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):

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

   

2. Click on the newest man page in section 3:

   

3. Download the linked .gz man page:

   

4. 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/
     

5. Enjoy:

   man 3 msgsnd
   
   MSGSND(P)                POSIX Programmer's Manual                MSGSND(P)
   
   NAME
          msgsnd - XSI message send operation
   (...)