[capnproto] Issue with Int8 and UInt8

[s***@gmail.com]

Hello,

I wanted to test the performance of serializing a single scaler type of
message. For example:
@0xf123cfa3565bb5a6;

struct TestBool {
value @0 :Bool;
}
struct TestInt8 {
value @0 :Int8;
}

When I was testing scaler types of Int8 and UInt8, by calling
auto r2 = message.getRoot<TestInt8>();
cout << r2.getValue() << "\nsize: " << size << endl;

returned nothing for the value:

size: 16


However, when I set the field to Int16, the correct value is returned.

127

size: 16

The complete code is pasted below:
capnp::MallocMessageBuilder message;
TestInt8::Builder r1 = message.getRoot<TestInt8>();
r1.setValue(127);
auto serialized = message.getSegmentsForOutput();
// auto serialized = capnp::messageToFlatArray(message);

//capnp::SegmentArrayMessageReader reader(serialized);
size_t size = 0;
for (auto segment : serialized) {
size += segment.asBytes().size();
}
auto r2 = message.getRoot<TestInt8>();
cout << r2.getValue() << "\nsize: " << size << endl;

Is there anything wrong? It only happened for Int8 and UInt8 cases.
Also, is the size I am getting the correct serialized size?

Best,
Shuo

['Kenton Varda' via Cap'n Proto]

Hi Shuo,

The problem you are seeing is a classic problem with C++'s iostream
classes. int8_t and uint8_t are aliases for `signed char` and `unsigned
char`. std::ostream::operator<<() treats both of these types as equivalent
to `char`. So, it writes the value as a single character rather than as an
integer. If the value is an unprintable character, it looks like nothing is
printed.

FWIW, the `kj::str()` universal stringification function does not have this
problem. It treats only bare `char` as a character. `signed char` and
`unsigned char` are treated as integers.

As to your second question, yes, a size of 16 is correct for both messages.
The first segment of a message always starts with an 8-byte root pointer
pointing to the root object (usually a struct). Object sizes are rounded up
to the nearest 8-byte boundary, so unless the object is totally empty, you
end up with at least 16 bytes per message. Of course, most of this size is
zero-valued so would compress away with capnp packing or any decent
compression algorithm.

-Kenton

Post by s***@gmail.com
Hello,
I wanted to test the performance of serializing a single scaler type of
@0xf123cfa3565bb5a6;
struct TestBool {
}
struct TestInt8 {
}
When I was testing scaler types of Int8 and UInt8, by calling
auto r2 = message.getRoot<TestInt8>();
cout << r2.getValue() << "\nsize: " << size << endl;
size: 16
However, when I set the field to Int16, the correct value is returned.
127
size: 16
capnp::MallocMessageBuilder message;
TestInt8::Builder r1 = message.getRoot<TestInt8>();
r1.setValue(127);
auto serialized = message.getSegmentsForOutput();
// auto serialized = capnp::messageToFlatArray(message);
//capnp::SegmentArrayMessageReader reader(serialized);
size_t size = 0;
for (auto segment : serialized) {
size += segment.asBytes().size();
}
auto r2 = message.getRoot<TestInt8>();
cout << r2.getValue() << "\nsize: " << size << endl;
Is there anything wrong? It only happened for Int8 and UInt8 cases.
Also, is the size I am getting the correct serialized size?
Best,
Shuo
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