using select() with pipe

Question

I am reading/writing to a pipe created by pipe(pipe_fds). So basically with following code, I am reading from that pipe:

fp = fdopen(pipe_fds[0], "r"); 

And when ever I get something, I print it out by:

while (fgets(buf, 200, fp)) {
    printf("%s", buf);
}

What I want is, when for certain amount of time nothing appears on the pipe to read from, I want to know about it and do:

printf("dummy");

Can this be achieved by select() ? Any pointers on how to do that will be great.

Answer 1

Let's say you wanted to wait 5 seconds and then if nothing was written to the pipe, you print out "dummy."

fd_set set;
struct timeval timeout;

/* Initialize the file descriptor set. */
FD_ZERO(&set);
FD_SET(pipe_fds[0], &set);

/* Initialize the timeout data structure. */
timeout.tv_sec = 5;
timeout.tv_usec = 0;

/* In the interest of brevity, I'm using the constant FD_SETSIZE, but a more
   efficient implementation would use the highest fd + 1 instead. In your case
   since you only have a single fd, you can replace FD_SETSIZE with
   pipe_fds[0] + 1 thereby limiting the number of fds the system has to
   iterate over. */
int ret = select(FD_SETSIZE, &set, NULL, NULL, &timeout);

// a return value of 0 means that the time expired
// without any acitivity on the file descriptor
if (ret == 0)
{
    printf("dummy");
}
else if (ret < 0)
{
    // error occurred
}
else
{
    // there was activity on the file descripor
}