Exploring Expect

Handling Multiple Inputs And More On Timeouts

In some cases, you do not want to wait inside of an expect function. Instead, you wait in select, poll, or an event manager such as those provided by window systems. In this case, give the file descriptor corresponding to the spawned process to the event loop. When the event loop detects that input can be read from the file descriptor, it calls back in some fashion, after which you can call an expect function to test the patterns. You can guarantee that the expect function returns immediately by setting exp_timeout to 0. If none of your patterns match, EXP_TIMEOUT is returned.

Here is an example using the select system call. select is directed to watch the file descriptor in fd. select returns when it finds that there is data to read. exp_expectl is then called and it reads the data and performs pattern matching. If none of the patterns match, exp_expectl returns EXP_TIMEOUT immediately rather than attempting to read more data.

#define WAIT_FOREVER (struct timeval *)0

FD_SET(fd,&rdrs);
select(fd_max, &rdrs, . . ., WAIT_FOREVER);

exp_timeout = 0;
exp_expectl(fd, . . ., exp_end);

If exp_timeout is nonzero, the expect function can block in the read system call while reading from a single file descriptor. Internally, an ALARM signal is used to interrupt the read. If you define signal handlers, y can choose to restart or abort the read. The integer variable exp_reading is 1 if and only if the read has been interrupted and 0 otherwise. The following ...