Exploring Expect

Pty Trapping

Some systems (notably HPs) require that ptys be trapped in order to detect an eof through select or poll. When trapping is enabled, all ioctls performed by the spawned process on the pty must be acknowledged. This acknowledgment is normally performed automatically when Expect is in one of its expect functions. But occasionally, you may need to explicitly deal with trapping. For example, you might want to change the mode of the slave’s pty after it has been started.

The trap and acknowledgment protocols are described in the documentation for your system. I will not describe them here because they can be avoided. This is fortunate, not because they are complex but because they cannot be performed while you are doing something else (e.g., in the middle of an ioctl call). The solution is to temporarily disable the trapping.

Trapping can be controlled with exp_slave_control. The first argument is the file descriptor corresponding to the spawned process. The second argument is a 0 if trapping is to be disabled and 1 if it is to be enabled.

/* disable trapping */
exp_slave_control(fd,0);

/* fiddle with mode of pty */
. . .

/* enable trapping */
exp_slave_control(fd,1);

On systems which do not use trapping, exp_trap_control turns into a no-op. Thus, if you are concerned about portability to systems which require trapping, use the trap control function.