Scalar types

Overview

The Protocol Buffers specification defines a multitude of signed and unsigned integer types (each with different implications for the underlying wire representation), as well as types for the representation of floating point numbers, booleans, bytes and strings.

Type mappings

The following table shows the type mappings from C types (both native and custom types like pb_string_t) to Protocol Buffer schema types:

Native type	Schema type
int32_t	int32, sint32, fixed32
int64_t	int64, sint64, fixed64
uint8_t	bool
uint32_t	uint32, fixed32
uint64_t	uint64, fixed64
float	float
double	double
pb_enum_t	enum
pb_string_t	bytes, string

See the Protocol Buffers specification for more information on types.

Strings and byte arrays

While the mappings of integer and floating point types should be quite self-explanatory, strings are worth discussing: in wire representation, strings and byte arrays are stored as a length-prefixed, unterminated list of unsigned chars. Because there's no nul-terminator, the length needs to be explicitly returned. This is what the datatype pb_string_t is for. The header file core/string.h defines a few useful functions for working with strings:

/* Initialize a string with explicit length */
pb_string_t string = pb_string_init("TEST", 4);

/* Initialize a string from a nul-terminated array of unsigned chars */
pb_string_t string = pb_string_init_from_chars("TEST");

/* Initialize an empty string - may be useful at times */
pb_string_t string = pb_string_init_empty();

/* Retrieve the underlying data and size of a string */
const uint8_t *data = pb_string_data(&string);
const size_t   size = pb_string_size(&string);

Strings are always allocated on the stack and do not take ownership of the provided array of unsigned chars, so there's no need for freeing them, but the caller must ensure that the array is not freed during operations.