Thread: pointers to pointers in struct

hey,

getting a bit confused with pointers, structs and pointers within structs. I'm trying to use gethostbyname to get resolve a hostname to an ip, which I will then store in the host struct. However, I'm doing something wrong before I even get to the stage of IP's; with copying the hostname.

I think hp->h_name is a pointer, which is being copied (as a pointer) into h->name. which is why when I call gethostbyname the second time (because it's using the same piece of memory) the 'facebook.com' name is being overwritten.

the problem I have is that I don't really understand if this is a pointer to a pointer (within the hostent struct), and how I actually assign the contents of h_name to h->name.

am I correct in thinking, as it stands, h->name now contains a pointer to the memory location used by gethostbyname ? if so, how do I print the memory address contained within h->name ?

thanks,

Code:

#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define MAXHOSTS 5

struct host {
    char *name;
    in_addr ip;
};

struct host hostlist[MAXHOSTS];

int main(int argc, char *argv[])
{

    struct hostent *hp;

    hp = gethostbyname("facebook.com");
    hostlist[0].name = hp->h_name;
    printf("hp->h_name %s is at %p, hostlist[0] %s is at %p\n", 
        hp->h_name, &hp->h_name,
        hostlist[0].name, hostlist[0].name);

    hp = gethostbyname("google.com");
    hostlist[1].name = hp->h_name;
    printf("hp->h_name %s is at %p, hostlist[1] %s is at %p\n",
        hp->h_name, &hp->h_name,
        hostlist[1].name, &hostlist[1].name);

    struct host *p;

    int i;
    for (i = 0; i <= MAXHOSTS;) {
        p = &hostlist[i];

        printf("%d : (%p) host : (%p) name = %s\n", i, p, p->name, p->name);
        ++i;
    }
    return 0;
}

Check your man pages:

Originally Posted by man gethostbyname

DESCRIPTION
The gethostbyname*() and gethostbyaddr*() functions are obsolete. Applications should use getaddrinfo(3) and getnameinfo(3) instead.
...
RETURN VALUE
The gethostbyname() and gethostbyaddr() functions return the hostent structure or a NULL pointer if an error occurs. On error, the h_errno variable holds an error number.
When non-NULL, the return value may point at static data, see the notes below.
...
NOTES
The functions gethostbyname() and gethostbyaddr() may return pointers to static data, which may be overwritten by later calls. Copying the struct hostent does not suf-
fice, since it contains pointers; a deep copy is required.

You should switch to getaddrinfo, which is more current. Also, if you printed out the address in hp (i.e. add a "%p" specifier, and pass it just plain old hp), you would see that after both calls to gethostbyname(), you end up with the same hostent structure (same address). Looks like your implementation returns a pointer to static data, as the man pages warns, so you need to use strdup or a malloc/strcpy (remember +1 for the nul) to copy the contents of the name field. This problem is hard for you to detect since you use two different diagnostic print statements:

Code:

    printf("hp->h_name %s is at %p, hostlist[0] %s is at %p\n", 
        hp->h_name, &hp->h_name,
        hostlist[0].name, hostlist[0].name);
...
    printf("hp->h_name %s is at %p, hostlist[1] %s is at %p\n",
        hp->h_name, &hp->h_name,
        hostlist[1].name, &hostlist[1].name);

That extra ampersand probably confused you.

Hi,

thanks. yeah that extra ampersand was just left there while playing around with the first statement trying different options to print the address in hostlist[0].name.

I get that gethostbyname uses static memory to save its results. however, I am not understanding why :

Code:

hostlist[0].name = hp->h_name;

doesn't work, and why when I come to print out hostlist[0].name it's printing out what's in hostlist[1].name. I suspect that both of these 'names' (via my assingment) are simply holding the address of the static memory used by gethostbyname, but I don't really understand 'why'.

cheers

Originally Posted by dougieb

the problem I have is that I don't really understand if this is a pointer to a pointer (within the hostent struct), and how I actually assign the contents of h_name to h->name.

If you are working with an array of structs, you probably should not use pointers inside the structs and especially you should not simply copy pointers around...

Try it like this...

Code:

#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define MAXHOSTS 5

struct host {
    char name[128];
    in_addr ip;
};

struct host hostlist[MAXHOSTS];

int main(int argc, char *argv[])
{

    struct hostent *hp;

    hp = gethostbyname("facebook.com");
    strcpy(hostlist[0].name, hp->h_name);
    printf("hp->h_name %s is at %p, hostlist[0] %s is at %p\n", 
        hp->h_name, &hp->h_name,
        hostlist[0].name, hostlist[0].name);

    hp = gethostbyname("google.com");
    strcpy(hostlist[1].name, hp->h_name);
     printf("hp->h_name %s is at %p, hostlist[1] %s is at %p\n",
        hp->h_name, &hp->h_name,
        hostlist[1].name, &hostlist[1].name);

    struct host *p;

    int i;
    for (i = 0; i <= MAXHOSTS;) {
        p = &hostlist[i];

        printf("%d : (%p) host : (%p) name = %s\n", i, p, p->name, p->name);
        ++i;
    }
    return 0;
}

This way the data is stored in the struct as individual strings for each element in the array... much simpler, far easier to manage.

