sendmail, 4th Edition

The Fork

When sendmail performs delivery, it cannot simply replace itself with the delivery agent program. Instead, it must fork(2), and the child will replace itself.

If sendmail is running in verbose mode (Verbose on page 1117), it shows that it is about to start this process:

Connecting to delivery agent

If a traffic-logging file was specified with the -X command-line switch (Log Transactions with -X on page 512), sendmail appends the following line to that file:

pid =  == EXEC the expanded A= here

Here, the A= delivery agent equate (A= on page 738) from the delivery agent’s declaration is printed with all its sendmail macros expanded and with the recipients listed.

Next, sendmail creates a pipe so that it will be able to print the email message to the delivery agent and so that it can read errors emitted by the delivery agent. See the -d11 debugging switch (-d11.1 on page 550) for a description of what can go wrong.

If all has gone well, sendmail fork(2)s a copy of itself. The parent then pipes the email message to the child.

When the entire message has been sent, the parent then wait(3)s for the child to complete its work and exit(2)s. The parent collects the exit(2) value from the child and determines delivery success based on that exit value.