When I specify a socket with FCGX_OpenSocket() instead of using the default 0, I get a 500 error code.
In looking at the log I see:

server_init() = {mutex=1708/6ac}
server_bind() = {pipe="\\.\pipe\abws-fcgi-0000000a-01c945d79c65451c00000180"}
PUID: 10 process_init() = {event=1696/6a0}
PUID: 10 process_start(cmdline=""C:\Program Files\Abyss Web SERVER\htdocs\FCGITest.EXE" ") = OK
PUID: 10 RUID: 0 CONNECT() = OK
PUID: 10 RUID: 0 send_table(TYPE=9, SIZE=56 bytes, items=3)
PUID: 10 RUID: 0 send_table_item(key="FCGI_MAX_CONNS", value="")
PUID: 10 RUID: 0 send_table_item(key="FCGI_MAX_REQS", value="")
PUID: 10 RUID: 0 send_table_item(key="FCGI_MPXS_CONNS", value="")
PUID: 10 RUID: 0 send_table_end()
PUID: 10 RUID: 0 send_table(TYPE=9, SIZE=8 bytes, items=0)
PUID: 10 RUID: 0 send_table_end()
PUID: 10 RUID: 0 Reading 8 bytes failed = The process cannot ACCESS the file because it is being used by another process.
PUID: 10 RUID: 0 TIMEOUT-header1!
PUID: 10 RUID: 0 disconnect()
PUID: 10 RUID: 1 request_process(uri="/FCGITest.exe")
PUID: 10 RUID: 1 CONNECT() = The system cannot find the file specified.
PUID: 10 RUID: 1 disconnect()
PUID: 10 RUID: 1 free_request()
PUID: 10 process_shutdown_now()
server_close()

Would greatly appreciate help in understanding:
Reading 8 bytes failed = The process cannot ACCESS the file because it is being used by another process.
and
The system cannot find the file specified.
I reading on this forum
http://www.aprelium.com/forum/viewtopic.php?t=11847
You do not say what Socket number we should use.
In reading the header file
http://www.fastcgi.com/devkit/include/fcgiapp.h .
We are only told specify a socket, not why.
ADDED:
Abyss MUST be in "Local - TCP/IP Sockets" mode not local pipes)
FCGX_OpenSocket() returns -12
yet on IIS 6.0 it returns 1
Added:
This is because libfcgi.dll does not support sockets on windows platforms and is not responding at all.
It also does not handle SIGTERM and so your app is terminated wihtout being able to perform any cleanup.