Where are the POSIX message functions (msgsnd, msgrcv, etc) man pages in Mac?
You can download them from http://manpages.ubuntu.com.

For example, to download man page msgsnd (3):


Search for msgsnd in the Google Custom Search field on the right hand side:


Click on the newest man page in section 3:


Download the linked .gz man page:


Copy it to the man directory:


If you want to keep system and additional man pages apart, copy it to /usr/local/share/man/man3/:

sudo mkdir -p /usr/local/share/man/man3/
sudo cp ~/Downloads/msgsnd.3posix.gz /usr/local/share/man/man3/


and add:

export MANPATH="/usr/local/share/man:$MANPATH"


to /etc/profile, if you wish the new man page to be available system-wide, or ~/.bash_profile, to be available to your user only (see man man, that is, man's man page, for more information on MANPATH.) 

Open a new Terminal window or tab for changes to take effect.
Otherwise, copy the man page to /usr/share/man/man3:

sudo cp /Users/jaume/Downloads/msgsnd.3posix.gz /usr/share/man/man3/


Enjoy:

man 3 msgsnd

MSGSND(P)                POSIX Programmer's Manual                MSGSND(P)

NAME
       msgsnd - XSI message send operation
(...)