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Default Values · protobuf-c/protobuf-c Wiki · GitHub

Default value handling in protobuf-c.

Default values are allowed for required and optional fields. This document describes their operational semantics.

The default value is used for initializing a fresh message. So the MESSAGE__INIT macro and message__init() function set the default values.

A required field is always serialized, even if it is set to the default value.

Strings and bytes have their default values stored as globals; occasionally you can use this to see if the value is still the default. For example,

message Foo {
  required string a = 1 [default = "blah"];
  required bytes b  = 2 [default = "\0 char"];
}

will yield some exposed globals in the generated C code (from the generated header file):

extern char foo__a__default_value[];
extern uint8_t foo_b__default_value_data[]
#define FOO__INIT { \
         PROTOBUF_C_MESSAGE_INIT(...), \
         foo__a__default_value, { 6, foo_b__default_value_data } }

So if you want to see if a is not set to the default value (for example, you may wish to free only non-default values), use:

if (foo.a != foo__a__default_value)
  /* handle non-default value */

Once again, the value is used for initializing a fresh instance, and it can be seen if you deserialize a message missing an optional field.

Except for string and messages, all optional fields have a corresponding has_field member of the generated C struct. In the case of being set to a default value, the has_field member will NOT be set, but the "field" member itself will be set to the default value. Such values will only be serialized if the has_field flag is set.

So, if you had a message like:

message Foo {
  optional int32 a = 1 [default = 42];
}

then the corresponding ___INIT macro would be:

#define FOO__INIT { \
         PROTOBUF_C_MESSAGE_INIT(...), \
         0,       /* !has_a */ \
         42       /* a */ \
       }

If you pack the default value of this message, it will be 0 bytes. To force it to pack the value of a, use code like:

Foo foo = FOO__INIT;
foo.has_a = 1;
/* pack foo */

For backwards compatibility, the situation with strings is more annoying. You see, strings have no corresponding has_field member, so the situation must be inferred from the string itself. If the string is NULL or is set to the default-value, it will not be serialized.

To determine if a string is set (and will therefore be serialized), use code like:

if (foo.a != NULL && foo.a != foo__a__default_value)
{
  /* foo.a is set to a non-default value. */
}

There are some kinds of default values not supported by protobuf itself. If protobuf is updated to support these kinds of default values, when we may need to follow suit.

• default values are ignored for 'repeated' fields
• default values are not specifiable for sub-message fields.

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