Question

    /*
** talker.c -- a datagram "client" demo
*/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <fstream>
#include <iostream>
#include <string>


using namespace std ;
#define SERVERPORT "3200775" // the port users will be connecting to

int main()
{ string s;

ifstream f1 ("queries1.txt");
if (f1.is_open()) 

{
while (!f1.eof()) 
{
getline(f1,s);
cout<<s<<endl;
        }

}
    int sockfd;
    //char ch [] = "hello";

struct addrinfo hints, *servinfo, *p;
int rv;
int numbytes;

// if (argc != 3) {
// fprintf(stderr,"usage: talker hostname message\n");
// exit(1);
//}

memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_DGRAM;

if ((rv = getaddrinfo("nunki.usc.edu", SERVERPORT, &hints, &servinfo)) != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
return 1;
}

// loop through all the results and make a socket
for(p = servinfo; p != NULL; p = p->ai_next) 
{
if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) 
{
perror("talker: socket");
continue;
}

break;
}

if (p == NULL) 
{
fprintf(stderr, "talker: failed to bind socket\n");
return 2;
}

//if ((numbytes = sendto(sockfd,ch, strlen(ch), 0,
// p->ai_addr, p->ai_addrlen)) == -1) {
//perror("talker: sendto");
//exit(1);

//for (f=0 ;f<15; f++ )
 //   {

    char* mess = malloc(20*sizeof(char));
    sprintf(mess,s);
         if ((numbytes = sendto(sockfd,mess, s.length(), 0, p->ai_addr, p->ai_addrlen)) == -1) 
{ cout<<s; 
  perror("talker: sendto");
  exit(1);

} 
printf("talker: sent %d bytes to \n", numbytes);
cout<<endl;
//}

freeaddrinfo(servinfo);

//printf("talker: sent %d bytes to \n", numbytes);

close(sockfd);
return 0;
}

Sorry for the sloppy way of coding . I am getting errors in this . How do i debug it ?

The errors are these

test.cpp:84: error: invalid conversion from ‘void*’ to ‘char*’
test.cpp:85: error: cannot convert ‘std::string’ to ‘const char*’ for argument ‘2’ to ‘int sprintf(char*, const char*, ...)’

Answer 1

First corrections to your code:

char* mess = malloc(20*sizeof(char));
sprintf(mess,s);
if ((numbytes = sendto(sockfd,mess, s.length(), 0, p->ai_addr, p->ai_addrlen)) == -1) 
    // ...

1. There’s no need to dynamically allocated such a little buffer, whose size is predictable at compile-time.
2. s is a string object, not a pointer to char*.
3. Even if it was a pointer to char*: it’s very unsuitable to use sprintf, since the source string may contain format codes (‘%’). Imagine what happens if it contains ‘%s’.
4. How do you know the string won’t be longer than 19 characters? You can’t know this at compile time.
5. Anyway, sprintf should be used if you want to do the string formatting. There’s no need to use it if you just need the string as-is.

This shame list can be continued. Simply speaking you should rewrite it this way:

if ((numbytes = sendto(sockfd, (char*) s.c_str(), s.length(), 0, p->ai_addr, p->ai_addrlen)) == -1) 
    // ...

Regarding your specific problem. If we assume the actual problem is not from the mentioned list – there’s usually complementary sockets functions that can give you the reach error information.

For instance, on Windows there’s a WSAGetLastError function that can be used immediately after you get the error.