Squid internals

The FD interface (src/fd.c)
A global array of fds - fd_state[SQUID_MAXFD]
Each FD has a plethora of state
Flags - open, close, closing, writing, eof ..
Connection - local/remote port, ip address
Description - type, desc
IO callbacks - ready for read, write, timeout, close, defer, comm_write state
Disk IO - pending write list (currently obselete)
Statistics - bytes read/written, persistent connection uses

Routines maintaining fd_state[]
open a FD - fd_open(fd, type, desc)
close a FD - fd_close(fd)
update FD description - fd_note(fd, desc)
update FD bytecount - fd_bytes(fd, len, type)
read from FD - FD_READ_METHOD
write to FD - FD_WRITE_METHOD