14 The following options can be specified in one of two ways:
16 1. Using the -D switch on the C compiler command line.
17 2. By #defining them at the top of pb.h.
19 You must have the same settings for the nanopb library and all code that
22 ============================ ================================================
23 __BIG_ENDIAN__ Set this if your platform stores integers and
24 floats in big-endian format. Mixed-endian
25 systems (different layout for ints and floats)
26 are currently not supported.
27 NANOPB_INTERNALS Set this to expose the field encoder functions
28 that are hidden since nanopb-0.1.3. Starting
29 with nanopb-0.2.4, this flag does nothing. Use
30 the newer functions that have better interface.
31 PB_MAX_REQUIRED_FIELDS Maximum number of required fields to check for
32 presence. Default value is 64. Increases stack
33 usage 1 byte per every 8 fields. Compiler
34 warning will tell if you need this.
35 PB_FIELD_16BIT Add support for tag numbers > 255 and fields
36 larger than 255 bytes or 255 array entries.
37 Increases code size 3 bytes per each field.
38 Compiler error will tell if you need this.
39 PB_FIELD_32BIT Add support for tag numbers > 65535 and fields
40 larger than 65535 bytes or 65535 array entries.
41 Increases code size 9 bytes per each field.
42 Compiler error will tell if you need this.
43 PB_NO_ERRMSG Disables the support for error messages; only
44 error information is the true/false return
45 value. Decreases the code size by a few hundred
47 PB_BUFFER_ONLY Disables the support for custom streams. Only
48 supports encoding and decoding with memory
49 buffers. Speeds up execution and decreases code
51 PB_OLD_CALLBACK_STYLE Use the old function signature (void\* instead
52 of void\*\*) for callback fields. This was the
53 default until nanopb-0.2.1.
54 PB_SYSTEM_HEADER Replace the standard header files with a single
55 header file. It should define all the required
56 functions and typedefs listed on the
57 `overview page`_. Value must include quotes,
58 for example *#define PB_SYSTEM_HEADER "foo.h"*.
59 ============================ ================================================
61 The PB_MAX_REQUIRED_FIELDS, PB_FIELD_16BIT and PB_FIELD_32BIT settings allow
62 raising some datatype limits to suit larger messages. Their need is recognized
63 automatically by C-preprocessor #if-directives in the generated .pb.h files.
64 The default setting is to use the smallest datatypes (least resources used).
66 .. _`overview page`: index.html#compiler-requirements
71 The generator behaviour can be adjusted using these options, defined in the
72 'nanopb.proto' file in the generator folder:
74 ============================ ================================================
75 max_size Allocated size for *bytes* and *string* fields.
76 max_count Allocated number of entries in arrays
78 type Type of the generated field. Default value
79 is *FT_DEFAULT*, which selects automatically.
80 You can use *FT_CALLBACK*, *FT_STATIC* or
81 *FT_IGNORE* to force a callback field, a static
82 field or to completely ignore the field.
83 long_names Prefix the enum name to the enum value in
84 definitions, i.e. *EnumName_EnumValue*. Enabled
86 packed_struct Make the generated structures packed.
87 NOTE: This cannot be used on CPUs that break
88 on unaligned accesses to variables.
89 ============================ ================================================
91 These options can be defined for the .proto files before they are converted
92 using the nanopb-generatory.py. There are three ways to define the options:
94 1. Using a separate .options file.
95 This is the preferred way as of nanopb-0.2.1, because it has the best
96 compatibility with other protobuf libraries.
97 2. Defining the options on the command line of nanopb_generator.py.
98 This only makes sense for settings that apply to a whole file.
99 3. Defining the options in the .proto file using the nanopb extensions.
100 This is the way used in nanopb-0.1, and will remain supported in the
101 future. It however sometimes causes trouble when using the .proto file
102 with other protobuf libraries.
104 The effect of the options is the same no matter how they are given. The most
105 common purpose is to define maximum size for string fields in order to
106 statically allocate them.
108 Defining the options in a .options file
109 ---------------------------------------
110 The preferred way to define options is to have a separate file
111 'myproto.options' in the same directory as the 'myproto.proto'. ::
115 required string name = 1;
116 repeated int32 ids = 4;
122 MyMessage.name max_size:40
123 MyMessage.ids max_count:5
125 The generator will automatically search for this file and read the
126 options from it. The file format is as follows:
128 * Lines starting with '#' or '//' are regarded as comments.
129 * Blank lines are ignored.
130 * All other lines should start with a field name pattern, followed by one or
131 more options. For example: *"MyMessage.myfield max_size:5 max_count:10"*.
