CODEWITHSUNDEEP
csocketsposix-select
qrtt1
qrtt1

Reputation: 7957

Check Socket File Descriptor is Available?

If I got a file descriptor (socket fd), how to check this fd is avaiable for read/write? In my situation, the client has connected to server and we know the fd. However, the server will disconnect the socket, are there any clues to check it ?

Upvotes: 3

Views: 9917

Answers (5)

qrtt1
qrtt1

Reputation: 7957

I found the recv can check. when socket fd is bad, some errno is set. 

ret = recv(socket_fd, buffer, bufferSize, MSG_PEEK); 
if(EPIPE == errno){
// something wrong
}

Upvotes: 0

dwc
dwc

Reputation: 24890

You want fcntl() to check for read/write settings on the fd:

#include <unistd.h>
#include <fcntl.h>

int    r;

r = fcntl(fd, F_GETFL);
if (r == -1)
        /* Error */
if (r & O_RDONLY)
    /* Read Only */
else if (r & O_WRONLY)
    /* Write Only */
else if (r & O_RDWR)
    /* Read/Write */

But this is a separate issue from when the socket is no longer connected. If you are already using select() or poll() then you're almost there. poll() will return status nicely if you specify POLLERR in events and check for it in revents.

If you're doing normal blocking I/O then just handle the read/write errors as they come in and recover gracefully.

Upvotes: 5

jesup
jesup

Reputation: 6984

In C#, this question is answered here

In general, socket disconnect is asynchronous and needs to be polled for in some manner. An async read on the socket will typically return if it's closed as well, giving you a chance to pick up on the status change quicker. Winsock (Windows) has the ability to register to receive notification of a disconnect (but again, this may not happen for a long time after the other side "goes away", unless you use some type of 'keepalive' (SO_KEEPALIVE, which by default may not notice for hours, or an application-level heartbeat).

Upvotes: 1

dicroce
dicroce

Reputation: 46760

Well, you could call select(). If the server has disconnected, I believe you'll eventually get an error code returned... If not, you can use select() to tell whether you're network stack is ready to send more data (or receive it).

Upvotes: -1

qrdl
qrdl

Reputation: 34948

You can use select() or poll() for this.

Upvotes: 0

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