hmm.. "especially you should not simply copy pointers around... " is that what 'hostlist[0].name = hp->name' is doing ? is that copying the pointer in hp->name, rather than (as I wanted) copying the contents ? I'm just trying to understand it, so I don't repeat the mistakes.

so, first output is the original. second is with Tatter's changes. certainly works :-)

Code:

$ ./test 
hp->h_name facebook.com is at 0x9e6da4, hostlist[0] facebook.com is at 0x848a0f5
hp->h_name google.com is at 0x9e6da4, hostlist[1] google.com is at 0x848a0ff
0 : (0x8049800) host : (0x848a0f5) name = oogle.com
1 : (0x8049808) host : (0x848a0ff) name = google.com
2 : (0x8049810) host : ((nil)) name = (null)
3 : (0x8049818) host : ((nil)) name = (null)
4 : (0x8049820) host : ((nil)) name = (null)
5 : (0x8049828) host : ((nil)) name = (null)
$ gcc -Wall -o test test.c
$ ./test 
hp->h_name facebook.com is at 0x9e6da4, hostlist[0] facebook.com is at 0x8049860
hp->h_name google.com is at 0x9e6da4, hostlist[1] google.com is at 0x80498e4
0 : (0x8049860) host : (0x8049860) name = facebook.com
1 : (0x80498e4) host : (0x80498e4) name = google.com
2 : (0x8049968) host : (0x8049968) name = 
3 : (0x80499ec) host : (0x80499ec) name = 
4 : (0x8049a70) host : (0x8049a70) name = 
5 : (0x8049af4) host : (0x8049af4) name =

Yes, you were simply copying the pointer, not the contents, so both [0] and [1] pointed to the same array in static memory, as Quzah pointed out. Tater's solution is a good one (certainly easiest to implement). My only suggestion might be to make the name field bigger (say, 256) since I think that's what the RFC specifies for host names, though I'm not sure so you should double check.

Originally Posted by anduril462

Yes, you were simply copying the pointer, not the contents, so both [0] and [1] pointed to the same array in static memory, as Quzah pointed out. Tater's solution is a good one (certainly easiest to implement). My only suggestion might be to make the name field bigger (say, 256) since I think that's what the RFC specifies for host names, though I'm not sure so you should double check.

I would have to look but I believe there's a #define in winsock2.h for the host size...

good tutorial Andrew. Although it makes sense, I'd never really considered that 'a pointer' is just variable that has it's own address as well. so &ptr is giving you a pointer to a pointer (assuming ptr has a value).

also I think (with a combination of what you guys have said and that tutorial) my problem was that

Code:

hostlist[0].name = "myspace.com"

worked, but

Code:

hostlist[0].name = hp->h_name

didn't. BUT, the "myspace.com" string constant, which is just a char array with a beginning address/pointer. but, memory is allocated as it's initialized. so, these two lines are effectively doing the same thing, copying a pointer. it's just that (now obvious, and as was said in the first reply) the string constant is not overwritten so hostlist[0].name can point to it as long as it exists without it being overwritten; unlike the static memory location of gethostbyname. but, crucially, for my understanding, both of the above assignments are doing the same thing, assigning pointers. and the strcpy is used to copy the contents out to a new memory location, and then it is copying the address of that new memory location into hostlist[i].name

thanks for all your help. I'll keep practicing :-)

Originally Posted by dougieb

also I think (with a combination of what you guys have said and that tutorial) my problem was that

Code:

hostlist[0].name = "myspace.com"

worked, but

Code:

hostlist[0].name = hp->h_name

didn't. BUT, the "myspace.com" string constant, which is just a char array with a beginning address/pointer. but, memory is allocated as it's initialized. so, these two lines are effectively doing the same thing, copying a pointer. it's just that (now obvious, and as was said in the first reply) the string constant is not overwritten so hostlist[0].name can point to it as long as it exists without it being overwritten; unlike the static memory location of gethostbyname. but, crucially, for my understanding, both of the above assignments are doing the same thing, assigning pointers. and the strcpy is used to copy the contents out to a new memory location, and then it is copying the address of that new memory location into hostlist[i].name

thanks for all your help. I'll keep practicing :-)

Ok, here's a little code sample I post from time to time to illustrate why you should not try to return arrays from functions... or in your case assign pointers to static arrays... Give this a run, I'll bet it explains your problem for you...

Code:

#include <stdio.h>

int* MyFunction(int a, int b, int c)
  {  static int array[3];
     array[0] = a;
     array[1] = b;
     array[2] = c;
     return array;  } // return a pointer.


int main (void)
  { int *a1, *a2;  // int pointers

    printf("calling a1 = MyFunction(10,20,30);\t");
    a1 = MyFunction(10,20,30);
    printf("a1 has %d %d %d\n",a1[0],a1[1],a1[2]);

    printf("calling a2 = MyFunction(100,200,300);\t");
    a2 = MyFunction(100,200,300);
    printf("a2 has %d %d %d\n",a2[0],a2[1],a2[2]);

    printf("\nLooks good, except...\t"); 
    printf("a1 now has %d %d %d\n",a1[0],a1[1],a1[2]);

    getchar();
    return 0; }

yep, that's it. makes sense when you understand it :-)

thanks again..