stream, const pb_field_t fields[], void *dest_struct);
+
+:stream: Input stream to read from.
+:fields: A field description array. Usually autogenerated.
+:dest_struct: Pointer to structure where data will be stored.
+:returns: True on success, false on IO error, on detectable errors in field description, if a field encoder returns false or if a required field is missing.
+
+In Protocol Buffers binary format, EOF is only allowed between fields. If it happens anywhere else, pb_decode will return *false*. If pb_decode returns false, you cannot trust any of the data in the structure.
+
+In addition to EOF, the pb_decode implementation supports terminating a message with a 0 byte. This is compatible with the official Protocol Buffers because 0 is never a valid field tag.
+
+For optional fields, this function applies the default value and sets *has_<field>* to false if the field is not present.
+
pb_decode_varint
----------------
Read and decode a varint_ encoded integer. ::
:stream: Input stream to read from.
:returns: True on success, false on IO error or length exceeding uint32_t.
-pb_decode
----------
-Read and decode all fields of a structure. Reads until EOF on input stream. ::
+pb_decode_tag
+-------------
+Decode the tag that comes before field in the protobuf encoding::
- bool pb_decode(pb_istream_t *stream, const pb_field_t fields[], void *dest_struct);
+ bool pb_decode_tag(pb_istream_t *stream, pb_wire_type_t *wire_type, int *tag, bool *eof);
:stream: Input stream to read from.
-:fields: A field description array. Usually autogenerated.
-:dest_struct: Pointer to structure where data will be stored.
-:returns: True on success, false on IO error, on detectable errors in field description, if a field encoder returns false or if a required field is missing.
+:wire_type: Pointer to variable where to store the wire type of the field.
+:tag: Pointer to variable where to store the tag of the field.
+:eof: Pointer to variable where to store end-of-file status.
+:returns: True on success, false on error or EOF.
-In Protocol Buffers binary format, EOF is only allowed between fields. If it happens anywhere else, pb_decode will return *false*.
+When the message (stream) ends, this function will return false and set *eof* to true. On other
+errors, *eof* will be set to false.
-In addition to EOF, the pb_decode implementation supports terminating a message with a 0 byte. This is compatible with the official Protocol Buffers because 0 is never a valid field tag.
+pb_skip_field
+-------------
+Remove the data for a field from the stream, without actually decoding it::
-For optional fields, this function applies the default value and sets *has_<field>* to false if the field is not present.
+ bool pb_skip_field(pb_istream_t *stream, pb_wire_type_t wire_type);
-Because of memory concerns, the detection of missing required fields is not perfect if the structure contains more than 32 fields.
+:stream: Input stream to read from.
+:wire_type: Type of field to skip.
+:returns: True on success, false on IO error.
.. sidebar:: Field decoders
Each field decoder reads and decodes a single value. For arrays, the decoder is called repeatedly.
- You can use the decoders from your callbacks.
+ You can use the decoders from your callbacks. Just be aware that the pb_field_t passed to the callback is not directly compatible
+ with the *varint* field decoders. Instead, you must create a new pb_field_t structure and set the data_size according to the data type
+ you pass to *dest*, e.g. *field.data_size = sizeof(int);*. Other fields in the *pb_field_t* don't matter.
+
+ The field decoder interface is a bit messy as a result of the interface required inside the nanopb library.
+ Eventually they may be replaced by separate wrapper functions with a more friendly interface.
pb_dec_varint
-------------
(parameters are the same as `pb_dec_varint`_)
-pb_dec_fixed
-------------
-Field decoder for PB_LTYPE_FIXED. ::
+pb_dec_fixed32
+--------------
+Field decoder for PB_LTYPE_FIXED32. ::
- bool pb_dec_fixed(pb_istream_t *stream, const pb_field_t *field, void *dest);
+ bool pb_dec_fixed32(pb_istream_t *stream, const pb_field_t *field, void *dest);
-(parameters are the same as `pb_dec_varint`_)
+:stream: Input stream to read from. 4 bytes will be read.
+:field: Not used.
+:dest: Pointer to destination *int32_t*, *uint32_t* or *float*.
+:returns: True on success, false on IO errors.
-This function reads *field->data_size* bytes from the input stream.
+This function reads 4 bytes from the input stream.
On big endian architectures, it then reverses the order of the bytes.
Finally, it writes the bytes to *dest*.
+pb_dec_fixed64
+--------------
+Field decoder for PB_LTYPE_FIXED64. ::
+
+ bool pb_dec_fixed(pb_istream_t *stream, const pb_field_t *field, void *dest);
+
+:stream: Input stream to read from. 8 bytes will be read.
+:field: Not used.
+:dest: Pointer to destination *int64_t*, *uint64_t* or *double*.
+:returns: True on success, false on IO errors.
+
+Same as `pb_dec_fixed32`_, except this reads 8 bytes.
+
pb_dec_bytes
------------
Field decoder for PB_LTYPE_BYTES. Reads a length-prefixed block of bytes. ::
bool pb_dec_bytes(pb_istream_t *stream, const pb_field_t *field, void *dest);
+**Note:** This is an internal function that is not useful in decoder callbacks. To read bytes fields in callbacks, use
+*stream->bytes_left* and `pb_read`_.
+
:stream: Input stream to read from.
:field: Field description structure. Only *field->data_size* matters.
:dest: Pointer to a structure similar to pb_bytes_array_t.
bool pb_dec_string(pb_istream_t *stream, const pb_field_t *field, void *dest);
+**Note:** This is an internal function that is not useful in decoder callbacks. To read string fields in callbacks, use
+*stream->bytes_left* and `pb_read`_.
+
:stream: Input stream to read from.
:field: Field description structure. Only *field->data_size* matters.
:dest: Pointer to a character array of size *field->data_size*.
bool pb_dec_submessage(pb_istream_t *stream, const pb_field_t *field, void *dest)
+**Note:** This is an internal function that is not useful in decoder callbacks. To read submessage fields in callbacks, use
+`pb_decode`_ directly.
+
:stream: Input stream to read from.
:field: Field description structure. Only *field->ptr* matters.
:dest: Pointer to the destination structure.
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