Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!prcrs!paul
From: paul@prcrs.UUCP (Paul Hite)
Newsgroups: comp.sys.hp
Subject: Re: System-configurable parameters
Message-ID: <1284@prcrs.UUCP>
Date: 8 Nov 90 20:03:42 GMT
References: <675@modus.sublink.ORG>
Organization: PRC Realty Systems, McLean, VA
Lines: 68

In article <675@modus.sublink.ORG>, luke@modus.sublink.ORG (Luciano Mannucci) writes:
> Does anyone know how a program (i.e. a C program) can test a system
> configurable parameter (such as shmseg or msgmax) in order to complain
> propperly whithout a need of generating a system error?
> 
> luke.

Well there's a problem with this approach.  You're trying to do 
something like:
	if(verify_that_a_resource_is_available())
		use_resource();
	else
		complain_nicely();


But between the "verify" and the "use", some other process could
consume the resource.  The standard approach is to simply check
the return code from shmget.  errno will tell you what the problem
is and your program can then issue whatever error message you like.
Am I missing something?  What's wrong with this approach?

Anyway, I've seen another request about obtaining system configurable
parameters.  There is no truly portable way to do this.  Basicly, you
just poke around /dev/kmem looking for them.  Sometimes you can find
what you're looking for.  In the case of shmseg, I first tried
"nm /hp-ux | grep shmseg".  No luck.  So then I tried 
"nm /hp-ux | grep shm"  and I found a bunch of stuff.  I ignored the
code and looked at the data.  I knew what my shm parameters were so
I could recognize them when I found them.  They were stored in a neat
little table called shminfo.  A program to do this would be:

	#include<stdio.h>
	#include<nlist.h>
	#include<fcntl.h>
	main()
	{
	int fdkern;
	static struct nlist nl[2] = { { "shminfo" } , { NULL } };
	int shmmax;
	int shmmin;
	int shmmni;
	int shmseg;
	char *address;

		nlist("/hp-ux", nl);
		address = nl[0].n_value;
		printf("address = %d\n", (int) address);

		fdkern = open("/dev/kmem", O_RDONLY);
		printf("fdkern = %d\n", fdkern);
		lseek(fdkern,(long) address,0);
		read(fdkern, (char *) &shmmax, sizeof(shmmax));
		printf("max shared memory segment size = %x\n", (int)shmmax);
		read(fdkern, (char *) &shmmin, sizeof(shmmin));
		printf("min shared memory segment size = %x\n", (int)shmmin);
		read(fdkern, (char *) &shmmni, sizeof(shmmni));
		printf("max shared memory ids = %d\n", (int)shmmni);
		read(fdkern, (char *) &shmseg, sizeof(shmseg));
		printf("max shared memory segments = %d\n", (int)shmseg);

		exit(0);
	}

But the rub is that this program must be able to read /dev/kmem.  Like ps,
it should be sgid'd to sys.  

Paul Hite   PRC Realty Systems  McLean,Va   uunet!prcrs!paul    (703) 556-2243
        You can't tell which way the train went by studying its tracks.