What is DC4 used for?

DC4 (ASCII 20) is a generic device control character that historically meant 'stop' or 'turn off a device function.' Like DC2, it has no universal meaning — each manufacturer assigned their own purpose to it.

How is DC4 different from the other DC characters?

DC1 (XON) and DC3 (XOFF) became standard flow control. DC2 and DC4 stayed free-form. DC4 was often paired with DC2 as an off/on toggle — DC2 starts a feature, DC4 stops it — but this was convention, not a rule.

What's the keyboard shortcut for DC4?

Ctrl+T generates DC4 (ASCII 20). In many modern shells, Ctrl+T is remapped to swap the two characters before the cursor (a readline/emacs binding), so the raw DC4 byte rarely reaches applications.

Should I filter DC4 from user input?

Yes. DC4 is a non-printable control character with no standard meaning in modern text. If it shows up in user input, it's either accidental or an injection attempt. Sanitize it out along with the other C0 control characters.

Device Control 4

DC4 (Device Control 4) is ASCII code point 20 (0x14), Unicode U+0014. It is a C0 control character that originally signaled an auxiliary device — such as a paper tape punch or reader — to stop or power down, complementing DC2's activation role. Like DC2, it never gained a universal protocol meaning and remained vendor-defined across hardware. Some early IBM terminals used 0x14 as a "restore normal mode" command after DC2 had engaged an alternate function. In modern systems it carries no standard interpretation, and most terminal emulators and parsers treat it as a harmless no-op byte to be silently consumed or logged. UTF-8 encodes it as the single byte 0x14; Unicode classifies it as Cc (Control) with the alias DEVICE CONTROL FOUR.

Symbol

Technical Details

Code Example

</>

unsigned char msg[] = {0x02, 0x14, 0x03};
// DC4 (0x14) as an in-band device-control opcode
int is_dc4 = (msg[1] == 0x14);
// Useful in serial parsers for state-machine transitions

Frequently Asked Questions

DC4 (ASCII 20) is a generic device control character that historically meant 'stop' or 'turn off a device function.' Like DC2, it has no universal meaning — each manufacturer assigned their own purpose to it.
DC1 (XON) and DC3 (XOFF) became standard flow control. DC2 and DC4 stayed free-form. DC4 was often paired with DC2 as an off/on toggle — DC2 starts a feature, DC4 stops it — but this was convention, not a rule.
Ctrl+T generates DC4 (ASCII 20). In many modern shells, Ctrl+T is remapped to swap the two characters before the cursor (a readline/emacs binding), so the raw DC4 byte rarely reaches applications.
Yes. DC4 is a non-printable control character with no standard meaning in modern text. If it shows up in user input, it's either accidental or an injection attempt. Sanitize it out along with the other C0 control characters.

Device Control 4

Symbol