The Linux Programming Interface

1336 Chapter 63

for (fd = 0; fd < nfds; fd++)

printf("%d: %s%s\n", fd, FD_ISSET(fd, &readfds)? "r" : "",

FD_ISSET(fd, &writefds)? "w" : "");

if (pto != NULL)

printf("timeout after select(): %ld.%03ld\n",

(long) timeout.tv_sec, (long) timeout.tv_usec / 10000);

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––––––––– altio/t_select.c

In the following shell session log, we demonstrate the use of the program in List-

ing 63-1. In the first example, we make a request to monitor file descriptor 0 for

input with a 10-second timeout:

$ ./t_select 10 0r

Press Enter, so that a line of input is available on file descriptor 0

ready = 1

0: r

timeout after select(): 8.003

$ Next shell prompt is displayed

The above output shows us that select() determined that one file descriptor was

ready. This was file descriptor 0, which was ready for reading. We can also see that

the timeout was modified. The final line of output, consisting of just the shell $

prompt, appeared because the t_select program didn’t read the newline character

that made file descriptor 0 ready, and so that character was read by the shell, which

responded by printing another prompt.

In the next example, we again monitor file descriptor 0 for input, but this time

with a timeout of 0 seconds:

$ ./t_select 0 0r

ready = 0

timeout after select(): 0.000

The select() call returned immediately, and found no file descriptor was ready.

In the next example, we monitor two file descriptors: descriptor 0, to see if

input is available, and descriptor 1, to see if output is possible. In this case, we specify

the timeout as NULL (the first command-line argument is a hyphen), meaning infinity:

$ ./t_select - 0r 1w

ready = 1

0:

1: w

The select() call returned immediately, informing us that output was possible on file

descriptor 1.