Nanopb: API reference

Compilation options

Compilation options affect the functionality included in the nanopb core C code. The options can be specified in one of two ways:

  1. Using the -D switch on the C compiler command line.
  2. Using a #define at the top of pb.h.

NOTE: You must have the same compilation options for the nanopb library and all code that includes nanopb headers.

  • PB_ENABLE_MALLOC: Enable dynamic allocation support in the decoder.
  • PB_MAX_REQUIRED_FIELDS: Maximum number of proto2 required fields to check for presence. Default value is 64. Compiler warning will tell if you need this.
  • PB_FIELD_32BIT: Add support for field tag numbers over 65535, fields larger than 64 kiB and arrays larger than 65535 entries. Compiler warning will tell if you need this.
  • PB_NO_ERRMSG: Disable error message support to save code size. Only error information is the true/false return value.
  • PB_BUFFER_ONLY: Disable support for custom streams. Only supports encoding and decoding with memory buffers. Speeds up execution and slightly decreases code size.
  • PB_SYSTEM_HEADER: Replace the standards header files with a single system-specific header file. Value must include quotes, for example #define PB_SYSTEM_HEADER "foo.h". See extra/pb_syshdr.h for an example.
  • PB_WITHOUT_64BIT: Disable support of 64-bit integer fields, for old compilers or for a slight speedup on 8-bit platforms.
  • PB_ENCODE_ARRAYS_UNPACKED: Encode scalar arrays in the unpacked format, which takes up more space. Only to be used when the decoder on the receiving side cannot process packed arrays, such as protobuf.js versions before 2020.
  • PB_CONVERT_DOUBLE_FLOAT: Convert doubles to floats for platforms that do not support 64-bit double datatype. Mainly AVR processors.
  • PB_VALIDATE_UTF8: Check whether incoming strings are valid UTF-8 sequences. Adds a small performance and code size penalty.
  • PB_C99_STATIC_ASSERT: Use C99 style negative array trick for static assertions. For compilers that do not support C11 standard.
  • PB_NO_STATIC_ASSERT: Disable static assertions at compile time. Only for compilers with limited support of C standards.

The PB_MAX_REQUIRED_FIELDS and PB_FIELD_32BIT settings allow raising some datatype limits to suit larger messages. Their need is recognized automatically by C-preprocessor #if-directives in the generated .pb.c files. The default setting is to use the smallest datatypes (least resources used).

Generator options

Generator options affect how the .proto files get converted to .pb.c and .pb.h. files.

Most options are related to specific message or field in .proto file. The full set of available options is defined in nanopb.proto. Here is a list of the most common options, but see the file for a full list:

  • max_size: Allocated maximum size for bytes and string fields. For strings, this includes the terminating zero.
  • max_length: Maximum length for string fields. Setting this is equivalent to setting max_size to a value of length + 1.
  • max_count: Allocated maximum number of entries in arrays (repeated fields).
  • type: Select how memory is allocated for the generated field. Default value is FT_DEFAULT, which defaults to FT_STATIC when possible and FT_CALLBACK if not possible. You can use FT_CALLBACK, FT_POINTER, FT_STATIC or FT_IGNORE to select a callback field, a dynamically allocate dfield, a statically allocated field or to completely ignore the field.
  • long_names: Prefix the enum name to the enum value in definitions, i.e. EnumName_EnumValue. Enabled by default.
  • packed_struct: Make the generated structures packed, which saves some RAM space but slows down execution. This can only be used if the CPU supports unaligned access to variables.
  • skip_message: Skip a whole message from generation. Can be used to remove message types that are not needed in an application.
  • no_unions: Generate oneof fields as multiple optional fields instead of a C union {}.
  • anonymous_oneof: Generate oneof fields as an anonymous union.
  • msgid: Specifies a unique id for this message type. Can be used by user code as an identifier.
  • fixed_length: Generate bytes fields with a constant length defined by max_size. A separate .size field will then not be generated.
  • fixed_count: Generate arrays with constant length defined by max_count.
  • package: Package name that applies only for nanopb generator. Defaults to name defined by package keyword in .proto file, which applies for all languages.
  • int_size: Override the integer type of a field. For example, specify int_size = IS_8 to convert int32 from protocol definition into int8_t in the structure. When used with enum types, the size of the generated enum can be specified (C++ only)