132 * The field name pattern is matched against a string of form *'Message.field'*.
133 For nested messages, the string is *'Message.SubMessage.field'*.
134 * The field name pattern may use the notation recognized by Python fnmatch():
136 - *\** matches any part of string, like 'Message.\*' for all fields
137 - *\?* matches any single character
138 - *[seq]* matches any of characters 's', 'e' and 'q'
139 - *[!seq]* matches any other character
141 * The options are written as *'option_name:option_value'* and several options
142 can be defined on same line, separated by whitespace.
143 * Options defined later in the file override the ones specified earlier, so
144 it makes sense to define wildcard options first in the file and more specific
147 If preferred, the name of the options file can be set using the command line
148 switch *-f* to nanopb_generator.py.
150 Defining the options on command line
151 ------------------------------------
152 The nanopb_generator.py has a simple command line option *-s OPTION:VALUE*.
153 The setting applies to the whole file that is being processed.
155 Defining the options in the .proto file
156 ---------------------------------------
157 The .proto file format allows defining custom options for the fields.
158 The nanopb library comes with *nanopb.proto* which does exactly that, allowing
159 you do define the options directly in the .proto file::
161 import "nanopb.proto";
164 required string name = 1 [(nanopb).max_size = 40];
165 repeated int32 ids = 4 [(nanopb).max_count = 5];
168 A small complication is that you have to set the include path of protoc so that
169 nanopb.proto can be found. This file, in turn, requires the file
170 *google/protobuf/descriptor.proto*. This is usually installed under
171 */usr/include*. Therefore, to compile a .proto file which uses options, use a
172 protoc command similar to::
174 protoc -I/usr/include -Inanopb/generator -I. -omessage.pb message.proto
176 The options can be defined in file, message and field scopes::
178 option (nanopb_fileopt).max_size = 20; // File scope
181 option (nanopb_msgopt).max_size = 30; // Message scope
182 required string fieldsize = 1 [(nanopb).max_size = 40]; // Field scope
198 Defines the encoder/decoder behaviour that should be used for a field. ::
200 typedef uint8_t pb_type_t;
202 The low-order nibble of the enumeration values defines the function that can be used for encoding and decoding the field data:
204 ==================== ===== ================================================
205 LTYPE identifier Value Storage format
206 ==================== ===== ================================================
207 PB_LTYPE_VARINT 0x00 Integer.
208 PB_LTYPE_SVARINT 0x01 Integer, zigzag encoded.
209 PB_LTYPE_FIXED32 0x02 32-bit integer or floating point.
210 PB_LTYPE_FIXED64 0x03 64-bit integer or floating point.
211 PB_LTYPE_BYTES 0x04 Structure with *size_t* field and byte array.
212 PB_LTYPE_STRING 0x05 Null-terminated string.
213 PB_LTYPE_SUBMESSAGE 0x06 Submessage structure.
214 ==================== ===== ================================================
216 The bits 4-5 define whether the field is required, optional or repeated:
218 ==================== ===== ================================================
219 HTYPE identifier Value Field handling
220 ==================== ===== ================================================
221 PB_HTYPE_REQUIRED 0x00 Verify that field exists in decoded message.
222 PB_HTYPE_OPTIONAL 0x10 Use separate *has_<field>* boolean to specify
223 whether the field is present.
224 (Unless it is a callback)
225 PB_HTYPE_REPEATED 0x20 A repeated field with preallocated array.
226 Separate *<field>_count* for number of items.
227 (Unless it is a callback)
228 ==================== ===== ================================================
230 The bits 6-7 define the how the storage for the field is allocated:
232 ==================== ===== ================================================
233 ATYPE identifier Value Allocation method
234 ==================== ===== ================================================
235 PB_ATYPE_STATIC 0x00 Statically allocated storage in the structure.
236 PB_ATYPE_CALLBACK 0x40 A field with dynamic storage size. Struct field
237 actually contains a pointer to a callback
239 ==================== ===== ================================================
244 Describes a single structure field with memory position in relation to others. The descriptions are usually autogenerated. ::
246 typedef struct _pb_field_t pb_field_t;
257 :tag: Tag number of the field or 0 to terminate a list of fields.
258 :type: LTYPE, HTYPE and ATYPE of the field.
259 :data_offset: Offset of field data, relative to the end of the previous field.
260 :size_offset: Offset of *bool* flag for optional fields or *size_t* count for arrays, relative to field data.
261 :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.
262 :array_size: Maximum number of entries in an array, if it is an array type.
263 :ptr: Pointer to default value for optional fields, or to submessage description for PB_LTYPE_SUBMESSAGE.
