emacs does not start - how to debug?

Question

After using emacs routinely for a long time, today I suddenly have problems starting emacs: When I type emacs in a terminal window, nothing at all happens, emacs seems to be running, but does not display anything, does not open a window, and does not react to any input. This phase takes 30 seconds (nearly exactly!), and then emacs suddenly starts up. Restarting the machine made emacs work again once, but 2 minutes later the problem was back again. Details are below. My questions: How can I make emacs start again without a 30 second delay? How can I debug such problems?

Operating System: Mac OS X, version 10.11.6 (on a MacBook Air)

Emacs version: The problem occurs both with emacs 22.1.1 (system provided) and emacs 24.5.1 (installed via homebrew). I can interrupt emacs 24.5.1 (in the non-working, hanging state) using Ctrl-C. Emacs 22.1.1 seems not to react to Ctrl-C, but if I press Ctrl-G twice it suspends itself.

Illustration of the problem:

>>> time emacs --batch

real    0m30.039s
user    0m0.013s
sys 0m0.010s

Things that still work:

• emacs --version and emacs --help work for both versions

• The computer seems otherwise happy, for example I still can start vi

Things that don't work ("hangs" below means that the 30 second wait is present):

• emacs -Q hangs, as does emacs -q

• emacs -nw hangs

• emacs --debug-init hangs

• Emacs seems to not complain about invalid command line options, for example emacs --tri-tra-tralala just hangs with no message

• The problem is not fixed by changing terminal emulators: emacs hangs both using the system-provided Terminal application, and using iTerm2.

Answer 1

Given that the problem occurs even with emacs -Q, Emacs's own debugging tools probably won't help. They're geared towards debugging Lisp problems but your Emacs hangs before problems in Lisp code get a chance to start.

You can see what a program is doing by running it under dtruss. From a terminal, run

sudo dtruss -deof emacs -Q  2>emacs.dtruss

Look at the resulting emacs.dtruss file and see what system calls took a long time. (Feel free to post the trace here if you don't understand it! This isn't meant to be comprehensible by end-users.)

Answer 2

As mentioned in the etc/PROBLEMS file in Emacs 24.5:

*** Emacs startup on GNU/Linux systems (and possibly other systems) is slow.

This can happen if the system is misconfigured and Emacs can't get the
full qualified domain name, FQDN.  You should have your FQDN in the
/etc/hosts file, something like this:

127.0.0.1   localhost
129.187.137.82  nuc04.t30.physik.tu-muenchen.de nuc04

The way to set this up may vary on non-GNU systems.

According to this answer, the DNS request that caused the delay was removed in Emacs 25, so upgrading Emacs might be a simple way to fix the problem.