How to check if stdin is still opened without blocking?

Question

I need my program written in pure C to stop execution when stdin is closed.

There is indefinite work done in program main cycle, and there is no way I can use blocking checks (like getc()) there (no data is supposed to arrive on stdin - it just stays opened for unknown time).

I intend to use described functionality in realization of network daemon hosted in inetd, xinetd or their analogs - it should emit data on stdout while connection stays opened and correctly finish work when it closes. Now my program is killed by hosting service as it won't stop after connection termination.

I wonder if fctntl() with O_NONBLOCK flag applied to stdin descriptor would allow me to use read() function in non-blocking mode? Should I use select() somehow?

P.S. The data is not supposed but might arrive to stdin. A way of non-blocking readout woould be an answer for the question.

Answer 1

select() does exactly what you want: signal that an operation (read, in this case) on a file descriptor (file, socket, whatever) will not block.

#include <stdio.h>
#include <sys/select.h>

int is_ready(int fd) {
    fd_set fdset;
    struct timeval timeout;
    int ret;
    FD_ZERO(&fdset);
    FD_SET(fd, &fdset);
    timeout.tv_sec = 0;
    timeout.tv_usec = 1;
    //int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
     struct timeval *timeout);
    return select(fd+1, &fdset, NULL, NULL, &timeout) == 1 ? 1 : 0;
}

You can now check a file descriptor before use, for instance in order to empty the file descriptor:

void empty_fd(int fd) {
    char buffer[1024];
    while (is_ready(fd)) {
        read(fd, buffer, sizeof(buffer));
    }
}

In your case, use fileno(stdin) to get the file descriptor of stdin:

if (is_ready(fileno(stdin))) {
    /* read stuff from stdin will not block */
}