How to set a non-standard baudrate on a serial port device on Linux?

Question

What are the ways of setting custom baudrates on Linux?

An answer to this question must be at a level of userland low-level APIs (ioctl, etc.) above the level of a syscall. It should be useful in these circumstances at least:

1. Writing low-level C-based userland code that uses serial ports,

2. Writing libraries that abstract the serial port functionality,

3. Writing kernel serial port drivers.

Answer 1

Things are, unfortunately, driver-dependent. Good drivers will implement all of the methods below. Bad drivers will implement only some of the methods. Thus you need to try them all. All of the methods below are implemented in the helper functions in linux/drivers/tty/serial/serial_core.c.

The following 4 choices are available.

1. Standard baud rates are set in tty->termios->c_cflag. You can choose from:

       B0
       B50
       B75
       B110
       B134
       B150
       B200
       B300
       B600
       B1200
       B1800
       B2400
       B4800
       B9600
       B19200
       B38400
       B57600
       B115200
       B230400
   

2. If you need rates not listed above, e.g. 460800 (this is a deprecated hack that the kernel developers wish to die, per the source code comments):

   • set tty->termios->c_cflag speed to B38400

   • call TIOCSSERIAL ioctl with (struct serial_struct) set as follows:

     serial->flags & ASYNC_SPD_MASK == ASYNC_SPD_[HI, VHI, SHI, WARP]
     // this is an assertion, i.e. what your code must achieve, not how
     

     This sets alternate speed to HI: 57600, VHI: 115200, SHI: 230400, WARP: 460800

3. You can set an arbitrary speed using alt_speed as follows:

   • Set tty->termios->c_cflag speed to B38400. This is unrelated to the speed you chose!

   • Set the intended speed in tty->alt_speed. It gets ignored when alt_speed==0.

4. You can also an arbitrary speed rate by setting custom divisor as follows:

   • Set tty->termios->c_cflag speed to B38400. This is unrelated to the speed you chose!

     bool set_baudrate(int fd, long baudrate) {
       struct termios term;
       if (tcgetattr(fd, &term)) return false;
       term.c_cflag &= ~(CBAUD | CBAUDEX);
       term.c_cflag |= B38400;
       if (tcsetattr(fd, TCSANOW, &term)) return false;
       // cont'd below
     

   • Call TIOCSSERIAL ioctl with struct serial_struct set as follows:

     serial->flags & ASYNC_SPD_MASK == ASYNC_SPD_CUST
     serial->custom_divisor == serial->baud_base / your_new_baudrate
     // these are assertions, i.e. what your code must achieve, not how
     

   How to do it? First get the structure filled (including baud_base you need) by calling TIOCGSERIAL ioctl. Then modify it to indicate the new baudrate and set it with TIOCSSERIAL:

         // cont'd
         struct serial_struct serial;
         if (ioctl(fd, TIOCGSERIAL, &serial)) return false;
         serial->flags &= ~ASYNC_SPD_MASK;
         serial->flags |= ASYNC_SPD_CUST;
         serial->custom_divisor = serial->baud_base / baudrate.
         if (ioctl(fd, TIOCSSERIAL, &serial)) return false;
         return true;
      }