http://cr.yp.to/ucspi-tcp.html

tcpserver is one of several deployment options for qpsmtpd though it is not used often now that forkserver and Apache::Qpsmtpd are available.

It turns sockets into file descriptors.

The qpsmtpd run script uses tcpserver and softlimit. Tcpserver and softlimit (included in the daemontools package) are part of a typical qmail install.

When installing from source on Linux, you should include errno.h if make fails with the following error:

undefined reference to `errno'

This is described at http://cr.yp.to/docs/unixport.html#errno and can be fixed with any of the following:

$ perl -pi.bak -e 's/extern int errno;/#include <errno.h>/' error.h

$ perl -pi.bak -e 's|^(gcc)|\1 --include /usr/include/errno.h|' conf-cc

$ echo gcc -O1 --include /usr/include/errno.h > conf-cc

The default conf-cc file sets the gcc optimization level to -O2 which can result in the following error when the run script is executed. This can be solved by either removing the optimization flag or setting it to -O1 in conf-cc.

$ ./run &
$ tcpserver: fatal: temporarily unable to figure out IP address for \
  0.0.0.0: file does not exist

If you wish to distribute daemontools, be sure to read http://cr.yp.to/distributors.html. Be aware that the distributors.html page has been said to be incorrect and the latest conditions may not be published.