Memory management with `addrinfo` structures

Question

I've just started to work with UDP sockets in C and I have a question relating to memory management. Often examples show something of the following

struct addrinfo *result;    //to store results
struct addrinfo hints;      //to indicate information we want
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if( (s = getaddrinfo(hostname, NULL, &hints, &result)) != 0){
  fprintf(stderr, "getaddrinfo: %s\n",gai_strerror(s));
  exit(1);
}
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:
:
//free the addrinfo struct
freeaddrinfo(result);

I've also seen the introduction of an additonal local addrinfo structure used in a for loop to cycle through the result for the sake of extracting a particular entry...something like this

struct addrinfo *entry;    //to store a result from linked list
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for (entry = result; entry != NULL; entry = entry->ai_next)
{
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}

My question is why is result the only structure that gets passed to freeaddrinfo and not hints or entry? In other words, I've never seen anyone call freeaddrinfo(hints).

One other thing, should I be preallocating memory for or initializing structures of the form addrinfo? If so, how do I do that?

[Man page for getaddrinfo and freeaddrinfo](https://linux.die.net/man/3/freeaddrinfo) has the answer to your question — , Jul 15 '18 at 07:32
Just skimmed through...it's because `getaddrinfo` dynamically allocates the memory? — ThatsRightJack, Jul 15 '18 at 07:36
If that's the case, should I be pre-allocating memory / initializing `entry` in the example above? Why doesn't that get passed to `freeaddrinfo` ? — ThatsRightJack, Jul 15 '18 at 07:38
Its difficult to answer your questions within the comments section. I'm going to reply with an answer — , Jul 15 '18 at 07:41
This question is not related to UDP at all, removing the tag. — alk, Jul 15 '18 at 08:21
It most often is a good idea to read the documentation prior to posting a more or less vague question on SO. — alk, Jul 15 '18 at 08:23

score 2 · Accepted Answer · 2018-07-15T07:55:41.870

Just skimmed through...it's because getaddrinfo dynamically allocates the memory?

Yes. More specifically, it allocates memory for the linkedlist pointed to by result pointer. Since the library is allocating memory on your behalf, library implementors have also providing you with an API to free the memory.

If that's the case, should I be pre-allocating memory / initializing entry in the example above? Why doesn't that get passed to freeaddrinfo

No. entry is just a variable used to iterate through the list. It is very much similar to

`for (i = 0; i < n; i++)`

loop you will find anywhere else. i is just an iterator.

You don't have to use entry variable at all if you don't want to. You can iterate through the linkedlist with just a while loop

while (result != NULL) {
    //Do what you have to do here

    result = result->ai_next;
}

If you observe closely, you will see that you are moving the result variable itself to the end of the list. If you iterate this way, there is no way for you to free the memory since you have lost the starting address which was returned to you by getaddrinfo(). To get around this, you can save the starting address in a temp variable and then use the while loop. Just add this line before the while loop.

struct addrinfo *entry = result; //save the starting address
while (...) {

}

You can call free on entry pointer now, instead of result. The effect would be the same.

freeaddrinfo(entry);

Interesting...I thought iterators were only integers. Sounds like a can of worms opening :) — ThatsRightJack, Jul 15 '18 at 07:52
read about pointer arithmetic. It is a bit tricky at first to understand it. But it is needed if you want to use C language — , Jul 15 '18 at 07:59

Memory management with `addrinfo` structures

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