To get started I used [sywtwpge] (https://github.com/johto/sywtwpge) which creates an extension based on a template. The code in pg_hello.c is adapted from the first article in the "Writing PostgreSQL Functions in C" series by Ron Peterson. Since the article was written the VARATT_SIZEP macro has been deprecated. I replaced this with a call to SET_VARSIZE instead.

Make sure you have development packages installed for postgres and build tools in general. For me on CentOS I was missing pg_config in my path. I got that installed using alternatives:

alternatives /usr/bin/pg_config pg_config /usr/pgsql-9.3/bin/pg_config 1

Once pg_config is in your path, do:

make; sudo make install

Then in a psql session issue:

CREATE extension pg_hello; SELECT hello('world');

There you go!