These special characters may be active in either canonical or noncanonical input mode, but their use is controlled by the flags IXON and IXOFF (see Input Modes).
termios.c_cc[VSTART] holds the character itself.
The START character is used to support the IXON and IXOFF input modes. If IXON is set, receiving a START character resumes suspended output; the START character itself is discarded. If IXANY is set, receiving any character at all resumes suspended output; the resuming character is not discarded unless it is the START character. IXOFF is set, the system may also transmit START characters to the terminal.
The usual value for the START character is C-q. You may not be able to change this value---the hardware may insist on using C-q regardless of what you specify.
termios.c_cc[VSTOP] holds the character itself.
The STOP character is used to support the IXON and IXOFF input modes. If IXON is set, receiving a STOP character causes output to be suspended; the STOP character itself is discarded. If IXOFF is set, the system may also transmit STOP characters to the terminal, to prevent the input queue from overflowing.
The usual value for the STOP character is C-s. You may not be able to change this value---the hardware may insist on using C-s regardless of what you specify.