265 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_FIELD_16BIT*.
269 An byte array with a field for storing the length::
276 In an actual array, the length of *bytes* may be different.
280 Part of a message structure, for fields with type PB_HTYPE_CALLBACK::
282 typedef struct _pb_callback_t pb_callback_t;
283 struct _pb_callback_t {
285 bool (*decode)(pb_istream_t *stream, const pb_field_t *field, void **arg);
286 bool (*encode)(pb_ostream_t *stream, const pb_field_t *field, void * const *arg);
292 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.
294 Previously the function received just the value of *arg* instead of a pointer to it. This old behaviour can be enabled by defining *PB_OLD_CALLBACK_STYLE*.
296 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.
300 Protocol Buffers wire types. These are used with `pb_encode_tag`_. ::
311 Defines the handler functions and auxiliary data for a field that extends
312 another message. Usually autogenerated by *nanopb_generator.py*::
315 bool (*decode)(pb_istream_t *stream, pb_extension_t *extension,
316 uint32_t tag, pb_wire_type_t wire_type);
317 bool (*encode)(pb_ostream_t *stream, const pb_extension_t *extension);
319 } pb_extension_type_t;
321 In the normal case, the function pointers are *NULL* and the decoder and
322 encoder use their internal implementations. The internal implementations
323 assume that *arg* points to a *pb_field_t* that describes the field in question.
325 To implement custom processing of unknown fields, you can provide pointers
326 to your own functions. Their functionality is mostly the same as for normal
327 callback fields, except that they get called for any unknown field when decoding.
331 Ties together the extension field type and the storage for the field value::
334 const pb_extension_type_t *type;
336 pb_extension_t *next;
339 :type: Pointer to the structure that defines the callback functions.
340 :dest: Pointer to the variable that stores the field value
341 (as used by the default extension callback functions.)
342 :next: Pointer to the next extension handler, or *NULL*.
346 Get the current error message from a stream, or a placeholder string if
347 there is no error message::
349 #define PB_GET_ERROR(stream) (string expression)
351 This should be used for printing errors, for example::
355 printf("Decode failed: %s\n", PB_GET_ERROR(stream));
358 The macro only returns pointers to constant strings (in code memory),
359 so that there is no need to release the returned pointer.
363 Set the error message and return false::
365 #define PB_RETURN_ERROR(stream,msg) (sets error and returns false)
367 This should be used to handle error conditions inside nanopb functions
368 and user callback functions::
372 PB_RETURN_ERROR(stream, "something went wrong");
375 The *msg* parameter must be a constant string.
382 pb_ostream_from_buffer
383 ----------------------
384 Constructs an output stream for writing into a memory buffer. This is just a helper function, it doesn't do anything you couldn't do yourself in a callback function. It uses an internal callback that stores the pointer in stream *state* field. ::
386 pb_ostream_t pb_ostream_from_buffer(uint8_t *buf, size_t bufsize);
388 :buf: Memory buffer to write into.
389 :bufsize: Maximum number of bytes to write.
390 :returns: An output stream.
392 After writing, you can check *stream.bytes_written* to find out how much valid data there is in the buffer.
396 Writes data to an output stream. Always use this function, instead of trying to call stream callback manually. ::
398 bool pb_write(pb_ostream_t *stream, const uint8_t *buf, size_t count);
400 :stream: Output stream to write to.
401 :buf: Pointer to buffer with the data to be written.
402 :count: Number of bytes to write.
403 :returns: True on success, false if maximum length is exceeded or an IO error happens.
405 If an error happens, *bytes_written* is not incremented. Depending on the callback used, calling pb_write again after it has failed once may be dangerous. 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.
409 Encodes the contents of a structure as a protocol buffers message and writes it to output stream. ::
411 bool pb_encode(pb_ostream_t *stream, const pb_field_t fields[], const void *src_struct);
413 :stream: Output stream to write to.
414 :fields: A field description array, usually autogenerated.
415 :src_struct: Pointer to the data that will be serialized.
416 :returns: True on success, false on IO error, on detectable errors in field description, or if a field encoder returns false.
418 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.
420 .. sidebar:: Encoding fields manually
422 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.
424 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.
426 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.
430 Starts a field in the Protocol Buffers binary format: encodes the field number and the wire type of the data. ::
432 bool pb_encode_tag(pb_ostream_t *stream, pb_wire_type_t wiretype, int field_number);
434 :stream: Output stream to write to. 1-5 bytes will be written.
435 :wiretype: PB_WT_VARINT, PB_WT_64BIT, PB_WT_STRING or PB_WT_32BIT
436 :field_number: Identifier for the field, defined in the .proto file. You can get it from field->tag.
