TCP server with select(), does not work because printf shows 3x to msg and msg comes with trash at the end and ends automatically

Question

void cria_server_tcp(int porto)
{
    struct sockaddr_in serveraddr, clientaddr;
    int fd_listen = cria_socket_tcp();
    int max_clientes = 20; //numero maximo de clientes em espera
    int addrlen;
    int *memset_r;
    int bind_r;
    int listen_r;

    memset_r=memset((void*)&serveraddr, (int)'\0',  sizeof(serveraddr));
        if(memset_r==NULL){
            printf("Erro no memset para cliente (TCP).\n");
        exit(EXIT_FAILURE);
        }

    serveraddr.sin_family= AF_INET;
    serveraddr.sin_addr.s_addr=htonl(INADDR_ANY);
    serveraddr.sin_port=htons((u_short)porto);

    bind_r=bind(fd_listen, (struct sockaddr*)&serveraddr, sizeof(serveraddr));
        if(bind_r == -1){
            printf("Erro no bind ao cliente (TCP).\n");
        exit(EXIT_FAILURE);
        }

    if(listen(fd_listen, max_clientes) < 0){
        printf("Erro a abrir socket para ouvir cliente (TCP).\n");
        exit(EXIT_FAILURE);
    }

        addrlen = sizeof(clientaddr);
        int fd;
        int n=0, len=0, maxlen=MAX_LEN;
        char buffer[MAX_LEN];
        char *auxbuffer = buffer;
        char msg[MAX_LEN];
        fd_set read_set;
        struct timeval timeout;
        int check_select;
        char ler[MAX_LEN];

        fd = accept(fd_listen, (struct sockaddr*)&clientaddr, &addrlen);
            if(fd == -1){
                printf("Erro a aceitar dados do cliente (TCP).\n");
                exit(EXIT_FAILURE);
            }

      timeout.tv_sec = 100;       /* timeout (secs.) */
      timeout.tv_usec = 0;      /* 0 microseconds */


        //corre indefinidamente
        while(1){

            FD_ZERO(&read_set);
            FD_SET(0, &read_set);
            FD_SET(fd, &read_set);
            check_select = select(fd + 1, &read_set, NULL, NULL, &timeout);

            if(check_select != 0 && check_select != -1)
            {
                if(FD_ISSET(fd, &read_set))
                {
                    n = recv(fd, buffer, maxlen, 0);
                    //printf("%d\n", n);

                    /*auxbuffer += n;
                    maxlen -= n;
                    len += n;*/

                    printf("Mensagem recebida: '%s'\n", buffer);
                    send(fd, msg, strlen(buffer), 0);
                    printf("entrei\n");
                }

                if(FD_ISSET(0, &read_set))
                {
                    scanf("%s", ler);
                    ler[strlen(ler)] = '\0';
                    read_stdin(ler, &fd);
                }
        }

        //corre enquanto a sessão do cliente estiver aberta
        /*while((n = recv(fd, auxbuffer, maxlen, 0)) > 0){

            auxbuffer += n;
            maxlen -= n;
            len += n;

            printf("Mensagem recebida: '%s'\n", buffer);

            send(fd, buffer, strlen(buffer), 0);
        }*/


    }

    close(fd);
    close(fd_listen);


}

Answer 1

An error you can see at first: you are receiving data in buffer using recv(), but you are using buffer in printf() as if it were a string ending in 0, and recv() does not guarantee this. A simple solution is place

buffer[n] = '\0';

immediately after recv(). You must also make the buffer size equal to (MAX_LEN+1) so that it fits this zero in case recv() has received MAX_LEN octets.

Another thing: a connection is marked as closed when recv() returns zero, i.e., it receives zero characters. It seems to me you’re not handling this case.