macOS / iOS

Macs use the usual termios TTY implementation that other POSIXes use, but support non-standard baud rates through the iossiospeed ioctl (as of Mac OS X 10.4). To support non-standard baud rates on Mac, there are three main approaches:

  1. Always use iossiospeed
  2. Use iossiospeed for non-standard bauds, but termios with standard bauds
  3. Use iossiospeed always by default and fail-over to the termios approach

Implementation notes

This library uses the first approach. Given that macOS as far back as 10.4 supports it (2005), there seem to be no downsides. Internally, baud rates within the termios struct are kept at 9600 when that struct is read & written. This means that anytime the termios struct is written back (using tcsetattr a call to iossiospeed follows it. Additionally, the termios struct is not cached and instead retrieved on every settings adjustment. While this can increase the number of system calls when changing port settings, it removes the need to keep state consistent and instead the kernel's state can always be considered the canonical source.

Platform notes

iossiospeed has no official documentation that can be found by searching https://developer.apple.com. However IOSerialTestLib.c can be found on Apple's open source code repository and has some example code for using this API.

Experimentation has shown that there are a few key features to using iossiospeed:

  • iossiospeed should be called after setting the termios struct via tcsetattr as that resets the baud rate and you cannot put custom baud rates in the termios struct.
  • Calling iossiospeed will modify the termios struct in the kernel such that you can no longer round-trip the termios struct. The following code will fail:
    struct termios t;
tcgetattr(fd, &t);
tcsetattr(fd, TCSANOW, &t)

Reference implementations

  • picocom follows the second approach. However they also cache the existing termios struct.

Additional References