437 :returns: True on success, false on IO error.
439 pb_encode_tag_for_field
440 -----------------------
441 Same as `pb_encode_tag`_, except takes the parameters from a *pb_field_t* structure. ::
443 bool pb_encode_tag_for_field(pb_ostream_t *stream, const pb_field_t *field);
445 :stream: Output stream to write to. 1-5 bytes will be written.
446 :field: Field description structure. Usually autogenerated.
447 :returns: True on success, false on IO error or unknown field type.
449 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.
451 Wire type mapping is as follows:
453 ========================= ============
455 ========================= ============
456 VARINT, SVARINT PB_WT_VARINT
458 STRING, BYTES, SUBMESSAGE PB_WT_STRING
460 ========================= ============
464 Encodes a signed or unsigned integer in the varint_ format. Works for fields of type `bool`, `enum`, `int32`, `int64`, `uint32` and `uint64`::
466 bool pb_encode_varint(pb_ostream_t *stream, uint64_t value);
468 :stream: Output stream to write to. 1-10 bytes will be written.
469 :value: Value to encode. Just cast e.g. int32_t directly to uint64_t.
470 :returns: True on success, false on IO error.
472 .. _varint: http://code.google.com/apis/protocolbuffers/docs/encoding.html#varints
476 Encodes a signed integer in the 'zig-zagged' format. Works for fields of type `sint32` and `sint64`::
478 bool pb_encode_svarint(pb_ostream_t *stream, int64_t value);
480 (parameters are the same as for `pb_encode_varint`_
484 Writes the length of a string as varint and then contents of the string. Works for fields of type `bytes` and `string`::
486 bool pb_encode_string(pb_ostream_t *stream, const uint8_t *buffer, size_t size);
488 :stream: Output stream to write to.
489 :buffer: Pointer to string data.
490 :size: Number of bytes in the string. Pass `strlen(s)` for strings.
491 :returns: True on success, false on IO error.
495 Writes 4 bytes to stream and swaps bytes on big-endian architectures. Works for fields of type `fixed32`, `sfixed32` and `float`::
497 bool pb_encode_fixed32(pb_ostream_t *stream, const void *value);
499 :stream: Output stream to write to.
500 :value: Pointer to a 4-bytes large C variable, for example `uint32_t foo;`.
501 :returns: True on success, false on IO error.
505 Writes 8 bytes to stream and swaps bytes on big-endian architecture. Works for fields of type `fixed64`, `sfixed64` and `double`::
507 bool pb_encode_fixed64(pb_ostream_t *stream, const void *value);
509 :stream: Output stream to write to.
510 :value: Pointer to a 8-bytes large C variable, for example `uint64_t foo;`.
511 :returns: True on success, false on IO error.
515 Encodes a submessage field, including the size header for it. Works for fields of any message type::
517 bool pb_encode_submessage(pb_ostream_t *stream, const pb_field_t fields[], const void *src_struct);
519 :stream: Output stream to write to.
520 :fields: Pointer to the autogenerated field description array for the submessage type, e.g. `MyMessage_fields`.
521 :src: Pointer to the structure where submessage data is.
522 :returns: True on success, false on IO errors, pb_encode errors or if submessage size changes between calls.
524 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.
526 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.
542 pb_istream_from_buffer
543 ----------------------
544 Helper function for creating an input stream that reads data from a memory buffer. ::
546 pb_istream_t pb_istream_from_buffer(uint8_t *buf, size_t bufsize);
548 :buf: Pointer to byte array to read from.
549 :bufsize: Size of the byte array.
550 :returns: An input stream ready to use.
554 Read data from input stream. Always use this function, don't try to call the stream callback directly. ::
556 bool pb_read(pb_istream_t *stream, uint8_t *buf, size_t count);
558 :stream: Input stream to read from.
559 :buf: Buffer to store the data to, or NULL to just read data without storing it anywhere.
560 :count: Number of bytes to read.
561 :returns: True on success, false if *stream->bytes_left* is less than *count* or if an IO error occurs.
563 End of file is signalled by *stream->bytes_left* being zero after pb_read returns false.
567 Read and decode all fields of a structure. Reads until EOF on input stream. ::
569 bool pb_decode(pb_istream_t *stream, const pb_field_t fields[], void *dest_struct);
571 :stream: Input stream to read from.
572 :fields: A field description array. Usually autogenerated.
573 :dest_struct: Pointer to structure where data will be stored.
574 :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.
576 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.
578 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.
580 For optional fields, this function applies the default value and sets *has_<field>* to false if the field is not present.
