VTIME

VTIME is a control parameter used in the POSIX termios interface on Unix-like systems. It is an index into the c_cc array and works together with VMIN to govern the behavior of read() when terminal input is in noncanonical mode (i.e., when ICANON is not set).

In practice, VTIME specifies a timeout for input in tenths of a second, stored as an unsigned char in c_cc[VTIME]. VMIN specifies the minimum number of characters to read. The combined settings determine how long read() blocks and how many characters it returns in noncanonical input.

How to use it. Programs typically obtain, modify, and apply terminal attributes with tcgetattr and tcsetattr (from <termios.h>). The VTIME value is interpreted in tenth-of-a-second units, and VMIN is the minimum character count required for a successful read. For example, a common pattern is to set VMIN and VTIME to achieve a desired balance of responsiveness and input completeness, then apply the settings with tcsetattr.

Behavior summary. When ICANON is disabled:

- If VMIN > 0 and VTIME > 0, read() blocks until at least VMIN characters are available or the timeout expires after the first character arrives; if the timeout expires first, read returns with the number of bytes currently available (which may be less than VMIN).

- If VTIME == 0, read() blocks until VMIN characters are available.

- If VMIN == 0 and VTIME > 0, read() may return as soon as data is available, or after the timeout, depending on the system; exact semantics can vary.

Note that exact behavior can differ slightly across systems, so consulting system-specific documentation is advised.

VTIME, together with VMIN, provides fine-grained control over input latency for interactive or serial-style programs.