Question

I am trying have non-blocking I/O between a server and client. After the two are connected, I am trying to use fork to handle the IO, but the server side gets an error when trying to read of “Transport endpoint is not connected” and it happens twice (because of fork I’m guessing?).

Server code

//includes taken out

#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;

    //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\n", gai_strerror(status));
        exit(1);
    }

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

    freeaddrinfo(servinfo);

    //listen
    if(listen(sock, 5) < 0) {
        printf("\nListen 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("\nAccept error %m", errno);
        exit(1);
    }

    cout<<"\nSuccessful Connection!";

    //set nonblock
    set_nonblock(new_sd);

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

    fork();
    pid = getpid();

    if(pid == 0) {

        while( !(out[0] == 'q' && out[1] == 'u' && out[2] == 'i' && out[3] == 't') ) {

            fgets(out, 255, stdin);
            numSent = send(sock, out, strlen(out), 0);

            if(numSent < 0) {
                printf("\nError sending %m", errno);
                exit(1);
            }   //end error
        }   //end while
    }   //end child

    else {
        numRead = recv(sock, in, 255, 0);
        if(numRead < 0) {
            printf("\nError reading %m", errno);
            exit(1);
        }   //end error
        else {
            cout<<in;
            for(int i=0;i<255;i++)
                in[i] = '\0';

        }   //end else
    }   //end parent

    cout<<"\n\nExiting normally\n";
    return 0;
}

Client code

//includes taken out

#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

    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\n", gai_strerror(status));
        exit(1);
    }

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

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

    cout<<"\nSuccessful connection!";

    //set nonblock
    set_nonblock(sock);

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

    fork();
    pid = getpid();

    if(pid == 0) {

        while( !(out[0] == 'q' && out[1] == 'u' && out[2] == 'i' && out[3] == 't') ) {

            fgets(out, 255, stdin);
            numSent = send(sock, out, strlen(out), 0);

            if(numSent < 0) {
                printf("\nError sending %m", errno);
                exit(1);
            }   //end error
        }   //end while
    }   //end child process

    else {
        while( !(in[0] == 'q' && in[1] == 'u' && in[2] == 'i' && in[3] == 't') ) {
        numRead = recv(sock, in, 255, 0);
            cout<<in;
            for(int i=0;i<255;i++)
                in[i] = '\0';
        }
    }   //end parent process

    cout<<"\n\nExiting normally\n";
    return 0;
}

I also tried using threads to do the I/O. The problem there is that when I run the program, its like the threads don’t occur. The program just runs with a “Successful Connection” and then “exiting normally.” I put some cout statements into the while(1) loops and they did print out a few times, but they just stop for some reason. I’m not sure if its a problem with my threads or my sockets. The code (very similar to above) for that is here –

Server

//includes taken out

#define PORT "4950"
#define STDIN 0

pthread_t readthread;
pthread_t sendthread;

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

struct sockaddr name;
int sock, new_sd;

void* readThread(void* threadid) {

    while(1) {

        numRead = recv(new_sd, in, 255, 0);

        if(numRead > 0) {
            cout<<"\n"<<in;
            for(int i=0;i<strlen(in);i++)
                in[i] = '\0';
        }   //end if
        else if(numRead < 0) {
            printf("\nError reading %m", errno);
            exit(1);
        }

    }   //end while
}   //END READTHREAD


void* sendThread(void* threadid) {

     while(1) {

        cin.getline(out, 255);

        numSent = send(new_sd, out, 255, 0);

        if(numSent > 0) {
            for(int i=0;i<strlen(out);i++)
                out[i] = '\0';
        }   //end if
        else if(numSent < 0) {
            printf("\nError sending %m", errno);
            exit(1);
        }
    }   //end while
}   //END SENDTHREAD

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, adrlen;

    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;

    //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\n", gai_strerror(status));
        exit(1);
    }

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

    freeaddrinfo(servinfo);

    //listen
    if(listen(sock, 5) < 0) {
        printf("\nListen 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("\nAccept error %m", errno);
        exit(1);
    }  

    cout<<"\nSuccessful Connection!";

    //set nonblock
    set_nonblock(new_sd);

    pthread_attr_t attr;
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

    pthread_create(&readthread, &attr, readThread, (void*)0);
    pthread_create(&sendthread, &attr, sendThread, (void*)1);

    cout<<"\n\nExiting normally\n";
    return 0;
}

Client

#define PORT "4950"

pthread_t readthread;
pthread_t sendthread;
char* in = new char[255];
char* out = new char[255];
int numSent;
int numRead;
struct sockaddr name;
int sock;

void* readThread(void* threadid) {

    while(1) {

        numRead = recv(sock, in, 255, 0);

        if(numRead > 0) {
            cout<<"\n"<<in;
            for(int i=0;i<strlen(in);i++)
                in[i] = '\0';
        }   //end if
        else if(numRead < 0) {
            printf("\nError reading %m", errno);
            exit(1);
        }
    }   //end while
}   //END READTHREAD

void* sendThread(void* threadid) {

    while(1) {

        cin.getline(out, 255);
        numSent = send(sock, out, 255, 0);

        if(numSent > 0) {
            for(int i=0;i<strlen(out);i++)
                out[i] = '\0';
        }   //end if
        else if(numSent < 0) {
            printf("\nError sending %m", errno);
            exit(1);
        }

    }   //end while
}   //END SENDTHREAD

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, adrlen;

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

    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\n", gai_strerror(status));
        exit(1);
    }

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

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

    cout<<"\nSuccessful connection!";

    //set nonblock
    set_nonblock(sock);

    pthread_attr_t attr;
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

    pthread_create(&readthread, &attr, readThread, (void*)0);
    pthread_create(&sendthread, &attr, sendThread, (void*)1);

    cout<<"\n\nExiting normally\n";
    return 0;
}

I apologize for the long post, but I have been at this for a few days now and am not sure how to proceed. I tried using select(), but that seems like a little much for just 1 client and a server I/O. If anyone can point out what may be going wrong above or any other tips (or I’m just plain wrong about select :)) I would greatly appreciate it.

Answer 1

I looked at the first two codes server and client. Unfortunately, you designed your program mistakenly. The server is supposed to do the following (in case of multi-processes):

1. Create a socket
2. Bind to a port number
3. Change to passive mode (by listen()).
4. Go to loop to accept incoming connection. Here sock is ONLY responsible to listen for new connection and it is not responsible to send/recv as you did.
   • accept a new connection. accept()
   • create a new process
   • The new process is responsible to do send/recv. The parent process should return to step 4 to listen for new connection.

Here is a pseudo-code

while (1) {
    new_sd = accept(....);
    if (new_sd < 0) continue; // or quit
    if (fork() == 0) {
         // do sendin and receiving USING new_sd
         exit(0);  // child terminates here
    }
}

UPDATE: since you are looking for non-blocking scenario, i would recommend you to use select() or poll() system calls.

I feel that you want to create two processes such that one process is for sending and another for receiving. If so, then you don’t need to set new_sd to non-blocking mode since these two processes are running at the same time. If you are planning to do that, then the child process will create another process such that the first child do sending and the second child do receiving. As follows:

while (1) {
    new_sd = accept(....);
    if (new_sd < 0) continue; // or quit
    if (fork() == 0) {
         pid = fork();
         if (pid == 0) then do_sending(new_sd);
         else do_receiving(new_sd);
         exit(0);  // child terminates here
    }
}