On Mon, 12 Sep 2011, Lukas Czerner wrote:
> On Sat, 10 Sep 2011, Michael Kerrisk wrote:
>
> > Hi Lukas,
> >
> > On Mon, Aug 29, 2011 at 2:26 PM, Lukas Czerner <lczerner@xxxxxxxxxx> wrote:
> > > When dealing with sockets, we have to be sure that there is no recv
> > > still blocking on it on another thread, otherwise it might block forever,
> > > since no more messages will be send via the socket. We should advice to
> > > use shutdown before closing socket.
> >
> > As far as I know, in the usual case, a simple close() is sufficient.
> > Can you say some more about how you determined the above statement and
> > in what circumstances it applies?
> >
> > Thanks,
> >
> > Michael
>
> Simply calling close() is not sufficient if the recv, or read is
> blocking on another thread. That is because the recv or read will not be
> notified that the descriptor has been closed. This can only be done via
> shutdown.
>
> We actually have an customer complaining about this behaviour being
> different than on Solaris where simple close() is sufficient to kill
> recv() or read() on another thread. It is simple the way it is in Linux,
> but it should at least be documented.
>
> I am not sure how to describe the problem more, so I have written really
> simple (and stupid) example, where shutdown should be used.
>
> Thanks!
> -Lukas
ping
>
>
> #include <sys/types.h>
> #include <sys/socket.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/un.h>
> #include <unistd.h>
> #include <pthread.h>
>
> #define BUFSIZE 1024
>
> void *close_socket(void *arg) {
> int sockfd = *(int *)arg;
>
> sleep(3);
> printf("Thread: closing socket %d\n", sockfd);
>
> /*
> * Shutdown should be used before close, otherwise the recv() in
> * client() will block forever.
> */
>
> /* shutdown(sockfd, SHUT_RDWR); */
> close(sockfd);
> }
>
> int client(void) {
> int sockfd;
> int len;
> struct sockaddr_un address;
> int ret;
> char *buffer=malloc(BUFSIZE);
> pthread_t thread;
>
> sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
>
> address.sun_family = AF_UNIX;
> strcpy(address.sun_path, "server_socket");
> len = sizeof(address);
>
> ret = connect(sockfd, (struct sockaddr *)&address, len);
> if (ret == -1) {
> perror("connect");
> return 1;
> }
> printf("client connected\n");
>
> ret = pthread_create(&thread, NULL, close_socket, (void *)&sockfd);
> if (ret != 0) {
> perror("Creating thread failed");
> return 1;
> }
>
> while (1) {
> ret = recv(sockfd,buffer,BUFSIZE,0);
> if (ret < 0) {
> perror("recv");
> return 1;
> }
> printf("Data received: %s\n", buffer);
> sleep(1);
> }
>
> close(sockfd);
> return 0;
> }
>
> int server(void) {
> char *message="This is the message I am sending to you";
> struct sockaddr_un server_addr, client_addr;
> int server_sockfd, client_sockfd;
> int server_len, client_len;
> int ret;
>
> unlink("server_socket");
> server_sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
>
> server_addr.sun_family = AF_UNIX;
> strcpy(server_addr.sun_path, "server_socket");
> server_len = sizeof(server_addr);
> bind(server_sockfd, (struct sockaddr *)&server_addr, server_len);
>
> listen(server_sockfd, 5);
>
> client_len = sizeof(client_addr);
> client_sockfd = accept(server_sockfd,
> (struct sockaddr *)&client_addr, &client_len);
>
> printf("Server: sending data...\n");
> ret = send(client_sockfd ,message,strlen(message),0);
> if (ret < 0) {
> perror("send");
> return 1;
> }
>
> /* simulate running server by not closing the client_socket socket */
> return 0;
> }
>
> int main() {
> pid_t pid;
> int n;
>
> pid = fork();
> if (pid < 0) {
> perror("fork failed");
> exit(1);
> }
> if (pid > 0) {
> printf(" - starting server\n");
> server();
> printf(" - exiting server\n");
> wait();
> } else {
> sleep(1);
> printf(" - starting client\n");
> client();
> printf(" - exiting client\n");
> }
> }
>
--
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