These options can be defined for the .proto files before they are converted using the There are three ways to define the options:

  1. Using a separate .options file. This allows using wildcards for applying same options to multiple fields.
  2. Defining the options on the command line of This only makes sense for settings that apply to a whole file.
  3. Defining the options in the .proto file using the nanopb extensions. This keeps the options close to the fields they apply to, but can be problematic if the same .proto file is shared with many projects.

The effect of the options is the same no matter how they are given. The most common purpose is to define maximum size for string fields in order to statically allocate them.

Defining the options in a .options file

The preferred way to define options is to have a separate file ‘myproto.options’ in the same directory as the ‘myproto.proto’. :

# myproto.proto
message MyMessage {
    required string name = 1;
    repeated int32 ids = 4;

# myproto.options         max_size:40
MyMessage.ids          max_count:5

The generator will automatically search for this file and read the options from it. The file format is as follows:

  • Lines starting with # or // are regarded as comments.
  • Blank lines are ignored.
  • All other lines should start with a field name pattern, followed by one or more options. For example: MyMessage.myfield max_size:5 max_count:10.
  • The field name pattern is matched against a string of form Message.field. For nested messages, the string is Message.SubMessage.field. A whole file can be matched by its filename dir/file.proto.
  • The field name pattern may use the notation recognized by Python fnmatch():
    • * matches any part of string, like Message.* for all fields
    • ? matches any single character
    • [seq] matches any of characters s, e and q
    • [!seq] matches any other character
  • The options are written as option_name:option_value and several options can be defined on same line, separated by whitespace.
  • Options defined later in the file override the ones specified earlier, so it makes sense to define wildcard options first in the file and more specific ones later.

To debug problems in applying the options, you can use the -v option for the nanopb generator. With protoc, plugin options are specified with --nanopb_opt:

nanopb_generator -v message.proto           # When invoked directly
protoc ... --nanopb_opt=-v --nanopb_out=. message.proto  # When invoked through protoc

Protoc doesn't currently pass include path into plugins. Therefore if your .proto is in a subdirectory, nanopb may have trouble finding the associated .options file. A workaround is to specify include path separately to the nanopb plugin, like:

protoc -Isubdir --nanopb_opt=-Isubdir --nanopb_out=. message.proto

If preferred, the name of the options file can be set using generator argument -f.

Defining the options in the .proto file

The .proto file format allows defining custom options for the fields. The nanopb library comes with nanopb.proto which does exactly that, allowing you do define the options directly in the .proto file:

import "nanopb.proto";

message MyMessage {
    required string name = 1 [(nanopb).max_size = 40];
    repeated int32 ids = 4   [(nanopb).max_count = 5];

A small complication is that you have to set the include path of protoc so that nanopb.proto can be found. Therefore, to compile a .proto file which uses options, use a protoc command similar to:

protoc -Inanopb/generator/proto -I. --nanopb_out=. message.proto

The options can be defined in file, message and field scopes:

option (nanopb_fileopt).max_size = 20; // File scope
message Message
    option (nanopb_msgopt).max_size = 30; // Message scope
    required string fieldsize = 1 [(nanopb).max_size = 40]; // Field scope

Defining the options on command line

The has a simple command line option -s OPTION:VALUE. The setting applies to the whole file that is being processed.

