Piping for msmtp

msmtp is a simple programme that can pretend to be sendmail on your system. Once configured it will take an input file (or stdin) and use the configured smtp server to send an email to the appropriate recipient.

Configuring msmtp

This is repeated in many, many places but for completeness here we go... this file is either /etc/msmtprc or ~/.msmtprc:

account bengreeneu
host localhost
from benjamin@example.com
port 25
auth on
user benjamin@example.com
password mypasswordissecret
logfile ~/.msmtp.log
tls on
tls_starttls on
tls_certcheck off

Adjust to taste ;-).

Spawn the msmtp process and pipe to it

To make this nice and easy, I have created a github repository that contains the example project and a Makefile.

The basic steps in this programme are:

• Create two pipes, one for STDIN and one for STDOUT they are named from the perspective of the parent process (wpipefd is write from parent to child and rpipefd is read from child to parent).
• Re-assign the file descriptors in the forked process so that STDIN_FILENO and STDOUT_FILENO are connected to the pipe.
• Replace the current executable image in the forked process with the execv system call. The new process will have the normal 3 file descriptors:
  • stdin (STDIN_FILENO / 0) connected to wpipefd
  • stdout (STDOUT_FILENO / 1) connected to rpipefd
  • stderr (STDERR_FILENO / 2) this is untouched so will be the same as the parent process... good for debugging!
• Write the email to the pipe, the Linux kernel will buffer the bytes written unless you have some funny kernel configuration.
• Read the output from rpipefd until the process closes the other side of the rpipefd (ie. the child process closes stdout or the process terminates).

Further Work

It might be cool to use the techniques learnt here to extend the little shell described here so it can pipe stuff like a more fully featured shell.

Many programmes act differently if stdin is connected to a pipe rather than a tty or pty. Fortunately there are several places that this is covered, please see Using pseudo-terminals (pty) to control interactive programmes. Although the tty and pty stuff is really quite old there are many modern uses, one of which is containers, see Implementing Container Runtime Shim for much more detail than perhaps you would like!