Thread: Questions about sockets and structs

Hi, I wrote this code

Code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>


int main() {
    char buffer[1000] = "";
    int fd = socket(AF_INET, SOCK_STREAM, 0);
    perror("socket");
    int i = 0, connfd;
    struct addrinfo *hints;
    struct addrinfo *res = malloc(sizeof(res));
    struct sockaddr *incoming;
    res->ai_addr = malloc(sizeof(res->ai_addr));
    hints->ai_flags = AI_PASSIVE;
    hints->ai_family = AF_INET;
    hints->ai_socktype = SOCK_STREAM;
    i = getaddrinfo(NULL, "80", hints, &res);
    perror("getaddrinfo");
    for(i = 0; i < 1; i++) {
        printf("%c", res->ai_addr->sa_data[i]);
    }
    i = bind(fd, res->ai_addr, res->ai_addrlen);
    perror("bind");
    listen(fd, 5);
    socklen_t sizeofIncoming = sizeof(incoming);
    while(connfd = accept(fd, incoming, &sizeofIncoming)) {
        perror("accept");
        read(connfd, buffer, sizeof(buffer));
        write(connfd, "HTTP/1.1 200 OK\nContent-Length: 29\nContent-Type: text/html\n\n", 65);
        sprintf(buffer, "Your ip is: %s", incoming->sa_data);
        write(connfd, buffer, 24);
        printf("SA_DATA\n%s", incoming->sa_data);
        printf("END OF SA_DATA\n");
        close(connfd);
    }


    return 0;
}

I have four questions
1) Why do I have to malloc *res but not *hints? When I tried to access *res variables it gave seg fault error
2) Why the line

Code:

 printf("%c", res->ai_addr->sa_data[i]);

doesn't print anything? is sa_data empty? why? isn't it supposed to contain an ip address?
3) In the lines

Code:

sprintf(buffer, "Your ip is: %s", incoming->sa_data);
write(connfd, buffer, 24);

I try to write "Your ip is: *some ip*" but the browser shows "YourYour ip is: keep-alive"
Also, what snprintf prints as incoming->sa_data differs from what

Code:

printf("SA_DATA\n%s", incoming->sa_data);

prints
4) if I change *hints into hints, change all the hints-> to hints. and in the getaddrinfo put a & before hints, it stops working why?

Thanks in advanced

I recoment you to read this Beej guide to network programming

Originally Posted by flp1969

I recoment you to read this Beej guide to network programming

I read some of it, it doesn't answer my questions.
1) is a about the language, not network programming
2) the guide says sa_data should contain a protocol address, but it doesn't
3) the same as two, except sa_data contains something but not an address
4) is about the language, not network programming

1) Why do I have to malloc *res but not *hints? When I tried to access *res variables it gave seg fault error

You don't have to malloc res. Read the Beej's guide for examples of how to use getaddrinfo().

2) Why the line

Code:

 printf("%c", res->ai_addr->sa_data[i]);

doesn't print anything? is sa_data empty? why? isn't it supposed to contain an ip address?
3) In the lines

Code:

sprintf(buffer, "Your ip is: %s", incoming->sa_data);
write(connfd, buffer, 24);

I try to write "Your ip is: *some ip*" but the browser shows "YourYour ip is: keep-alive"
Also, what snprintf prints as incoming->sa_data differs from what

Code:

printf("SA_DATA\n%s", incoming->sa_data);

prints

sa_data is not a string. It's a raw data type. Use inet_ntop() to convert it to a human-readable string. Beej's guide has example code to do that too.

4) if I change *hints into hints, change all the hints-> to hints. and in the getaddrinfo put a & before hints, it stops working why?

Your original code doesn't allocate memory for hints or even initialize it, so it's pointing to who knows where, so you're overwriting some memory that you may or may not have access to. hints doesn't have to be a pointer; you just have to pass its address to getaddrinfo().

Originally Posted by qqsszz

I read some of it, it doesn't answer my questions.
1) is a about the language, not network programming
2) the guide says sa_data should contain a protocol address, but it doesn't
3) the same as two, except sa_data contains something but not an address
4) is about the language, not network programming

My sugestion was about your code... It is ALL wrong and you should study this well writen simplified guide.
But... be my guest and I wish you good luck...

Originally Posted by flp1969

Because getaddinfo() will returnt (in the last argument) the pointer of a linked list allocated by itself.
Notice the prototype:

Code:

int getaddrinfo(const char *node, const char *service,
                const struct addrinfo *hints,
                struct addrinfo **res);

That's why you need to call freeaddrinfo() afterwards.

About 'hints' you don't need to declare as a pointer:

Code:

struct addrinfo hints = { .ai_flags = ... };

And the third arguments is passed as &hints.

But in your code you don't need to call getaddrinfo!

I think you misunderstood the question because I forgot to mention which pointer. I was talking about hints, not res.
And I don't understand why you say I don't need to call getaddrinfo, I took that part of the code from here System Calls or Bust. I don't know what to give to bind if I don't call getaddrinfo

Originally Posted by laserlight

You do need to cause the pointer to point to something valid.


That's undefined behaviour: it might work for you, then mysteriously fail when you're demonstrating your program to your big boss.

Thanks

@qqsszz, you are trying to create a server for connections, so your code will bind to 0.0.0.0, the address of your network interface or 127.0.0.1. getaddrinfo() is used to query DNS (or hosts file) to translate a NAME to an IP address. For your purposes you don't need this. An example of getaddrinfo() usage:

Code:

#include <stdlib.h>
#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netdb.h>

int main ( void )
{
  // I'll try to get debian.org IP addresses...
  char *name = "debian.org";
  struct addrinfo *res, *p;
  struct addrinfo hint = { .ai_flags = AI_PASSIVE,
                           .ai_family = AF_INET,
                           .ai_socktype = SOCK_STREAM,
                           .ai_protocol = IPPROTO_TCP };
  int status;

  // getaddrinfo() returns 0 if success.
  if ( status = getaddrinfo ( name, NULL, &hint, &res ) )
  {
    // perror() will not work here!
    fprintf ( stderr, "getaddrinfo: %s\n", gai_strerror ( status ) );
    return EXIT_FAILURE;
  }

  // res is a linked list allocated by getaddrinfo!
  for ( p = res; p; p = p->ai_next )
  {
    char buffer[16];  //           1111111
                      //  1234567890123456
                      // "xxx.xxx.xxx.xxx\0"

    if ( ! inet_ntop ( AF_INET,
                       & ( ( struct sockaddr_in * ) p->ai_addr )->sin_addr,
                       buffer, sizeof buffer ) )
    {
      perror ( "inet_ntop" );
      freeaddrinfo ( res );
      return EXIT_FAILURE;
    }

    printf ( "Found: %s\n", buffer );
  }

  // Need to free the linked list!
  freeaddrinfo ( res );

  return EXIT_SUCCESS;
}

And yes... I was talking about res AND hints, as I show above.