Nanopb: Basic concepts

The things outlined here are the underlying concepts of the nanopb design.


Proto files

All Protocol Buffers implementations use .proto files to describe the message format. The point of these files is to be a portable interface description language.

Compiling .proto files for nanopb

Nanopb uses the Google's protoc compiler to parse the .proto file, and then a python script to generate the C header and source code from it:

user@host:~$ protoc -omessage.pb message.proto
user@host:~$ python ../generator/ message.pb
Writing to message.h and message.c

Modifying generator behaviour

Using generator options, you can set maximum sizes for fields in order to allocate them statically. The preferred way to do this is to create an .options file with the same name as your .proto file:

# Foo.proto
message Foo {
   required string name = 1;
# Foo.options max_size:16

For more information on this, see the Proto file options section in the reference manual.


Nanopb uses streams for accessing the data in encoded format. The stream abstraction is very lightweight, and consists of a structure (pb_ostream_t or pb_istream_t) which contains a pointer to a callback function.

There are a few generic rules for callback functions:

  1. Return false on IO errors. The encoding or decoding process will abort immediately.
  2. Use state to store your own data, such as a file descriptor.
  3. bytes_written and bytes_left are updated by pb_write and pb_read.
  4. Your callback may be used with substreams. In this case bytes_left, bytes_written and max_size have smaller values than the original stream. Don't use these values to calculate pointers.
  5. Always read or write the full requested length of data. For example, POSIX recv() needs the MSG_WAITALL parameter to accomplish this.

Output streams

struct _pb_ostream_t
   bool (*callback)(pb_ostream_t *stream, const uint8_t *buf, size_t count);
   void *state;
   size_t max_size;
   size_t bytes_written;

The callback for output stream may be NULL, in which case the stream simply counts the number of bytes written. In this case, max_size is ignored.

Otherwise, if bytes_written + bytes_to_be_written is larger than max_size, pb_write returns false before doing anything else. If you don't want to limit the size of the stream, pass SIZE_MAX.

Example 1:

This is the way to get the size of the message without storing it anywhere:

Person myperson = ...;
pb_ostream_t sizestream = {0};
pb_encode(&sizestream, Person_fields, &myperson);
printf("Encoded size is %d\n", sizestream.bytes_written);

Example 2:

Writing to stdout:

bool callback(pb_ostream_t *stream, const uint8_t *buf, size_t count)
   FILE *file = (FILE*) stream->state;
   return fwrite(buf, 1, count, file) == count;

pb_ostream_t stdoutstream = {&callback, stdout, SIZE_MAX, 0};

Input streams

For input streams, there is one extra rule:

  1. You don't need to know the length of the message in advance. After getting EOF error when reading, set bytes_left to 0 and return false. Pb_decode will detect this and if the EOF was in a proper position, it will return true.

Here is the structure:

struct _pb_istream_t
   bool (*callback)(pb_istream_t *stream, uint8_t *buf, size_t count);
   void *state;
   size_t bytes_left;

The callback must always be a function pointer. Bytes_left is an upper limit on the number of bytes that will be read. You can use SIZE_MAX if your callback handles EOF as described above.


This function binds an input stream to stdin:

bool callback(pb_istream_t *stream, uint8_t *buf, size_t count)
   FILE *file = (FILE*)stream->state;
   bool status;

   if (buf == NULL)
       while (count-- && fgetc(file) != EOF);
       return count == 0;

   status = (fread(buf, 1, count, file) == count);

   if (feof(file))
       stream->bytes_left = 0;

   return status;

pb_istream_t stdinstream = {&callback, stdin, SIZE_MAX};

Data types

Most Protocol Buffers datatypes have directly corresponding C datatypes, such as int32 is int32_t, float is float and bool is bool. However, the variable-length datatypes are more complex:

  1. Strings, bytes and repeated fields of any type map to callback functions by default.
  2. If there is a special option (nanopb).max_size specified in the .proto file, string maps to null-terminated char array and bytes map to a structure containing a char array and a size field.
  3. If (nanopb).fixed_length is set to true and (nanopb).max_size is also set, then bytes map to an inline byte array of fixed size.
  4. If there is a special option (nanopb).max_count specified on a repeated field, it maps to an array of whatever type is being repeated. Another field will be created for the actual number of entries stored.
  5. If (nanopb).fixed_count is set to true and (nanopb).max_count is also set, the field for the actual number of entries will not by created as the count is always assumed to be max count.
field in .proto autogenerated in .h
required string name = 1; pb_callback_t name;
required string name = 1 [(nanopb).max_size = 40]; char name[40];
repeated string name = 1 [(nanopb).max_size = 40]; pb_callback_t name;
repeated string name = 1 [(nanopb).max_size = 40, (nanopb).max_count = 5];
size_t name_count;
char name[5][40];
required bytes data = 1 [(nanopb).max_size = 40];
typedef struct {
size_t size;
pb_byte_t bytes[40];
} Person_data_t;
Person_data_t data;
required bytes data = 1 [(nanopb).max_size = 40, (nanopb).fixed_length = true];
pb_byte_t data[40];
repeated int32 data = 1 [(nanopb).max_count = 5, (nanopb).fixed_count true];
int32_t data[5];

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.

Note: When using the fixed_count option, the decoder assumes the repeated elements are received sequentially or that repeated elements for a non-packed field will not be interleaved with another fixed_count non-packed field.

