When sendmail forks a copy of itself to
process a queue, it does so twice to prevent the creation of a zombie
process. A zombie process is one that has lost its parent, and has
not yet died. It continues to exist as though alive, yet it cannot be
killed, hence it is a zombie.
This USE_DOUBLE_FORK compile-time macro is defined by default as
1 to enable the double fork to prevent zombies. In
the rare instance that you are porting to a new system, you can
redefine USE_DOUBLE_FORK like this:
If you are running a precompiled sendmail
binary, you can use the -d0.10 debugging
command-line switch (-d0.10) to determine if
USE_DOUBLE_FORK is defined (if it appears in the list, it is
defined). New ports should be reported to
email@example.com so that they can be folded
into future releases.