Help on sockets needed!

[John Ioannidis]

In article <507 at asuvax.UUCP>, system at asuvax.UUCP (Marc Lesure) writes:
> Here at Arizona State University we have a class being taught which requires
> the students to use sockets.  The problem we have is that when these users
> start running their software, our VAX hangs and the only way to recover is
> to hard crash the VAX.  I've looked at the routines the students have
> written and they seem to follow what the documentation says (I'm not that
> familiar with the socket software).
> 
>      Is there an error(s) in the documentation?
	Are you kidding? errors or omissions in the Unix documentation?

>      Is there a limit on the number of sockets that can used at any on time?
	There is only that limit of the #of processes trying to connect
	to a specific socket. (As specified in the listen(2) call)

>      Do sockets prevent dead-locking situations?  
	Most probably they create dlock's inside the kernel :-)

> 
> We are running a VAX 11/780 with 4.2bsd.
> 

>From the description, I understand that the problem is with Unix domain
(AF_UNIX) sockets. 

Two programs that demonstrate the use of unix-domain sockets are the
following. Compile the as follows:

$ cc und_send.c -o us
$ cc und_recv.c -o ur

and execute them :

$ ur&
$ us

Then, you'll (hopefully!) get the following output, which comes from ur:

Accepted!
	Lenght:110
	Family: 1
	Path: yourself

Got peer!
	Lenght:110
	Family: 0
	Path: 

Read: Hello, yourself!

Of course, like it says in the manual, the peer's family is always 0 and
the path doesn't make sense.
Sorry that the programs are not very well commented (the code is obvious :-) )
but I just don't have the time to add them right now.
So, enjoy:


----------CUT HERE: und_send.c ------------

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>

/* 
 * I included <sys/un.h> here and changed the path length from 109 to 108.
 * Don't ask me why -- it works this way, but didn't with 109!!!
 */

/*	un.h	6.1	83/07/29	*/

/*
 * Definitions for UNIX IPC domain.
 */
struct	sockaddr_un {
	short	sun_family;		/* AF_UNIX */
	char	sun_path[108];		/* path name (gag) [mod by ji] */
};

#ifdef KERNEL
int	unp_discard();
#endif

main()
{
	int sm;
	struct sockaddr_un myself;
	struct sockaddr_un yourself;

	bzero( &myself, sizeof( myself ) );
	bzero( &yourself, sizeof( yourself ) );

	myself.sun_family = AF_UNIX;
	yourself.sun_family = AF_UNIX;

	strcpy( myself.sun_path, "myself" );
	strcpy( yourself.sun_path, "yourself" );

	sm = socket( AF_UNIX, SOCK_STREAM, 0, 0 );
	if( sm < 0 )
	{
		perror( "und_send: socket" );
		exit(1);
	}

	unlink( "myself" );
	if( bind( sm, &myself, sizeof( myself ) ) < 0 )
	{
		perror( "und_send: bind" );
		exit( 1 );
	}

	if( connect( sm, &yourself, sizeof( yourself ) ) < 0 )
	{
		perror( "und_send: connect" );
		exit( 1 );
	}

	if( write( sm, "Hello, yourself!", 17 )<0 )
		perror( "und_send: write" );
}

-------------CUT HERE: und_recv.c -----------


#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>

/*	un.h	6.1	83/07/29	*/

/*
 * Definitions for UNIX IPC domain.
 */
struct	sockaddr_un {
	short	sun_family;		/* AF_UNIX */
	char	sun_path[108];		/* path name (gag) [mod by ji] */
};

#ifdef KERNEL
int	unp_discard();
#endif

main()
{

/* Now, I am YOURSELF!!! */

	int i, sm, as, rfds, mylen, hislen;
	char buf[80];
	struct sockaddr_un myself;
	struct sockaddr_un yourself;
	struct sockaddr_un himself;

	bzero( &myself, sizeof( myself ) );
	bzero( &yourself, sizeof( yourself ) );
	bzero( &himself, sizeof( himself ) );

	yourself.sun_family = AF_UNIX;

	strcpy( yourself.sun_path, "yourself" );

	sm = socket( AF_UNIX, SOCK_STREAM, 0, 0 );
	if( sm < 0 )
	{
		perror( "und_recv: socket" );
		exit(1);
	}

	unlink( "yourself" );
	if( bind( sm, &yourself, sizeof( yourself ) ) < 0 )
	{
		perror( "und_recv: bind" );
		exit( 1 );
	}

	if( listen( sm, 5 ) < 0 )
	{
		perror( "und_recv: listen" );
		exit( 1 );
	}

	/* 
	 * SCREAM OF JOY !!!!!!!!!!
	 * The key to success is the following select statement
	 * I saw that in a program our system administrator had
	 * given me quite some time ago, but which did not otherwise
	 * work like I wanted it to. Also note that the select(2) 
	 * call is somewhat changed in 4.3, so beware.
	 */

	rfds = 1 << sm;
	select( 20, &rfds, 0, 0, 0 );

	mylen = sizeof( myself );
	if( ( as=accept( sm, &myself, &mylen )) < 0 )
	{
		perror( "und_recv: accept" );
		exit( 1 );
	}
	printf( "Accepted!\n\tLenght:%d\n\tFamily: %d\n\tPath: %s\n\n",
			mylen, myself.sun_family, myself.sun_path );
	
	hislen = sizeof( himself );
	getpeername( as, &himself, &hislen );
	printf( "Got peer!\n\tLenght:%d\n\tFamily: %d\n\tPath: ",
			hislen, himself.sun_family );
	for( i=0; i<hislen; i++ )
		putchar( himself.sun_path[i]);
	printf( "\n\n" );
	

	if( read( as, buf, 80 ) < 0 )
	{
		perror( "und_recv: read" );
		exit( 1 );
	}
	printf( "Read: %s\n", buf );
}

----------------CUT HERE-------------

So, that's all, folks. I'm posting this instead of directly mailing
it because I'm pretty sure that other people have also had problems
with unix-domain sockets. 

____________

#include <appropriate_disclaimers>

VOICE: 	+1 212 280 5510			ARPA: ioannidis at cs.columbia.EDU
USnail:	John Ioannidis			      ji at garfield.columbia.EDU
	450 Computer Science
	Columbia University,		USENET: ...{seismo|topaz}!
	New York, NY 10027			   columbia!garfield!ji

			... It's all Greek to me!