on UNIX domain connection, the server can pass the blob content as an [anciliary file descriptor](http://man7.org/linux/man-pages/man7/unix.7.html). The file descriptor can later be mmapped. In that case, the xml message will have the attached property set to true (and no enclen by the way):
Some example of low level functions for these tasks can be found in the indi source file integs/test.cpp (integration tests), which is somewhat decoupled from indi so probably easier to read for a start:
Tests codes in the integs directory may also be useful as they show the plain text version of the messages for various cases.