TCSANOW

TCSANOW is a constant defined by the POSIX termios interface used with the tcsetattr function. It specifies how and when changes to terminal attributes should take effect.

In practical terms, tcsetattr(fd, TCSANOW, termios_p) applies the settings described in the termios structure immediately to the terminal associated with the file descriptor fd. This means that the new control modes, input and output processing flags, and other terminal attributes take effect without waiting for any pending I/O operations.

TCSANOW is one of several optional_actions values supported by tcsetattr. The other common options are TCSADRAIN and TCSAFLUSH. TCSADRAIN applies changes after all output written to the terminal has been transmitted, whereas TCSAFLUSH discards any unread input before applying the changes. These semantics allow a program to coordinate attribute changes with ongoing input or output.

Usage of TCSANOW typically occurs when a program needs to switch modes, such as turning off canonical input processing or toggling local echo, without delaying the change. It is commonly used in conjunction with tcgetattr to retrieve current settings, modify them, and then apply them back to the terminal.

TCSANOW is defined in the termios.h header and is available on POSIX-compliant systems, including Linux, macOS, and BSD variants. It is part of the standard terminal I/O configuration API used by many C and C++ programs to control terminal behavior. See also tcgetattr, tcsetattr, TCSADRAIN, and TCSAFLUSH.