There are also a few command line options that cannot be applied using the other mechanisms, as they affect the whole generation:

  • --c-style: Modify symbol names to better match C naming conventions.
  • --no-timestamp: Do not add timestamp to generated files.
  • --strip-path: Remove relative path from generated #include directives.
  • --cpp-descriptors: Generate extra convenience definitions for use from C++



Type used for storing byte-sized data, such as raw binary input and bytes-type fields.

typedef uint_least8_t pb_byte_t;

For most platforms this is equivalent to uint8_t. Some platforms however do not support 8-bit variables, and on those platforms 16 or 32 bits need to be used for each byte.


Type used for storing tag numbers and sizes of message fields. By default the type is 16-bit:

typedef uint_least16_t pb_size_t;

If tag numbers or fields larger than 65535 are needed, PB_FIELD_32BIT option can be used to change the type to 32-bit value.


Type used to store the type of each field, to control the encoder/decoder behaviour.

typedef uint_least8_t pb_type_t;

The low-order nibble of the enumeration values defines the function that can be used for encoding and decoding the field data:

LTYPE identifierValueStorage format
PB_LTYPE_UVARINT0x02Unsigned integer.
PB_LTYPE_SVARINT0x03Integer, zigzag encoded.
PB_LTYPE_FIXED320x0432-bit integer or floating point.
PB_LTYPE_FIXED640x0564-bit integer or floating point.
PB_LTYPE_BYTES0x06Structure with size_t field and byte array.
PB_LTYPE_STRING0x07Null-terminated string.
PB_LTYPE_SUBMESSAGE0x08Submessage structure.
PB_LTYPE_SUBMSG_W_CB0x09Submessage with pre-decoding callback.
PB_LTYPE_EXTENSION0x0APointer to pb_extension_t.
PB_LTYPE_FIXED_LENGTH_BYTES0x0BInline pb_byte_t array of fixed size.

The bits 4-5 define whether the field is required, optional or repeated. There are separate definitions for semantically different modes, even though some of them share values and are distinguished based on values of other fields:

HTYPE identifierValueField handling
PB_HTYPE_REQUIRED0x00Verify that field exists in decoded message.
PB_HTYPE_OPTIONAL0x10Use separate has_<field> boolean to specify whether the field is present.
PB_HTYPE_SINGULAR0x10Proto3 field, which is present when its value is non-zero.
PB_HTYPE_REPEATED0x20A repeated field with preallocated array. Separate <field>_count for number of items.
PB_HTYPE_FIXARRAY0x20A repeated field that has constant length.
PB_HTYPE_ONEOF0x30Oneof-field, only one of each group can be present.

The bits 6-7 define the how the storage for the field is allocated:

ATYPE identifierValueAllocation method
PB_ATYPE_STATIC0x00Statically allocated storage in the structure.
PB_ATYPE_POINTER0x80Dynamically allocated storage. Struct field contains a pointer to the storage.
PB_ATYPE_CALLBACK0x40A field with dynamic storage size. Struct field contains a pointer to a callback function.


Autogenerated structure that contains information about a message and pointers to the field descriptors. Use functions defined in pb_common.h to process the field information.

typedef struct pb_msgdesc_s pb_msgdesc_t;
struct pb_msgdesc_s {
    pb_size_t field_count;
    const uint32_t *field_info;
    const pb_msgdesc_t * const * submsg_info;
    const pb_byte_t *default_value;

    bool (*field_callback)(pb_istream_t *istream, pb_ostream_t *ostream, const pb_field_iter_t *field);
field_countTotal number of fields in the message.
field_infoPointer to compact representation of the field information.
submsg_infoPointer to array of pointers to descriptors for submessages.
default_valueDefault values for this message as an encoded protobuf message.
field_callbackFunction used to handle all callback fields in this message. By default pb_default_field_callback() which loads per-field callbacks from a pb_callback_t structure.


