Date: Sat, 2 Dec 2000 15:36:06 -0500 From: Ryan Younce <ryan@manunkind.org> To: Sue Blake <sue@welearn.com.au> Cc: Kent Stewart <kstewart@urx.com>, freebsd-questions@FreeBSD.ORG Subject: Re: unpseakable speaker device? Message-ID: <20001202153606.B40048@cheshire.manunkind.org> In-Reply-To: <20001202231152.M377@welearn.com.au>; from sue@welearn.com.au on Sat, Dec 02, 2000 at 11:11:54PM %2B1100 References: <20001202222840.K377@welearn.com.au> <3A28E1E2.8D703D3@urx.com> <20001202231152.M377@welearn.com.au>
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> 1.
> test (secretly) that PC speaker device is accessible by the user
> if not privileged, provide instructions for changing permissions
> and exit the program
> if the user is allowed to write to /dev/speaker, continue with test 2
>
> 2.
> test for kernel PC speaker support
> if unsupported, provide instructions for building it into the kernel
> and exit the program
> if supported, run the rest of the program as if nothing had been
> going on, without mentioning the testing at all
>
> I can do both of these tests OK, but there might be a better way.
> The second of the two tests is the one that I think I could improve.
Sue, your best bet (as far as I can tell) will probably entail calling a
wrapper program (written in C or possibly Perl) which has the capability
of understanding errno. A call to open() may fail for multiple reasons,
but by checking errno, you can retrieve the exact reason.
For instance, using C:
open("/dev/speaker", O_WRONLY);
will fail in both of the above situations, but errno will be set to 6 if
it is because the device is not configured, and 13 if the permissions don't
allow it. The problem with errno and shell scripts is that errno is a
variable inside the process, and relatively few apps will return the errno
value as the actual program return code. Going on this basis, you could
write the wrapper as:
#include <fcntl.h>
#include <errno.h>
int main(void)
{
int fd, error = 0;
if ( (fd = open("/dev/speaker", O_WRONLY)) < 0)
error = errno;
else
close(fd);
return error;
}
in which case this short program will return 6 as the return code in the
event that the device is not available, and 13 if permissions don't exist,
and 0 if there is no error.
If somebody knows of a way to do this sort of thing with just shell scripts,
I'll be interested in hearing it. So far, I know of no way.
--
Ryan Younce, Cat Herder / ryan@manunkind.org / http://www.manunkind.org/~ryan/
A weird imagination is most useful to gain full advantage of all the features.
--Manual Page for the 4.4 BSD Automounter, amd(8), Caveats Section
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