TCSADRAIN

TCSADRAIN is a constant used with the tcsetattr function in POSIX-compliant systems to specify when terminal attribute changes should take effect. It is one of several optional_actions values defined for applying modifications to a terminal device as described in the termios interface.

With TCSADRAIN, the requested changes are applied after all output written to the terminal has been transmitted. This ensures that ongoing output is not interrupted or corrupted by mid-change changes to attributes that affect output processing, such as local modes (echo, canonical mode) or signal handling. If there is no pending output, the change is applied as soon as tcsetattr returns.

Other actions include TCSANOW, which applies changes immediately, and TCSAFLUSH, which applies changes after discarding any pending input and output data in the terminal's queues. The choice depends on the desired effect: TCSADRAIN preserves current output, TCSANOW is fastest, and TCSAFLUSH resets state more aggressively.

In practice, a program obtains the current terminal settings with tcgetattr, modifies the desired fields in a struct termios, and applies them with tcsetattr(fd, TCSADRAIN, &termios_p). This pattern is common in Unix-like systems for implementing features such as character-based input modes or turning off echo, while avoiding disruption to in-flight output.