Non-blocking call for reading descriptor

Question

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?

Answer 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.

Answer 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 */