c - Using select() for non-blocking sockets

Question

I am trying to use the select function to have non-blocking i/o between a server and 1 client (no more) where the communication flows nicely (can send at any time and the other will receive without waiting to send). I found a tutorial with some code and tried to adapt it to mine. This is what I have -

Server

#define PORT "4950"
#define STDIN 0

struct sockaddr name;

void set_nonblock(int socket) {
    int flags;
    flags = fcntl(socket,F_GETFL,0);
    assert(flags != -1);
    fcntl(socket, F_SETFL, flags | O_NONBLOCK);
}


// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa) {
    if (sa->sa_family == AF_INET)
        return &(((struct sockaddr_in*)sa)->sin_addr);

    return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(int agrc, char** argv) {
    int status, sock, adrlen, new_sd;

    struct addrinfo hints;
    struct addrinfo *servinfo;  //will point to the results

    //store the connecting address and size
    struct sockaddr_storage their_addr;
    socklen_t their_addr_size;

    fd_set read_flags,write_flags; // the flag sets to be used
    struct timeval waitd;          // the max wait time for an event
    int sel;                      // holds return value for select();

    //socket infoS
    memset(&hints, 0, sizeof hints); //make sure the struct is empty
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM; //tcp
    hints.ai_flags = AI_PASSIVE;     //use local-host address

    //get server info, put into servinfo
    if ((status = getaddrinfo("127.0.0.1", PORT, &hints, &servinfo)) != 0) {
        fprintf(stderr, "getaddrinfo error: %s
", gai_strerror(status));
        exit(1);
    }

    //make socket
    sock = socket(servinfo->ai_family, servinfo->ai_socktype, servinfo->ai_protocol);
    if (sock < 0) {
        printf("
server socket failure %m", errno);
        exit(1);
    }

    //allow reuse of port
    int yes=1;
    if (setsockopt(sock,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(int)) == -1) {
        perror("setsockopt");
        exit(1);
    }

    //unlink and bind
    unlink("127.0.0.1");
    if(bind (sock, servinfo->ai_addr, servinfo->ai_addrlen) < 0) {
        printf("
Bind error %m", errno);
        exit(1);
    }

    freeaddrinfo(servinfo);

    //listen
    if(listen(sock, 5) < 0) {
        printf("
Listen error %m", errno);
        exit(1);
    }
    their_addr_size = sizeof(their_addr);

    //accept
    new_sd = accept(sock, (struct sockaddr*)&their_addr, &their_addr_size);
    if( new_sd < 0) {
        printf("
Accept error %m", errno);
        exit(1);
    }

    set_nonblock(new_sd);
    cout<<"
Successful Connection!";

    char* in = new char[255];
    char* out = new char[255];
    int numSent;
    int numRead;


    while(1) {

        waitd.tv_sec = 10;
        FD_ZERO(&read_flags);
        FD_ZERO(&write_flags);
        FD_SET(new_sd, &read_flags);

        if(strlen(out) != 0)
            FD_SET(new_sd, &write_flags);

        sel = select(new_sd+1, &read_flags, &write_flags, (fd_set*)0, &waitd);
        if(sel < 0)
            continue;

        //socket ready for reading
        if(FD_ISSET(new_sd, &read_flags)) {
            FD_CLR(new_sd, &read_flags);

            memset(&in, 0, sizeof(in));

            if(recv(new_sd, in, sizeof(in), 0) <= 0) {
                close(new_sd);
                break;
            }
            else
                cout<<"
"<<in;
        }   //end if ready for read

        //socket ready for writing
        if(FD_ISSET(new_sd, &write_flags)) {

            FD_CLR(new_sd, &write_flags);
            send(new_sd, out, strlen(out), 0);
            memset(&out, 0, sizeof(out));
        }
    }   //end while

    cout<<"

Exiting normally
";
    return 0;
}

Client (basically the same just minus an accept call) -

#define PORT "4950"

struct sockaddr name;

void set_nonblock(int socket) {
    int flags;
    flags = fcntl(socket,F_GETFL,0);
    assert(flags != -1);
    fcntl(socket, F_SETFL, flags | O_NONBLOCK);
}

// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa) {
    if (sa->sa_family == AF_INET)
        return &(((struct sockaddr_in*)sa)->sin_addr);

    return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(int agrc, char** argv) {
    int status, sock, adrlen;

    struct addrinfo hints;
    struct addrinfo *servinfo;  //will point to the results

    fd_set read_flags,write_flags; // the flag sets to be used
    struct timeval waitd;          // the max wait time for an event
    int sel;                      // holds return value for select();

    memset(&hints, 0, sizeof hints); //make sure the struct is empty
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM; //tcp
    hints.ai_flags = AI_PASSIVE;     //use local-host address

    //get server info, put into servinfo
    if ((status = getaddrinfo("127.0.0.1", PORT, &hints, &servinfo)) != 0) {
        fprintf(stderr, "getaddrinfo error: %s
", gai_strerror(status));
        exit(1);
    }

    //make socket
    sock = socket(servinfo->ai_family, servinfo->ai_socktype, servinfo->ai_protocol);
    if (sock < 0) {
        printf("
server socket failure %m", errno);
        exit(1);
    }

    if(connect(sock, servinfo->ai_addr, servinfo->ai_addrlen) < 0) {
        printf("
client connection failure %m", errno);
        exit(1);
    }

    cout<<"
Successful connection!";

    set_nonblock(sock);

    char* out = new char[255];
    char* in = new char[255];
    int numRead;
    int numSent;

    while(1) {

        waitd.tv_sec = 10;
        FD_ZERO(&read_flags);
        FD_ZERO(&write_flags);
        FD_SET(sock, &read_flags);

        if(strlen(out) != 0)
            FD_SET(sock, &write_flags);


        sel = select(sock+1, &read_flags, &write_flags, (fd_set*)0, &waitd);
        if(sel < 0)
            continue;

        //socket ready for reading
        if(FD_ISSET(sock, &read_flags)) {
            FD_CLR(sock, &read_flags);

            memset(&in, 0, sizeof(in));

            if(recv(sock, in, sizeof(in), 0) <= 0) {
                close(sock);
                break;
            }
            else
                cout<<"
"<<in;
        }   //end if ready for read

        //socket ready for writing
        if(FD_ISSET(sock, &write_flags)) {
            FD_CLR(sock, &write_flags);
            send(sock, out, strlen(out), 0);
            memset(&out, 0, sizeof(out));
        }
    }   //end while

    cout<<"

Exiting normally
";
    return 0;
}

The problem is that when I run them, nothing happens. I can type into one and hit enter and nothing shows up on the other screen (and vice versa). Thats not a whole of information for me to debug and this is my first real attempt at using select so I thought maybe I am just unaware of something simple. If anything can be spotted as wrong or weird please point it out, any help is appreciated.

Answer 1

The problem is that when I run them, nothing happens.

The real problem is that people have been pasting stuff from Beej for years without understanding it. That's why I don't really like that guide; it gives large blocks of code without really explaining them in detail.

You're not reading anything and not sending anything; no fgets, scanf, cin, etc. Here's what I would do:

FD_SET(sock, &read_flags);
FD_SET(STDIN_FILENO, &read_flags);

/* .. snip .. */

if(FD_ISSET(STDIN_FILENO, &read_flags)) {
    fgets(out, len, stdin);
}

This will monitor stdin and read from it when input is available; then, when the socket is writeable (FD_ISSET(sock, &write_flags)), it will send the buffer.