Describes a single structure field with memory position in relation to others. The field information is stored in a compact format and loaded into pb_field_iter_t by the functions defined in pb_common.h.

typedef struct pb_field_iter_s pb_field_iter_t;
struct pb_field_iter_s {
    const pb_msgdesc_t *descriptor;
    void *message;

    pb_size_t index;
    pb_size_t field_info_index;
    pb_size_t required_field_index;
    pb_size_t submessage_index;

    pb_size_t tag;
    pb_size_t data_size;
    pb_size_t array_size;
    pb_type_t type;

    void *pField;
    void *pData;
    void *pSize;

    const pb_msgdesc_t *submsg_desc;
descriptorPointer to pb_msgdesc_t for the message that contains this field.
messagePointer to the start of the message structure.
indexIndex of the field inside the message
field_info_indexIndex to the internal field_info array
required_field_indexIndex that counts only the required fields
submessage_indexIndex that counts only submessages
tagTag number defined in .proto file for this field.
data_sizesizeof() of the field in the structure. For repeated fields this is for a single array entry.
array_sizeMaximum number of items in a statically allocated array.
typeType (pb_type_t) of the field.
pFieldPointer to the field storage in the structure.
pDataPointer to data contents. For arrays and pointers this can be different than pField.
pSizePointer to count or has field, or NULL if this field doesn't have such.
submsg_descFor submessage fields, points to the descriptor for the submessage.

By default pb_size_t is 16-bit, limiting the sizes and tags to 65535. The limit can be raised by defining PB_FIELD_32BIT.


An byte array with a field for storing the length:

typedef struct {
    pb_size_t size;
    pb_byte_t bytes[1];
} pb_bytes_array_t;

In an actual array, the length of bytes may be different. The macros PB_BYTES_ARRAY_T() and PB_BYTES_ARRAY_T_ALLOCSIZE() are used to allocate variable length storage for bytes fields.


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_iter_t *field, void **arg);
        bool (*encode)(pb_ostream_t *stream, const pb_field_iter_t *field, void * const *arg);
    } funcs;

    void *arg;

A pointer to the arg is passed to the callback when calling. It can be used to store any information that the callback might need. Note that this is a double pointer. If you set field.arg to point to &data in your main code, in the callback you can access it like this:

myfunction(*arg);           /* Gives pointer to data as argument */
myfunction(*(data_t*)*arg); /* Gives value of data as argument */
*arg = newdata;             /* Alters value of field.arg in structure */

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.


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;


Defines the handler functions and auxiliary data for a field that extends another message. Usually autogenerated by

typedef struct {
    bool (*decode)(pb_istream_t *stream, pb_extension_t *extension,
               uint32_t tag, pb_wire_type_t wire_type);
    bool (*encode)(pb_ostream_t *stream, const pb_extension_t *extension);
    const void *arg;
} pb_extension_type_t;

In the normal case, the function pointers are NULL and the decoder and encoder use their internal implementations. The internal implementations assume that arg points to a pb_field_iter_t that describes the field in question.

To implement custom processing of unknown fields, you can provide pointers to your own functions. Their functionality is mostly the same as for normal callback fields, except that they get called for any unknown field when decoding.


Ties together the extension field type and the storage for the field value. For message structs that have extensions, the generator will add a pb_extension_t* field. It should point to a linked list of extensions.

typedef struct {
    const pb_extension_type_t *type;
    void *dest;
    pb_extension_t *next;
    bool found;
} pb_extension_t;
typePointer to the structure that defines the callback functions.
destPointer to the variable that stores the field value (as used by the default extension callback functions.)
nextPointer to the next extension handler, or NULL for last handler.
foundDecoder sets this to true if the extension was found.


Get the current error message from a stream, or a placeholder string if there is no error message:

#define PB_GET_ERROR(stream) (string expression)

This should be used for printing errors, for example:

if (!pb_decode(...))
    printf("Decode failed: %s\n", PB_GET_ERROR(stream));

The macro only returns pointers to constant strings (in code memory), so that there is no need to release the returned pointer.


Set the error message and return false:

#define PB_RETURN_ERROR(stream,msg) (sets error and returns false)

This should be used to handle error conditions inside nanopb functions and user callback functions:

if (error_condition)
    PB_RETURN_ERROR(stream, "something went wrong");

The msg parameter must be a constant string.


