I came across a cryptic error while trying to create another database instance for my local Postgresql server. The error was:

creating template1 database in data/base/1 ...
FATAL: could not create shared memory segment: Cannot allocate memory
DETAIL: Failed system call was shmget(key=1, size=1318912, 03600).
HINT: This error usually means that PostgreSQL's request for a shared<br />memory segment exceeded available memory or swap space. To reduce the <br />request size (currently 1318912 bytes), reduce PostgreSQL's shared_buffers <br />parameter (currently 50) and/or its max_connections parameter (currently 10).
The PostgreSQL documentation contains more information about shared memory <br />configuration.
child process exited with exit code 1
initdb: removing contents of data directory "data"

The issue is that your Mac is not configured for "server" level resource usage. In order to check what your kernel settings are for shared memory, type:

sysctl -a

Look for keys that start with `kern.sysv.sh____`. Note that the numbers are somewhat related, so you have to change them together. Some helpful detail was found on this post:

Note that (kern.sysv.shmall * 4096) should be greater than or equal to kern.sysv.shmmax. kern.sysv.shmmax must also be a multiple of 4096.

You can set these values temporarily with `sysctl -w [keyname] [value]`, but to make them permanent, you can write the values to `/etc/sysctl.conf`. Here are the values I used:

kern.sysv.shmmax=1610612736
kern.sysv.shmall=393216
kern.sysv.shmmin=1
kern.sysv.shmmni=32
kern.sysv.shmseg=8
kern.maxprocperuid=512
kern.maxproc=2048

You'll need to reboot, but once you're back up you should be able to create more Postgres databases with ease.