Tuesday, June 15, 2021

Compiling a C++ application to run on Heroku

TL;DR Use a buildpack and set "LDFLAGS=--static" --disable-shared

I use Heroku to host most of my websites, and since I mostly use PHP for web development this has worked fine. However, every so often I write an app that calls an external program written in, say, C++. Up until now I've had to host these apps on my own web servers. Today I finally bit the bullet and learned how to add a C++ program to a Heroku-hosted site.

In this case I wanted to add CRF++ to an app for parsing citations. I'd read on Stack Overflow that you could simply log into your Heroku instance using

heroku run bash
and compile the code there. I tried that for CRF++ but got a load of g++ errors, culminating in:

configure: error: Your compiler is not powerful enough to compile CRF++.

Turns out that the g++ compiler is only available at build time, that is, when the Heroku instance is being built before it is deployed. Once it is deployed g++ is no longer available (I'm assuming because Heroku tries to keep each running instance as small as possible).

So, next I tried using a buildpack, specifically felkr/heroku-buildpack-cpp. I forked this buildpack, and added it to my Heroku app (using the "Settings" tab). I put the source code for CRF++ into the root folder of the GitHub repository for the app (which makes things messy but this is where the buildpack looks for either Makefile or configure) then when the app is deployed CRF++ is compiled. Yay!

Not so fast, I then did

heroku run bash
and tried running the executable:

heroku run bash -a <my app name>
./crf_learn
/app/.libs/crf_learn: error while loading shared libraries: libcrfpp.so.0: cannot open shared object file: No such file or directory

For whatever reason the executable is looking for a shared library which doesn’t exist (this brought back many painful memories of dealing with C++ compilers on Macs, Windows, and Linux back in the day). To fix this I edited the buildpack compile script to set the "LDFLAGS=--static" --disable-shared flags for configure. This tells the compiler to build static versions of the libraries and executable. Redeploying the app once again everything now worked!

The actual website itself is a mess at the moment so I won't share the link just yet, but it's great to know that I can have both a C++ executable and a PHP script hosted together without (too much) pain. As always, Google and Stack Overflow are your friends.