The p macro
contains the process ID of the sendmail that
executes the delivery agent. Every process (running program) under
Unix has a unique identification number associated with it (a process
ID). Process IDs are necessary to differentiate one incantation of a
program from another. The sendmail program
fork(2)s often to perform tasks (such as
delivery) while performing other tasks (such as listening for
incoming SMTP connections). All copies share the name
sendmail; each has a unique process ID number.
$p is intended for use in header definitions but
can also be used in the A= equate (A=) of delivery agents.
$p is transient. If it is defined in the
configuration file or in the command line, that definition can be
ignored by sendmail. Note that a
$& prefix is necessary when you reference this
macro in rules (that is, use $&p, not
$p).
