Non-blocking call for reading descriptor

c unix

I have a fd descriptor, which I can use to read from by calling read(fd, buffer,...). Now, I want to check if there is anything to read before actually making the call, because the call is blocking. How do I do this?

Solution 1:

int flags = fcntl(fd, F_GETFL, 0);
fcntl(fd, F_SETFL, flags | O_NONBLOCK);

The code snippet above will configure such a descriptor for non-blocking access. If data is not available when you call read, then the system call will fail with a return value of -1 and errno is set to EAGAIN. See the fnctl man pages for more information.

Alternatively, you can use select with a configurable timeout to check and/or wait a specified time interval for more data. This method is probably what you want and can be much more efficient.

Solution 2:

Use select or poll to query whether the file descriptor has data available for read:

fd_set fds;
FD_ZERO(&fds);
FD_SET(&fds, fd);
if (select(fd+1, &fds, 0, 0)==1) /* there is data available */

Solution 3:

Depending on what you're doing you might be able to turn the problem inside out and use select to tell you when your file descriptor has something to read.

Solution 4:

use poll for timeout:

struct pollfd p;
            int n;
            while ((n = poll(&p, 1, iTo)) < 0) 
              {
               if (errno == EAGAIN || errno == EINTR)
                   continue;
            }

            if (!n) {
                 errno = ETIMEDOUT;
                   }

        while ((len = read(Fd, anyBuff, sizeof(anyenter code hereBuff))) < 0) {
            if (errno == EAGAIN || errno == EINTR)
                continue;

        }

Solution 5:

I think you should use select or poll functions to check if there are something to read from the descriptor.