Programming

I'm writing a function right now, and I want to set an alarm to avoid a timeout, here's the general idea of my code:


int amt = -2;
alarm(10);
amt = read(fd, &t->buf[tailpos], TASKBUFSIZ - tailpos); //do a read

when the alarm goes off, i want to check the value of "amt"

basically, i want to check to see that the read call was completed in under 10 seconds, however in a signal handler, i dont think you can pass in the value of amt. so is my approach possible to implement? or would i need to a fork a process and do it that way? I'd prefer not to do that, I ran into some problems with that approach..

here's the entirety of the function as well:

taskbufresult_t read_to_taskbuf(int fd, task_t *t)
{
unsigned headpos = (t->head % TASKBUFSIZ);
unsigned tailpos = (t->tail % TASKBUFSIZ);
ssize_t amt;

if (t->head == t->tail || headpos < tailpos)
amt = read(fd, &t->buf[tailpos], TASKBUFSIZ - tailpos);
else
amt = read(fd, &t->buf[tailpos], headpos - tailpos);

if (amt == -1 && (errno == EINTR || errno == EAGAIN
|| errno == EWOULDBLOCK))
return TBUF_AGAIN;
else if (amt == -1)
return TBUF_ERROR;
else if (amt == 0)
return TBUF_END;
else {
t->tail += amt;
return TBUF_OK;
}
}

help would be much appreciated, my TAs suck...

I think your approach using alarm and checking the return value of read makes sense. You should get back EINTR because the alarm signal handler went off.

Also, have you considered using select for this? Some implementations of select modify the timeout variable to indicate the amount of time not slept. You could use this to keep track of how much time you're willing to continue to sleep for the next select in the case of partial reads. Linux behaves this way. If you're not using Linux, you could use another function to track how long you've been in the function.

HTH