Simulink Desktop real-time, external mode, serial read, OSX

Hi, I'm trying to implement a custom s function to read packets sent to a serial interface on my Mac. The application has strict real time constraints and so I need to use the real time kernel module and External mode. I read from the Documentation Simulink Desktop Real-Time User Guide that "Software you use to write I/O drivers must not make calls to the operating system API." I was wondering if libraries like stdio.h, unistd.h,fcntl.h, errno.h, termios.h, string.h, sys/ioctl.h, which I use to read the serial input, belong to those that can't be used. Thank you.