This macro generates the pb_msgdesc_t and associated arrays, based on a list of fields in X-macro format. :

#define PB_BIND(msgname, structname, width) ...
msgnameName of the message type. Expects msgname_FIELDLIST macro to exist.
structnameName of the C structure to bind to.
widthNumber of words per field descriptor, or AUTO to use minimum size possible.

This macro is automatically invoked inside the autogenerated .pb.c files. User code can also call it to bind message types with custom structures or class types.



Constructs an output stream for writing into a memory buffer. It uses an internal callback that stores the pointer in stream state field. :

pb_ostream_t pb_ostream_from_buffer(pb_byte_t *buf, size_t bufsize);
bufMemory buffer to write into.
bufsizeMaximum number of bytes to write.
returnsAn output stream.

After writing, you can check stream.bytes_written to find out how much valid data there is in the buffer. This should be passed as the message length on decoding side.


Writes data to an output stream. Always use this function, instead of trying to call stream callback manually. :

bool pb_write(pb_ostream_t *stream, const pb_byte_t *buf, size_t count);
streamOutput stream to write to.
bufPointer to buffer with the data to be written.
countNumber of bytes to write.
returnsTrue on success, false if maximum length is exceeded or an IO error happens.

NOTE: If an error happens, bytes_written is not incremented. Depending on the callback used, calling pb_write again after it has failed once may cause undefined behavior. Nanopb itself never does this, instead it returns the error to user application. The builtin pb_ostream_from_buffer is safe to call again after failed write.


Encodes the contents of a structure as a protocol buffers message and writes it to output stream. :

bool pb_encode(pb_ostream_t *stream, const pb_msgdesc_t *fields, const void *src_struct);
streamOutput stream to write to.
fieldsMessage descriptor, usually autogenerated.
src_structPointer to the message structure. Must match fields descriptor.
returnsTrue on success, false on any error condition. Error message is set to stream->errmsg.

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.


Encodes the message, with extended behavior set by flags:

bool pb_encode_ex(pb_ostream_t *stream, const pb_msgdesc_t *fields, const void *src_struct, unsigned int flags);
streamOutput stream to write to.
fieldsMessage descriptor, usually autogenerated.
src_structPointer to the message structure. Must match fields descriptor.
flagsExtended options, see below.
returnsTrue on success, false on any error condition. Error message is set to stream->errmsg.

The options that can be defined are:

  • PB_ENCODE_DELIMITED: Indicate the length of the message by prefixing with a varint-encoded length. Compatible with parseDelimitedFrom in Google's protobuf library.
  • PB_ENCODE_NULLTERMINATED: Indicate the length of the message by appending a zero tag value after it. Supported by nanopb decoder, but not by most other protobuf libraries.


Calculates the length of the encoded message.

bool pb_get_encoded_size(size_t *size, const pb_msgdesc_t *fields, const void *src_struct);
sizeCalculated size of the encoded message.
fieldsMessage descriptor, usually autogenerated.
src_structPointer to the data that will be serialized.
returnsTrue on success, false on detectable errors in field description or if a field encoder returns false.

Callback field encoders

The functions with names pb_encode_<datatype> 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_<datatype> functions repeatedly to write out values.

The tag of a field must be encoded first 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.

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.

See Google Protobuf Encoding Format Documentation for background information on the Protobuf wire format.


Starts a field in the Protocol Buffers binary format: encodes the field number and the wire type of the data.

bool pb_encode_tag(pb_ostream_t *stream, pb_wire_type_t wiretype, uint32_t field_number);
streamOutput stream to write to. 1-5 bytes will be written.
field_numberIdentifier for the field, defined in the .proto file. You can get it from field->tag.
returnsTrue on success, false on IO error.


