How do I wake select() on a socket close?

Asked 2009-08-25T16:37:11.197

I am currently using select loop to manage sockets in a proxy. One of the requirements of this proxy is that if the proxy sends a message to the outside server and does not get a response in a certain time, the proxy should close that socket and try to connect to a secondary server. The closing happens in a separate thread, while the select thread blocks waiting for activity.

I am having trouble figuring out how to detect that this socket closed specifically, so that I can handle the failure. If I call close() in the other thread, I get an EBADF, but I can't tell which socket closed. I tried to detect the socket through the exception fdset, thinking it would contain the closed socket, but I'm not getting anything returned there. I have also heard calling shutdown() will send a FIN to the server and receive a FIN back, so that I can close it; but the whole point is me trying to close this as a result of not getting a response within the timeout period, so I cant do that, either.

If my assumptions here are wrong, let me know. Any ideas would be appreciated.

EDIT: In response to the suggestions about using select time out: I need to do the closing asynchronously, because the client connecting to the proxy will time out and I can't wait around for the select to be polled. This would only work if I made the select time out very small, which would then constantly be polling and wasting resources which I don't want.

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2 Answers

-1

Use a timeout for the select, and if the read-ready/write-ready/had-error sequences are all empty (w.r.t that socket), check if it was closed.

answered 2009-08-25T17:24:38.200

-1

Just run a "test select" on every single socket that might have closed with a zero timeout and check the select result and errno until you found the one that has closed.

The following piece of demo code starts two server sockets on separate threads and creates two client sockets to connect to either server socket. Then it starts another thread, that will randomly kill one of the client sockets after 10 seconds (it will just close it). Closing either client socket causes select to fail with error in the main thread and the code below will now test which of the two sockets has actually closed.

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <stdint.h>
#include <pthread.h>
#include <stdbool.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/select.h>
#include <sys/socket.h>


static void * serverThread ( void * threadArg )
{
    int res;
    int connSo;
    int servSo;
    socklen_t addrLen;
    struct sockaddr_in soAddr;
    uint16_t * port = threadArg;

    servSo = socket(PF_INET, SOCK_STREAM, 0);
    assert(servSo >= 0);

    memset(&soAddr, 0, sizeof(soAddr));
    soAddr.sin_family = AF_INET;
    soAddr.sin_port = htons(*port);

    // Uncommend line below if your system offers this field in the struct
    // and also needs this field to be initialized correctly.
//  soAddr.sin_len = sizeof(soAddr);

    res = bind(servSo, (struct sockaddr *)&soAddr, sizeof(soAddr));
    assert(res == 0);

    res = listen(servSo, 10);
    assert(res == 0);

    addrLen = 0;
    connSo = accept(servSo, NULL, &addrLen);
    assert(connSo >= 0);

    for (;;) {
        char buffer[2048];
        ssize_t bytesRead;

        bytesRead = recv(connSo, buffer, sizeof(buffer), 0);
        if (bytesRead <= 0) break;


                        
                        
                            answered 2012-12-21T12:14:22.123



        
        
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