does rssl*ToString add 0 in the end of returned string in RsslBuffer.data

Some do, some do not, it is not documented in the reference guide. Some can only be inferred from the example usage.

One function returning a C String in a RsslBuffer:

rsslDateTimeToString(&fidDateTimeBuf, RSSL_DT_DATETIME, &fidDateTimeValue);
printf("%s\n", fidDateTimeBuf.data);

Another returning only a buffer:

fidStateBuf.data = (char*)alloca(stateBufLen);
fidStateBuf.length = stateBufLen;
rsslStateToString(&fidStateBuf, &fidStateValue);
printf("%.*s\n", fidStateBuf.length, fidStateBuf.data);

Also some API completely ignore the apparent RsslBuffer standard signature and do return a C-String.

const char* rsslQosTimelinessToString(RsslUInt8 value)
{
    switch (value)
    {
    case RSSL_QOS_TIME_UNSPECIFIED:     return "Unspecified";
    case RSSL_QOS_TIME_REALTIME:        return "Realtime";
    case RSSL_QOS_TIME_DELAYED_UNKNOWN: return "DelayedByUnknown";
    case RSSL_QOS_TIME_DELAYED:         return "DelayedByTimeInfo";
    default:                            return "Unknown QosTimeliness";
    }
}