Same as pb_encode_tag, except takes the parameters from a pb_field_iter_t structure.

bool pb_encode_tag_for_field(pb_ostream_t *stream, const pb_field_iter_t *field);
streamOutput stream to write to. 1-5 bytes will be written.
fieldField iterator for this field.
returnsTrue on success, false on IO error or unknown field type.

This function only considers the PB_LTYPE of the field. You can use it from your field callbacks, because the source generator writes correct LTYPE also for callback type fields.

Wire type mapping is as follows:

LTYPEsWire type


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_varint(pb_ostream_t *stream, uint64_t value);
streamOutput stream to write to. 1-10 bytes will be written.
valueValue to encode, cast to uint64_t.
returnsTrue on success, false on IO error.

NOTE: Value will be converted to uint64_t in the argument. To encode signed values, the argument should be cast to int64_t first for correct sign extension.


Encodes a signed integer in the zig-zagged format. Works for fields of type sint32 and sint64:

bool pb_encode_svarint(pb_ostream_t *stream, int64_t value);

(parameters are the same as for pb_encode_varint


Writes the length of a string as varint and then contents of the string. Works for fields of type bytes and string:

bool pb_encode_string(pb_ostream_t *stream, const pb_byte_t *buffer, size_t size);
streamOutput stream to write to.
bufferPointer to string data.
sizeNumber of bytes in the string. Pass strlen(s) for strings.
returnsTrue on success, false on IO error.


Writes 4 bytes to stream and swaps bytes on big-endian architectures. Works for fields of type fixed32, sfixed32 and float:

bool pb_encode_fixed32(pb_ostream_t *stream, const void *value);
streamOutput stream to write to. 4 bytes will be written.
valuePointer to a 4-bytes large C variable, for example uint32_t foo;.
returnsTrue on success, false on IO error.


Writes 8 bytes to stream and swaps bytes on big-endian architecture. Works for fields of type fixed64, sfixed64 and double:

bool pb_encode_fixed64(pb_ostream_t *stream, const void *value);
streamOutput stream to write to. 8 bytes will be written.
valuePointer to a 8-bytes large C variable, for example uint64_t foo;.
returnsTrue on success, false on IO error.


Encodes a 32-bit float value so that it appears like a 64-bit double in the encoded message. This is sometimes needed when platforms like AVR that do not support 64-bit double need to communicate using a message type that contains double fields.

bool pb_encode_float_as_double(pb_ostream_t *stream, float value);
streamOutput stream to write to. 8 bytes will be written.
valueFloat value to encode.
returnsTrue on success, false on IO error.


Encodes a submessage field, including the size header for it. Works for fields of any message type.

bool pb_encode_submessage(pb_ostream_t *stream, const pb_msgdesc_t *fields, const void *src_struct);
streamOutput stream to write to.
fieldsPointer to the autogenerated message descriptor for the submessage type, e.g. MyMessage_fields.
srcPointer to the structure where submessage data is.
returnsTrue 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 garbage will be written to the output before the problem is detected.



Helper function for creating an input stream that reads data from a memory buffer.

pb_istream_t pb_istream_from_buffer(const pb_byte_t *buf, size_t bufsize);
bufPointer to byte array to read from.
bufsizeSize of the byte array. Typically length of the message to be decoded.
returnsAn input stream ready to use.


Read data from input stream. Always use this function, don't try to call the stream callback directly.

bool pb_read(pb_istream_t *stream, pb_byte_t *buf, size_t count);
streamInput stream to read from.
bufBuffer to store the data to, or NULL to just read data without storing it anywhere.
countNumber of bytes to read.
returnsTrue on success, false if stream->bytes_left is less than count or if an IO error occurs.

End of file is signalled by stream->bytes_left being zero after pb_read returns false.


Read and decode all fields of a structure. Reads until EOF on input stream.

bool pb_decode(pb_istream_t *stream, const pb_msgdesc_t *fields, void *dest_struct);
streamInput stream to read from.
fieldsMessage descriptor, usually autogenerated.
dest_structPointer to message structure where data will be stored.
returnsTrue on success, false on any error condition. Error message will be in stream->errmsg.

