Errno code 88 in send() function (Socket programming)

Question

I'm doing an implementation of FTP, now I'm implementing the RETR command, I take the errno code 88 when I try to do "get FILENAME".

I think that the error can be the conversion of unsigned to uint32_t and uint16_t in the port command.

#include <cstring>
#include <cstdarg>
#include <cstdio>
#include <cerrno>
#include <netdb.h>

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <pwd.h>
#include <grp.h>
#include <time.h>
#include <locale.h>
#include <langinfo.h>
 #include <fcntl.h>

#include <sys/stat.h> 
#include <iostream>
#include <dirent.h>

#include "common.h"

#include "ClientConnection.h"




ClientConnection::ClientConnection(int s) {
    int sock = (int)(s);

    char buffer[MAX_BUFF];

    control_socket = s;
    // Consultar la documentación para conocer el funcionamiento de fdopen.
    fd = fdopen(s, "a+");
    if (fd == NULL){
    std::cout << "Connection closed" << std::endl;

    fclose(fd);
    close(control_socket);
    ok = false;
    return ;
    }

    ok = true;
    data_socket = -1;



};


ClientConnection::~ClientConnection() {
    fclose(fd);
    close(control_socket); 

}


int connect_TCP(uint32_t address, uint16_t port) {
    struct sockaddr_in sin;
    struct hostent
      *hent;
    int s;
    memset(&sin, 0, sizeof(sin));
    sin.sin_family = AF_INET;
    sin.sin_port = htons(port);
    sin.sin_addr.s_addr = address;

    //if(hent = gethostbyname(address)) 

    //memcpy(&sin.sin_addr,hent->h_addr,hent->h_length);

    //else if ((sin.sin_addr.s_addr = inet_addr((char*)address)) == INADDR_NONE)
         //errexit("No puedo resolver el nombre \"%s\"\n", address);


    s = socket(AF_INET, SOCK_STREAM, 0);
    if(s < 0){
       printf("No se puede crear el socket\n");
       return 0;   
    }
    if(connect(s, (struct sockaddr *)&sin, sizeof(sin)) < 0){
        printf("No se puede conectar con %u\n", address);
        return 0;
    }
    return s;
}



void ClientConnection::stop() {
    close(data_socket);
    close(control_socket);
    parar = true;

}


#define COMMAND(cmd) strcmp(command, cmd)==0


void ClientConnection::WaitForRequests() {
    if (!ok) {
     return;
    }

    fprintf(fd, "220 Service ready\n");

    while(!parar) {

      fscanf(fd, "%s", command);
      if (COMMAND("USER")) {

           fscanf(fd, "%s", arg);
           fprintf(fd, "331 User name ok, need password\n");

      }
      else if (COMMAND("PWD")) {

      }
      else if (COMMAND("PASS")) {
        char pass[30];
        fscanf(fd,"%s",pass);

        fprintf(fd,"230 User logged in\n");

      }

      else if (COMMAND("PORT")) {

        unsigned ip[4];
        unsigned port[2];

        fscanf(fd,"%u,%u,%u,%u,%u,%u",&ip[0],&ip[1],&ip[2],&ip[3],&port[0],&port[1]);


        uint32_t aux1;
        uint16_t aux2;

        aux1 = ip[3] << 24 | ip[2] << 16 | ip[1] << 8 | ip[0];

        aux2 = port[1]*256 + port[0];

        data_socket = connect_TCP(aux1,aux2);

        fprintf(fd,"200 OK\n");

      }

      else if (COMMAND("PASV")) {

      }
      else if (COMMAND("CWD")) {

      }
      else if (COMMAND("STOR") ) { //put

        FILE* fp = fopen("filename","w+");

        int size_buffer = 512;

        char buffer[size_buffer];

        int recived_datas;

        while(recived_datas == size_buffer){

          datos_recibidos = recv(data_socket,buffer,size_buffer,0);
          fwrite(buffer,1,recived_datas,fp);

        }
        close(data_socket);
        fclose(fp);

      }
      else if (COMMAND("SYST")) {

        fprintf(fd,"SYSTEM DETAILS\n");

      }
      else if (COMMAND("TYPE")) {

        fprintf(fd,"type 1");

      }
      else if (COMMAND("RETR")) {

        fscanf(fd,"%s",arg);
        std::cout << "Argument: " << arg << std::endl;
        FILE* fp = fopen(arg,"r+");

        int sent_datas;

        int size_buffer = 512;

        char buffer[size_buffer];

         std::cout << "Buffer size = " << size_buffer << std::endl;

        do{

          sent_datas = fread(buffer,size_buffer,1,fp);
          printf("Code %d |  %s\n",errno,strerror(errno));
          send(data_socket,buffer,sent_datas,0);
            printf("Code %d |  %s\n",errno,strerror(errno));

        }while(sent_datas == size_buffer);

        close(data_socket);
        fclose(fp);

        fprintf(fd,"Transferencia completada");
      }
      else if (COMMAND("QUIT")) {

      }
      else if (COMMAND("LIST")) {

      }
      else  {
        fprintf(fd, "502 Command not implemented.\n"); fflush(fd);
        printf("Comando : %s %s\n", command, arg);
        printf("Error interno del servidor\n");

      }

    }

    fclose(fd);


    return;

};

Answer 1

Error code 88 is ENOTSOCK meaning that your tried to do a socket operation on "not a socket".

The offending line, I believe is:

 send(data_socket,buffer,sent_datas,0);

It looks like in your RETR section you never set data_socket to a valid socket with your connect_TCP function, as you did in PORT. Are you certain that data_socket is a valid fd when you call your RETR function?