How do I access members of this structure in C?

Question

I'm working with this structure in C:

/** This structure describes an Internet host address. */
typedef struct pj_hostent
{
    char    *h_name;        /**< The official name of the host. */
    char   **h_aliases;     /**< Aliases list. */
    int      h_addrtype;    /**< Host address type. */
    int      h_length;      /**< Length of address. */
    char   **h_addr_list;   /**< List of addresses. */
} pj_hostent;

I can access the h_name part of the structure fine like this:

strcpy(test1, he->h_name); // copy part of struct into char[] array

and it contains a meaningful "sip2" value. However, when I try to access the elements of h_addr_list like this:

strcpy(test1, he->h_addr_list[0]);

I get meaningless jibberish.

What's the correct way in C to access values like this?

Answer 1

struct elements are accessed with the dot operator

strcpy(test1, he.h_addr_list[0]);

elements of structs that are "pointed" to are accessed with the -> operator

strcpy(test1, he->h_addr_list->[0]);

If you're getting gibberish, then either test1 isn't big enough, h_addr_list isn't pointing to an array of char*, or the first element of the array is actually gibberish.

Answer 2

You are accessing it correctly. That field really does contain gibberish.

Answer 3

strcpy assumes that you have already allocated memory to the character pointer. So, probably, the pointer is still having a NULL value.

Answer 4

h_addr_list[0] is not a string, it is a pj_in_addr, which is a 32-bit integer which is not null terminated.

printf() it with %d or %x, not %s.

See PJLIB Reference: Network Address Resolution for an example of use.

Answer 5

The struct only contains a pointer. You need to allocate some storage for that pointer to point at before you can use it. As it is, the pointer almost certainly just contains a (more or less) random value. When you use that as a target in your call to strcpy, you're overwriting that random memory location -- which can and will lead to major problems elsewhere.

h_addr_list is pretty much the same way -- it's just a pointer to a pointer. That'll (normally) be used to refer to an array of strings, but something has to allocate that array of strings before it can be put to any meaningful use. Given the similarity to a hostent structure, along with this structure there's presumably some analog to (or wrapper around) gethostbyname and/or gethostbyaddress that initializes this. In the case of the wrapper (which I'd judge the more likely of the two), it probably gets the correct data in a hostent, and then copies data from the hostent to this structure.

Answer 6

A short Example that does work. Remember that allocating for the struct does not allocate for the pointer.

typedef struct pj_hostent
{
    char    *h_name;        /**< The official name of the host. */
    char   **h_aliases;     /**< Aliases list. */
    int      h_addrtype;    /**< Host address type. */
    int      h_length;      /**< Length of address. */
    char   **h_addr_list;   /**< List of addresses. */
} pj_hostent;

int _tmain(int argc, _TCHAR* argv[])
{
    char* s[]= {"this","is","couple of words"};
    pj_hostent hst = {"high",s,3,4,s };
    char  buff[255] = {};
    //Access as a local initialized struct
    strcpy(buff,hst.h_addr_list[0]);
    printf("%s\n",buff);
    pj_hostent * he = & hst; 
    //Access as a pointer 
    strcpy(buff,he->h_addr_list[0]);
    printf("%s\n",buff);
    gets(buff);
}

Answer 7

If you never initialized/allocated memory for those pointers, you would get jibberish and/or segmentation faults because they are pointing to random points in memory.

Answer 8

Just because it is declared as a char** does NOT mean that it is an array of strings.

It may be a pointer to a byte-buffer of unknown format. (ie. jibberish)

You should check any available documentation or other source code for details of exactly what h_addr_list is, and how it should be interpreted.