Field callbacks

When a field has dynamic length, nanopb cannot statically allocate storage for it. Instead, it allows you to handle the field in whatever way you want, using a callback function.

The pb_callback_t structure contains a function pointer and a void pointer called arg you can use for passing data to the callback. If the function pointer is NULL, the field will be skipped. A pointer to the arg is passed to the function, so that it can modify it and retrieve the value.

The actual behavior of the callback function is different in encoding and decoding modes. In encoding mode, the callback is called once and should write out everything, including field tags. In decoding mode, the callback is called repeatedly for every data item.

Encoding callbacks

bool (*encode)(pb_ostream_t *stream, const pb_field_t *field, void * const *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.

This callback writes out a dynamically sized string:

bool write_string(pb_ostream_t *stream, const pb_field_t *field, void * const *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.

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] = {
   PB_FIELD(  1, STRING  , REQUIRED, STATIC, Person_PhoneNumber, number, number, 0),
   PB_FIELD(  2, ENUM    , OPTIONAL, STATIC, Person_PhoneNumber, type, number, &Person_PhoneNumber_type_default),


Protocol Buffers supports oneof sections. Here is an example of oneof usage:

message MsgType1 {
    required int32 value = 1;

message MsgType2 {
    required bool value = 1;

message MsgType3 {
    required int32 value1 = 1;
    required int32 value2 = 2;

message MyMessage {
    required uint32 uid = 1;
    required uint32 pid = 2;
    required uint32 utime = 3;

    oneof payload {
        MsgType1 msg1 = 4;
        MsgType2 msg2 = 5;
        MsgType3 msg3 = 6;

Nanopb will generate payload as a C union and add an additional field which_payload:

typedef struct _MyMessage {
  uint32_t uid;
  uint32_t pid;
  uint32_t utime;
  pb_size_t which_payload;
  union {
      MsgType1 msg1;
      MsgType2 msg2;
      MsgType3 msg3;
  } payload;
/* @@protoc_insertion_point(struct:MyMessage) */
} MyMessage;

which_payload indicates which of the oneof fields is actually set. The user is expected to set the filed manually using the correct field tag:

MyMessage msg = MyMessage_init_zero;
msg.payload.msg2.value = true;
msg.which_payload = MyMessage_msg2_tag;

Notice that neither which_payload field nor the unused fileds in payload will consume any space in the resulting encoded message.

When a C union is used to represent a oneof section, the union cannot have callback fields or nested callback fields. Otherwise, the decoding process may fail. If callbacks must be used inside a oneof section, the generator option no_unions should be set to true for that section.

Extension fields

Protocol Buffers supports a concept of extension fields, which are additional fields to a message, but defined outside the actual message. The definition can even be in a completely separate .proto file.

The base message is declared as extensible by keyword extensions in the .proto file:

message MyMessage {
    .. fields ..
    extensions 100 to 199;

For each extensible message, declares an additional callback field called extensions. The field and associated datatype pb_extension_t forms a linked list of handlers. When an unknown field is encountered, the decoder calls each handler in turn until either one of them handles the field, or the list is exhausted.

The actual extensions are declared using the extend keyword in the .proto, and are in the global namespace:

extend MyMessage {
    optional int32 myextension = 100;

For each extension, creates a constant of type pb_extension_type_t. To link together the base message and the extension, you have to:

  1. Allocate storage for your field, matching the datatype in the .proto. For example, for a int32 field, you need a int32_t variable to store the value.
  2. Create a pb_extension_t constant, with pointers to your variable and to the generated pb_extension_type_t.
  3. Set the message.extensions pointer to point to the pb_extension_t.

An example of this is available in tests/test_encode_extensions.c and tests/test_decode_extensions.c.

Default values

Protobuf has two syntax variants, proto2 and proto3. Of these proto2 has user definable default values that can be given in .proto file:

message MyMessage {
    optional bytes foo = 1 [default = "ABC\x01\x02\x03"];
    optional string bar = 2 [default = "åäö"];

Nanopb will generate both static and runtime initialization for the default values. In myproto.pb.h there will be a #define MyMessage_init_default that can be used to initialize whole message into default values:

MyMessage msg = MyMessage_init_default;

In addition to this, pb_decode() will initialize message fields to defaults at runtime. If this is not desired, pb_decode_noinit() can be used instead.

Message framing

Protocol Buffers does not specify a method of framing the messages for transmission. This is something that must be provided by the library user, as there is no one-size-fits-all solution. Typical needs for a framing format are to:

  1. Encode the message length.
  2. Encode the message type.
  3. Perform any synchronization and error checking that may be needed depending on application.

For example UDP packets already fullfill all the requirements, and TCP streams typically only need a way to identify the message length and type. Lower level interfaces such as serial ports may need a more robust frame format, such as HDLC (high-level data link control).

Nanopb provides a few helpers to facilitate implementing framing formats:

  1. Functions pb_encode_delimited and pb_decode_delimited prefix the message data with a varint-encoded length.
  2. Union messages and oneofs are supported in order to implement top-level container messages.
  3. Message IDs can be specified using the (nanopb_msgopt).msgid option and can then be accessed from the header.

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, i.e. stack overflow.
  2. Invalid field descriptors (would usually mean a bug in the generator).
  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/max_count of a string or array field
  7. Invalid protocol buffers binary message.