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##### 17.4.9.3 Special Characters for Flow Control

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).

— Macro: int VSTART

> This is the subscript for the START character in the special control character array. 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.

— Macro: int VSTOP

> This is the subscript for the STOP character in the special control character array. 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.