How to see if a pipe is empty

Question

Assuming a pipe,

int pipe_fd[2];
pipe(pipe_fd);

We fork, and expect that one process will write into the pipe at an arbitrary time. In one of the processes, we want to be able to check the contents of the pipe without blocking.

i.e. While a typical read will block if nothing is present and the write end remains open. I want to go do other stuff and potentially even read a bit at a time, do some stuff, and then check back to see if there's more, a la:

close(pipe_fd[1]);

while(1){
    if(/**Check pipe contents**/){
        int present_chars = 0;    

        while( read(pipe_fd[0],&buffer[present_chars],1) != 0)
            ++present_chars;

        //do something

    }
    else
        //do something else
}

Answer 1

Your logic is wrong in that read will not return 0 when it runs out of characters; instead, it will block until it receives more, unless you put the file in non-blocking mode, but then it will return -1 and set errno to EWOULDBLOCK or EAGAIN rather than returning 0. The only time read can ever return 0 is when the size argument was 0 or end-of-file has been reached. And, for pipes, end-of-file means the writing end of the pipe has been closed; end-of-file status does not occur just because there's not any input available yet.

With that said, the simplest way to check is:

if (poll(&(struct pollfd){ .fd = fd, .events = POLLIN }, 1, 0)==1) {
    /* data available */
}

but unless you're using nonblocking mode, you'll need to make this check before every single read operation. Passing a larger buffer to read rather than doing it a byte-at-a-time would eliminate most of the cost of checking.