Date: Thu, 13 Mar 2008 10:45:15 -0700 From: Patrick Mahan <mahan@mahan.org> To: Andrew Falanga <af300wsm@gmail.com> Cc: FreeBSD Questions <freebsd-questions@freebsd.org> Subject: Re: Network programming question Message-ID: <47D9682B.5060402@mahan.org> In-Reply-To: <340a29540803130910l2a5badacxe50cd81ace87e1f7@mail.gmail.com> References: <340a29540803130910l2a5badacxe50cd81ace87e1f7@mail.gmail.com>
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Andrew Falanga presented these words - circa 3/13/08 9:10 AM->
> Hi,
>
> I'd like to know why the inet_pton(3) doesn't fill in the address
> family of the proper structure passed into it. I'm at a complete loss
> for why. Here's the prototype:
>
> int inet_pton(int af, const char * restrict src, void * restrict dst);
>
>
> Three arguments only. The address family, hm, I'm passing it in; the
> address string in printable ASCII text, and a void pointer to the
> address structure to put the address into, presumably one of the
> sockaddr_* family structures for AF_INET or AF_INET6 (further, the man
> page says that this function is only valid for these two families now
> anyway).
>
>>From some coding for a program, I did find that this function,
> inet_pton(3), *does* in fact mangle the sin_family member of the
> sockaddr_in structure, so why not "mangle" it to what it should be? I
> was doing something like this:
>
> // valid code above
> sockaddr_in sa;
>
> sa.sin_family = AF_INET;
> sa.sin_port = htons(3252);
>
> inet_pton(AF_INET, "192.168.0.1", &sa);
>
> sendto(sa, msg, strlen(msg), 0, (struct sockaddr*)&sa, sizeof(sa));
>
See man inet_pton . . . for details.
Briefly, inet_pton() doesn't understand sockaddr structures. Instead,
it only understands in_addr or in6_addr structures which are included
inside the sockaddr structure. So your above example should be changed
to
// valid code above
sockaddr_in sa;
int res;
sa.sin_family = AF_INET;
sa.sin_port = htons(3252);
if ((res = inet_pton(AF_INET, "192.168.0.1", &sa.sin_addr)) < 0)
perror("inet_pton");
if (!res) // error occurred
fprintf(stderr, "Address notation incorrect for AF_INET address\n");
>
> The call to sendto is wrapped in an if an was failing for errno code
> 47, Address family not supported by protocol (I was using UDP). I
> changed the assignment of AF_INET to the sa.sin_family member to
> *after* the call to inet_pton(3) and suddenly everything worked. Why?
> Since the address family was used by inet_pton(3) to figure out how
> to read the address and assign it to sa.sin_addr.s_addr, why not
> simply assign AF_INET to the address family member in inet_pton(3)?
>
Because it is treating the sockaddr_in structure as an in_addr structure
which is clobbering the sin_family field.
> I'm not trying to be argumentative. I'm just curious. It seems like
> redundancy. I've used the address family to tell inet_pton(3) how to
> operate, and then this function can't assign it to the sockaddr_in
> structure passed to it? This makes little sense. In case it's
> because I'm using older FBSD libraries that had a flaw fixed, I'm
> using FreeBSD 6.2-RELEASE-p4. Is this because that's how POSIX
> defined it to work? Is this the right venue or should I try one of
> the other mailing lists?
>
RTM,
Patrick
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