In Protocol Buffers binary format, end-of-file 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.

For optional fields, this function applies the default value and sets has_<field> to false if the field is not present.

If PB_ENABLE_MALLOC is defined, this function may allocate storage for any pointer type fields. In this case, you have to call pb_release to release the memory after you are done with the message. On error return pb_decode will release the memory itself.


Same as pb_decode, but allows extended options.

bool pb_decode_ex(pb_istream_t *stream, const pb_msgdesc_t *fields, void *dest_struct, unsigned int flags);
streamInput stream to read from.
fieldsMessage descriptor, usually autogenerated.
dest_structPointer to message structure where data will be stored.
flagsExtended options, see below
returnsTrue on success, false on any error condition. Error message will be in stream->errmsg.

The following options can be defined and combined with bitwise | operator:

  • PB_DECODE_NOINIT: Do not initialize structure before decoding. This can be used to combine multiple messages, or if you have already initialized the message structure yourself.

  • PB_DECODE_DELIMITED: Expect a length prefix in varint format before message. The counterpart of PB_ENCODE_DELIMITED.

  • PB_DECODE_NULLTERMINATED: Expect the message to be terminated with zero tag. The counterpart of PB_ENCODE_NULLTERMINATED.

If PB_ENABLE_MALLOC is defined, this function may allocate storage for any pointer type fields. In this case, you have to call pb_release to release the memory after you are done with the message. On error return pb_decode_ex will release the memory itself.


Releases any dynamically allocated fields:

void pb_release(const pb_msgdesc_t *fields, void *dest_struct);
fieldsMessage descriptor, usually autogenerated.
dest_structPointer to structure where data is stored. If NULL, function does nothing.

This function is only available if PB_ENABLE_MALLOC is defined. It will release any pointer type fields in the structure and set the pointers to NULL.

This function is safe to call multiple times, calling it again does nothing.


Decode the tag that comes before field in the protobuf encoding:

bool pb_decode_tag(pb_istream_t *stream, pb_wire_type_t *wire_type, uint32_t *tag, bool *eof);
streamInput stream to read from.
wire_typePointer to variable where to store the wire type of the field.
tagPointer to variable where to store the tag of the field.
eofPointer to variable where to store end-of-file status.
returnsTrue on success, false on error or EOF.

When the message (stream) ends, this function will return false and set eof to true. On other errors, eof will be set to false.


Remove the data for a field from the stream, without actually decoding it:

bool pb_skip_field(pb_istream_t *stream, pb_wire_type_t wire_type);
streamInput stream to read from.
wire_typeType of field to skip.
returnsTrue on success, false on IO error.

This function determines the amount of bytes to read based on the wire type. For PB_WT_STRING, it will read the length prefix of a string or submessage to determine its length.

Callback field decoders

The functions with names pb_decode_<datatype> 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_decode will call your callback function repeatedly, which can then store the values into e.g. filesystem in the order received in.

For decoding numeric (including enumerated and boolean) values, use pb_decode_varint, pb_decode_svarint, pb_decode_fixed32 and pb_decode_fixed64. They take a pointer to a 32- or 64-bit C variable, which you may then cast to smaller datatype for storage.

For decoding strings and bytes fields, the length has already been decoded and the callback function is given a length-limited substream. You can therefore check the total length in stream->bytes_left and read the data using pb_read.

Finally, for decoding submessages in a callback, use pb_decode and pass it the SubMessage_fields descriptor array.


Read and decode a varint encoded integer.

bool pb_decode_varint(pb_istream_t *stream, uint64_t *dest);
streamInput stream to read from. 1-10 bytes will be read.
destStorage for the decoded integer. Value is undefined on error.
returnsTrue on success, false if value exceeds uint64_t range or an IO error happens.


