Question

I’m in the process of porting some (socket-related) Windows C code to Linux/Android and I’m running into an issue with the ioctl command:

unsigned long   u;

if(sockfd != -1 && !ioctl(sockfd, FIONREAD, &u))

{
    return((long) u);
}
    ...
// throw exception

When I check errno, I see EINVAL, but I don’t see why the call to ioctl failed with these arguments. I even tried to declare u as an int instead and it still failed. I have no idea what is wrong. This code works perfectly fine on Windows (ioctlsocket instead of ioctl).

Here’s an strace from Linux:

socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
bind(3, {sa_family=AF_INET, sin_port=htons(9099), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
listen(3, 5)                            = 0
ioctl(3, FIONREAD, [1])                 = -1 EINVAL (Invalid argument)
write(2, "Exception code: 00000503, data: "..., 52Exception code: 00000503, data: 00000000 ((null):0)
) = 52
shutdown(3, 2 /* send and receive */)   = 0
close(3)                                = 0
exit_group(1)                           = ?

Answer 1

Linux doesn’t support FIONREAD/SIOCINQ on listening sockets. See tcp(7):

SIOCINQ
       Returns the amount of queued unread data in the receive buffer.  The
       socket must not be in LISTEN state, otherwise an error (EINVAL) is
       returned.  SIOCINQ is defined in <linux/sockios.h>.  Alternatively, you
       can use the synonymous FIONREAD, defined in <sys/ioctl.h>.