Why does this pointer-fu segfault?

Question 1

The code doesn't follow the structure layout. The layout is:

 struct tt__IPAddress
 {
    enum tt__IPType Type;   /* required element of type tt:IPType */
    char *IPv4Address;  /* optional element of type tt:IPv4Address */
    char *IPv6Address;  /* optional element of type tt:IPv6Address */
 };

meaning: IPv4Address is a char pointer. yet this:

DNSInformation->DNSManual->IPv4Address = (char **)soap_malloc(soap, sizeof(char *));

is assigning a char ** cast to it. but the type is still char * so this:

strncpy(*DNSInformation->DNSManual->IPv4Address, dns_string, LARGE_INFO_LENGTH-1);

is dereferencing said pointer to a single char, which I can assure you is NOT compatible with a char * on your platform (and likely any other for that matter).

There should be warnings running amok on this compilation at a minimum, and outright errors if your compiler has any brains at all. This looks like it was original intended to be this:

 struct tt__IPAddress
 {
    enum tt__IPType Type; 
    char **IPv4Address;  /* note ptr to ptr */
    char **IPv6Address;
 };

for having a dynamic pointer array, each pointer being dynamically allocated memory for a single IP address. if it were like this, it would make much more sense. That said, if you intend on only a single IPv4 address per structure then this should be changed:

DNSInformation->DNSManual = soap_malloc(soap, sizeof(struct tt__IPAddress)));
if (DNSInformation->DNSManual)
{
    DNSInformation->DNSManual->IPv4Address = soap_malloc(soap, sizeof(char) * LARGE_INFO_LENGTH);
    if (DNSInformation->DNSManual->IPv4Address)
    {
        strncpy(DNSInformation->DNSManual->IPv4Address, dns_string, LARGE_INFO_LENGTH-1);
        DNSInformation->DNSManual->IPv4Address[LARGE_INFO_LENGTH-1] = 0;
    }
}

Or something similar to that.

Question 2

I think it looks broken.

This:

char *IPv4Address;  /* optional element of type tt:IPv4Address */

says IPv4Address is a single pointer to character data, i.e. a string.

But then it's used like this:

DNSInformation->DNSManual->IPv4Address = (char **)soap_malloc(soap,
                                                              sizeof(char *));

This is just wrong. Assuming a sane return value for soap_malloc() (i.e. void * to comply with malloc()), no cast should be necessary, but the fact that the cast differs from the actual type signal some kind of error.

It treats the IPv4Address struct field as a pointer to pointer, which it clearly is not.

Question 3

I'm sure it should look similar to this:

DNSInformation->DNSManual = soap_malloc(soap, sizeof(struct tt__IPAddress)));
DNSInformation->DNSManual->IPv4Address = soap_malloc(soap, sizeof(char) * LARGE_INFO_LENGTH);

strncpy(DNSInformation->DNSManual->IPv4Address, dns_string, LARGE_INFO_LENGTH-1);

Your struct contains pointers to strings, but first it is allocating a array of pointers (char**) and then allocating memory for the first pointer in this array.

and don't forget to set the binary zero after you used strncpy() as it doesn't set it itself.

//Edit: First part was wrong, sorry

Question 4

Here is a working solution (I used malloc instead of soap_malloc etc.):

#include <stdio.h>
#include <stdlib.h>

#define LARGE_INFO_LENGTH 1024

enum tt__IPType { tt__IPv4, tt__IPv6 };

struct tt__IPAddress
{
  enum tt__IPType Type;   /* required element of type tt:IPType */
  char *IPv4Address;  /* optional element of type tt:IPv4Address */
  char *IPv6Address;  /* optional element of type tt:IPv6Address */
};

struct tt__DNSInformation
{
  struct tt__IPAddress* DNSManual;
};

int main()
{
  struct tt__DNSInformation* DNSInformation;
  char dns_string[] = "192.168.2.254";

  DNSInformation = malloc(sizeof(struct tt__DNSInformation));
  DNSInformation->DNSManual = malloc(sizeof(struct tt__IPAddress));
  DNSInformation->DNSManual->IPv4Address = malloc(sizeof(char) * LARGE_INFO_LENGTH);
  strncpy(DNSInformation->DNSManual->IPv4Address, dns_string, LARGE_INFO_LENGTH - 1);

  printf("%s\n", DNSInformation->DNSManual->IPv4Address);
  return 0;
}