Same as pb_decode_varint, but limits the value to 32 bits:

bool pb_decode_varint32(pb_istream_t *stream, uint32_t *dest);

Parameters are the same as pb_decode_varint. This function can be used for decoding lengths and other commonly occurring elements that you know shouldn't be larger than 32 bit. It will return an error if the value exceeds the uint32_t datatype.


Similar to pb_decode_varint, except that it performs zigzag-decoding on the value. This corresponds to the Protocol Buffers sint32 and sint64 datatypes. :

bool pb_decode_svarint(pb_istream_t *stream, int64_t *dest);

(parameters are the same as pb_decode_varint)


Decode a fixed32, sfixed32 or float value.

bool pb_decode_fixed32(pb_istream_t *stream, void *dest);
streamInput stream to read from. 4 bytes will be read.
destPointer to destination int32_t, uint32_t or float.
returnsTrue on success, false on IO errors.

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.


Decode a fixed64, sfixed64 or double value. :

bool pb_decode_fixed64(pb_istream_t *stream, void *dest);
streamInput stream to read from. 8 bytes will be read.
destPointer to destination int64_t, uint64_t or double.
returnsTrue on success, false on IO errors.

Same as pb_decode_fixed32, except this reads 8 bytes.


Decodes a 64-bit double value into a 32-bit float variable. Counterpart of pb_encode_float_as_double. :

bool pb_decode_double_as_float(pb_istream_t *stream, float *dest);
streamInput stream to read from. 8 bytes will be read.
destPointer to destination float.
returnsTrue on success, false on IO errors.


Decode the length for a field with wire type PB_WT_STRING and create a substream for reading the data.

bool pb_make_string_substream(pb_istream_t *stream, pb_istream_t *substream);
streamOriginal input stream to read the length and data from.
substreamStorage for a new substream that has limited length. Filled in by the function.
returnsTrue on success, false if reading the length fails.

This function uses pb_decode_varint to read an integer from the stream. This is interpreted as a number of bytes, and the substream is set up so that its bytes_left is initially the same as the length, and its callback function and state the same as the parent stream.


Close the substream created with pb_make_string_substream.

void pb_close_string_substream(pb_istream_t *stream, pb_istream_t *substream);
streamOriginal input stream to read data from.
substreamSubstream to close

This function copies back the state from the substream to the parent stream, and throws away any unread data from the substream. It must be called after done with the substream.



Begins iterating over the fields in a message type:

bool pb_field_iter_begin(pb_field_iter_t *iter, const pb_msgdesc_t *desc, void *message);
iterPointer to destination pb_field_iter_t variable.
descAutogenerated message descriptor.
messagePointer to message structure.
returnsTrue on success, false if the message type has no fields.


Advance to the next field in the message:

bool pb_field_iter_next(pb_field_iter_t *iter);
iterPointer to pb_field_iter_t previously initialized by pb_field_iter_begin.
returnsTrue on success, false after last field in the message.

When the last field in the message has been processed, this function will return false and initialize iter back to the first field in the message.


Find a field specified by tag number in the message:

bool pb_field_iter_find(pb_field_iter_t *iter, uint32_t tag);
iterPointer to pb_field_iter_t previously initialized by pb_field_iter_begin.
tagTag number to search for.
returnsTrue if field was found, false otherwise.

This function is functionally identical to calling pb_field_iter_next() until iter.tag equals the searched value. Internally this function avoids fully processing the descriptor for intermediate fields.


Validates an UTF8 encoded string:

bool pb_validate_utf8(const char *s);
sPointer to beginning of a string.
returnsTrue, if string is valid UTF-8, false otherwise.

The protobuf standard requires that string fields only contain valid UTF-8 encoded text, while bytes fields can contain arbitrary data. When the compilation option PB_VALIDATE_UTF8 is defined, nanopb will automatically validate strings on both encoding and decoding.

User code can call this function to validate strings in e.g. custom callbacks.