+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.
+
+**Example:**
+
+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 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.
+
+=============================================================================== =======================
+ 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;
+ | uint8_t bytes[40];
+ | } Person_data_t;
+ | Person_data_t data;
+=============================================================================== =======================
+
+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.
+
+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.
+
+.. _`pb_callback_t`: reference.html#pb-callback-t
+
+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.
+
+.. _`pb_encode`: reference.html#pb-encode
+.. _`pb_encode_tag_for_field`: reference.html#pb-encode-tag-for-field
+.. _`pb_encode_tag`: reference.html#pb-encode-tag
+
+This callback writes out a dynamically sized string::
+
+ bool write_string(pb_ostream_t *stream, const pb_field_t *field, 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.
+
+.. _`pb_decode`: reference.html#pb-decode
+
+This callback reads multiple integers and prints them::
+
+ bool read_ints(pb_istream_t *stream, const pb_field_t *field, void **arg)
+ {
+ while (stream->bytes_left)
+ {
+ uint64_t value;
+ if (!pb_decode_varint(stream, &value))
+ return false;
+ printf("%lld\n", value);
+ }
+ return true;
+ }
+
+Field description array
+=======================
+
+For using the *pb_encode* and *pb_decode* functions, you need an array of pb_field_t constants describing the structure you wish to encode. This description is usually autogenerated from .proto file.
+
+For example this submessage in the Person.proto file::
+
+ message Person {
+ message PhoneNumber {
+ required string number = 1 [(nanopb).max_size = 40];
+ optional PhoneType type = 2 [default = HOME];
+ }
+ }
+
+generates this field description array for the structure *Person_PhoneNumber*::
+
+ const pb_field_t Person_PhoneNumber_fields[3] = {
+ 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),
+ PB_LAST_FIELD
+ };
+
+
+Return values and error handling
+================================
+
+Most functions in nanopb return bool: *true* means success, *false* means failure. There is also some support for error messages for debugging purposes: the error messages go in *stream->errmsg*.