584 Same as `pb_decode`_, except does not apply the default values to fields. ::
586 bool pb_decode_noinit(pb_istream_t *stream, const pb_field_t fields[], void *dest_struct);
588 (parameters are the same as for `pb_decode`_.)
590 The destination structure should be filled with zeros before calling this function. Doing a *memset* manually can be slightly faster than using `pb_decode`_ if you don't need any default values.
594 Skip a varint_ encoded integer without decoding it. ::
596 bool pb_skip_varint(pb_istream_t *stream);
598 :stream: Input stream to read from. Will read 1 byte at a time until the MSB is clear.
599 :returns: True on success, false on IO error.
603 Skip a varint-length-prefixed string. This means skipping a value with wire type PB_WT_STRING. ::
605 bool pb_skip_string(pb_istream_t *stream);
607 :stream: Input stream to read from.
608 :returns: True on success, false on IO error or length exceeding uint32_t.
612 Decode the tag that comes before field in the protobuf encoding::
614 bool pb_decode_tag(pb_istream_t *stream, pb_wire_type_t *wire_type, int *tag, bool *eof);
616 :stream: Input stream to read from.
617 :wire_type: Pointer to variable where to store the wire type of the field.
618 :tag: Pointer to variable where to store the tag of the field.
619 :eof: Pointer to variable where to store end-of-file status.
620 :returns: True on success, false on error or EOF.
622 When the message (stream) ends, this function will return false and set *eof* to true. On other
623 errors, *eof* will be set to false.
627 Remove the data for a field from the stream, without actually decoding it::
629 bool pb_skip_field(pb_istream_t *stream, pb_wire_type_t wire_type);
631 :stream: Input stream to read from.
632 :wire_type: Type of field to skip.
633 :returns: True on success, false on IO error.
635 .. sidebar:: Decoding fields manually
637 The functions with names beginning with *pb_decode_* 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.
639 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.
641 For decoding strings and bytes fields, the length has already been decoded. You can therefore check the total length in *stream->bytes_left* and read the data using `pb_read`_.
643 Finally, for decoding submessages in a callback, simply use `pb_decode`_ and pass it the *SubMessage_fields* descriptor array.
647 Read and decode a varint_ encoded integer. ::
649 bool pb_decode_varint(pb_istream_t *stream, uint64_t *dest);
651 :stream: Input stream to read from. 1-10 bytes will be read.
652 :dest: Storage for the decoded integer. Value is undefined on error.
653 :returns: True on success, false if value exceeds uint64_t range or an IO error happens.
657 Similar to `pb_decode_varint`_, except that it performs zigzag-decoding on the value. This corresponds to the Protocol Buffers *sint32* and *sint64* datatypes. ::
659 bool pb_decode_svarint(pb_istream_t *stream, int64_t *dest);
661 (parameters are the same as `pb_decode_varint`_)
665 Decode a *fixed32*, *sfixed32* or *float* value. ::
667 bool pb_decode_fixed32(pb_istream_t *stream, void *dest);
669 :stream: Input stream to read from. 4 bytes will be read.
670 :dest: Pointer to destination *int32_t*, *uint32_t* or *float*.
671 :returns: True on success, false on IO errors.
673 This function reads 4 bytes from the input stream.
674 On big endian architectures, it then reverses the order of the bytes.
675 Finally, it writes the bytes to *dest*.
679 Decode a *fixed64*, *sfixed64* or *double* value. ::
681 bool pb_dec_fixed(pb_istream_t *stream, const pb_field_t *field, void *dest);
683 :stream: Input stream to read from. 8 bytes will be read.
685 :dest: Pointer to destination *int64_t*, *uint64_t* or *double*.
686 :returns: True on success, false on IO errors.
688 Same as `pb_decode_fixed32`_, except this reads 8 bytes.
690 pb_make_string_substream
691 ------------------------
692 Decode the length for a field with wire type *PB_WT_STRING* and create a substream for reading the data. ::
694 bool pb_make_string_substream(pb_istream_t *stream, pb_istream_t *substream);
696 :stream: Original input stream to read the length and data from.
697 :substream: New substream that has limited length. Filled in by the function.
698 :returns: True on success, false if reading the length fails.
700 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.
702 pb_close_string_substream
703 -------------------------
704 Close the substream created with `pb_make_string_substream`_. ::
706 void pb_close_string_substream(pb_istream_t *stream, pb_istream_t *substream);
708 :stream: Original input stream to read the length and data from.
709 :substream: Substream to close
711 This function copies back the state from the substream to the parent stream.
712 It must be called after done with the substream.