kdb: guid encoding in c results in invalid serialization - kdb

Question

I'm trying to manipulate guid from C++. Whenever I attempt to serialize a guid, I get a null pointer.
U g={0};
auto k = ku(g);
auto p = ::b9(2, k);
First two lines are straight from the manual for creating a null guid. This will result in p == 0.
Really what I was attempting to do was creating a list of guid and then serializing:
k = ktn(UU, 3)
kU(k)[0] = <an instance of U with the g bytes initialized>
kU(k)[1] = <an instance of U with the g bytes initialized>
kU(k)[2] = <an instance of U with the g bytes initialized>
That did not work when attempting to serialize.

Answer 1

I believe you should be using 3 as the first argument to b9. For example:
jmcmurray#homer ~/c $ more test.c
#include"k.h"
K f(K x)
{
K k = ktn(UU,3);I j=0;
for(j=0;j<3;j++){
U g={0};I i=0;
for(i=j;i<j+16;i++){
g.g[i] = (unsigned char)i;
}
kU(k)[0] = g;
}
return b9(3,k);
}
jmcmurray#homer ~/c $ gcc -shared -fPIC -DKXVER=3 test.c -o test.so
jmcmurray#homer ~/c $ q
KDB+ 3.5 2017.11.30 Copyright (C) 1993-2017 Kx Systems
l64/ 8()core 16048MB jmcmurray homer.aquaq.co.uk 192.168.1.57 EXPIRE 2019.06.30 AquaQ #52428
q)f:`:./test 2:(`f;1)
q)f[]
0x010000003e000000020003000000000002030405060708090a0b0c0d0e0f00ae67af727f000..
q)-9!f[]
00000203-0405-0607-0809-0a0b0c0d0e0f 001868af-727f-0000-6062-67af727f0000 a0a..
q)
Here I am able to return a serialised list of GUIDs from my shared object & deserialize on the q side. When I tried with 2 as in your example I got a 'type error when running the function in q.
According to https://code.kx.com/q/interfaces/capiref/#b9-serialize 3 means
unenumerate, compress, allow serialization of timespan and timestamp
2 is the same without "compress". So I guess you must compress GUIDs?