Weird order after select() (with FD_SET())

Question

I am developing a multi-client Unix Domain Socket to transfer data through multiple processes. I found some code that implements chat between every client and stuff but I want that once a client send something to the server, the server reply back and the client disconnect.

Having that said, I don't want while(fgets()) but I want (on client side):

int main() {
    int sockfd;
    struct sockaddr_un remote;
    fd_set readfds;
    char buf[1024];
    char buf2[1024];
    int len;

    sockfd = socket(AF_UNIX, SOCK_STREAM, 0);

    remote.sun_family = AF_UNIX;
    strcpy(remote.sun_path, SOCK_PATH);
    len = strlen(remote.sun_path) + sizeof(remote.sun_family);

    if(connect(sockfd, (struct sockaddr*)&remote, len) == -1)
        /* handle error */

    FD_ZERO(&readfds);
    FD_SET(0, &readfds);
    FD_SET(sockfd, &readfds);

    if(select(sockfd+1, &readfds, NULL, NULL, NULL) == -1)
        /* handle error */

    if(FD_ISSET(0, &readfds)) {
        fgets(buf, 1024, stdin);
        if(write(sockfd, buf, 1024) <= 0)
            /* handle error */
    }

    if(FD_ISSET(sockfd, &readfds)) {
        if(read(sockfd, &buf2, 1024) <= 0)
            /* handle error */
    }

    printf("%s\n", buf2);

    close(sockfd);
}

In this order, it works if I do everything after connect() twice (with a loop) but I want to do it only once. Without this loop, my server (which is a daemon) crash and I don't know why.

Furthermore, I added printf() from the code above to understand how it works:

(...)
printf("before select\n");
fflush(stdout);

if(select(sockfd+1, &readfds, NULL, NULL, NULL) == -1)
    /* handle error */

printf("before select\n");
fflush(stdout);

if(FD_ISSET(0, &readfds)) {
    fgets(buf, 1024, stdin);
    if(write(sockfd, buf, 1024) <= 0)
        /* handle error */
}
(...)

And I have this output:

before select
"input to fgets"
after select

And I don't understand why I have the input BEFORE "after select". It doesn't make any sense to me since I call fgets() after printf().

I hope this is understandable enough. What's wrong with my code ? Did I miss something ?

you're missing declarations of all your variables, please provide a [mcve] — Chris Turner, Mar 17 '17 at 12:02
and a lot of your function calls seem to be missing parameters ie `write(sockfd, str)` should have 3 parameters — Chris Turner, Mar 17 '17 at 12:04
`printf` is line buffered when connected to a terminal. Either add `\n` to your printfs or `fflush(stdout);` after each. — Klas Lindbäck, Mar 17 '17 at 12:20
I added everything you need, I wanted to write the less possible code. I just tried to add `fflush(stdout)` and it doesn't change anything — Raphaël, Mar 17 '17 at 13:18
`if(read(sockfd, &buf2, 1024) <= 0) /* handle error */` read returning -1 is not always *an error*. check errno, and decide what to do next for instance on EAGAIN / EWOULDBLOCK — joop, Mar 17 '17 at 13:36

score 1 · Accepted Answer · answered Mar 17 '17 at 12:55

1

The first time through, you call select() before the server has responded. The result is that sockfd won't be ready for reading.

In your case, the client might not need select() on the sockfd. You know that if you wrote something to the server you want to wait for the reply, right?

answered Mar 17 '17 at 12:55

Klas Lindbäck

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What is the point of `select()` then ? I just tried and it works perfectly without calling `select()`. But it has been called before `read()` or `write()` in every socket example I've seen – Raphaël Mar 17 '17 at 13:24
`select()` or the newer `poll()` are used to monitor several sockets/file descriptors at once. The typical case is a single-threaded server with multiple client connections. – Klas Lindbäck Mar 17 '17 at 14:12
So it is only useful on the server side ? Or if a client connects to multiple servers ? – Raphaël Mar 17 '17 at 14:24
1

Yes. You can also use it to implement a timeout on a read. If you call `select()` with the timeout parameter set, you can wait a predetermined time for a response. – Klas Lindbäck Mar 17 '17 at 14:40

Weird order after select() (with FD_SET())

1 Answers1