Re: Bug#187391: PortingIssues sockaddr_un

[Ognyan Kulev]

Ognyan Kulev wrote:
> This discussion is about
> http://hurd.gnufans.org/bin/view/Distrib/PortingIssues#Uncompliant_use_of_sockaddr_un_t 
>
> We need an agreement about what should be in this page.
>
> Robert Millan wrote:
>
> > On Sat, Apr 05, 2003 at 06:01:16PM +0300, Ognyan Kulev wrote:
> >
> > > sub.sun_path[sizeof(sun.sun_path) - 1] = '\0';

I've changed my mind about this, so I've added it again.

I understand that my changes are somewhat intrusive to what Robert 
Millan wrote, but I wouldn't do it if I didn't beleive they are right. 
Please say your mind about the current text.

 Uncompliant use of sockaddr_un

The following declaration:

sockaddr_un sun = { AF_UNIX, "/path/to/socket" };

won't work on GNU/Hurd. The Glibc API requires that the second parameter 
of a sockaddr_un struct is array of chars, but NOT pointer to array of 
chars. So we have to copy them directly into the sun_path address. Glibc 
wants string of chars there that doesn't need to end in nul character 
_and_ correct size of sockaddr_un passed to socket functions. When 
calling socket functions one should always use SUN_LEN (su) for the 
sockaddr length argument. An example:

sockaddr_un su;
/* AF_LOCAL is the canonical flag in Glibc.
   Use AF_UNIX if you want compatibility.  */
su.sun_family = AF_LOCAL;
/* Copy the string into sun_path. It must be
   no longer than approximately 100 characters. */
strcpy (su.sun_path, "/path/to/socket");

SUN_LEN macro is not defined in The Single UNIX Specification Version 3 
(see sys/un.h manpage). You can use the following definition to handle 
this case (definition taken from NetBSD):

#ifndef SUN_LEN
#define SUN_LEN(su) \
   (sizeof(*(su)) - sizeof((su)->sun_path) + \
   strlen((su)->sun_path))
#endif

If you have pointer to array of characters as file name, you'd better 
use the following code to set sun_path:

strncpy (su.sun_path, filename, sizeof (su.sun_path));
su.sun_path[sizeof (su.sun_path) - 1] = '\0';

If you expect file name longer than approximately 100 characters, use 
the following code. This is the recommended code for GNU systems. You 
can include it whenever __GLIBC__ is defined.

/* `offsetof', `alloca' and `AF_LOCAL' may not
   be available everywhere.  */
socklen_t su_len = offsetof (struct sockaddr_un, sun_path)
                   + strlen (filename) + 1;
struct sockaddr_un *su = alloca (su_len);
su->sun_family = AF_LOCAL;
strcpy (su->sun_path, filename);

NOTE: the current API is subject to change, see the notes in Glibc's 
docs ("info libc" and search for sockaddr_un) and Debian bug #187391.

Regards
--
Ognyan Kulev <ogi@fmi.uni-sofia.bg>, "\"Programmer\""