Determing the number of bytes ready to be recv()'d

I can use select() to determine if a call to recv() would block, but once I've determined that there are bytes to be read, is there a way to query how many bytes are currently available before I actually call recv()?

If your OS provides it (and most do), you can use ioctl(..,FIONREAD,..):

int get_n_readable_bytes(int fd) {
    int n = -1;
    if (ioctl(fd, FIONREAD, &n) < 0) {
        perror("ioctl failed");
        return -1;
    }
    return n;
}

Windows provides an analogous ioctlsocket(..,FIONREAD,..), which expects a pointer to unsigned long:

unsigned long get_n_readable_bytes(SOCKET sock) {
    unsigned long n = -1;
   if (ioctlsocket(sock, FIONREAD, &n) < 0) {
       /* look in WSAGetLastError() for the error code */
       return 0;
   }
   return n;
}

The ioctl call should work on sockets and some other fds, though not on all fds. I believe that it works fine with TCP sockets on nearly any free unix-like OS you are likely to use. Its semantics are a little different for UDP sockets: for them, it tells you the number of bytes in the next datagram.

The ioctlsocket call on Windows will (obviously) only work on sockets.