Date: Thu, 19 Dec 2002 19:22:24 +0200 From: Giorgos Keramidas <keramida@ceid.upatras.gr> To: Aaron Burke <aburke@nullplusone.com> Cc: freebsd-questions@FreeBSD.ORG Subject: Re: Problems with a C application that changes users and run 'screen -x' Message-ID: <20021219172224.GB985@gothmog.gr> In-Reply-To: <NGBBLCIHCLNJAIGIFFHJEEOMCOAA.aburke@nullplusone.com> References: <NGBBLCIHCLNJAIGIFFHJEEOMCOAA.aburke@nullplusone.com>
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On 2002-12-19 02:50, Aaron Burke <aburke@nullplusone.com> wrote:
> I have an application that simply logs in as another user and runs
> "screen -x". The problem I am having with the followin code is that
> the results of execution is a message from (I am guessing the shell)
> saying that I dont have access to the "/dev/ttyp?" where ? is the
> current virtual terminal that I am running on.
>
> Here is my application:
>
> #include <stdlib.h>
> int main(int argc, char* pszArgs[])
> {
> int result;
> result= system("/usr/bin/su ppp -m --login -c " &
> "/usr/local/bin/screen -x");
> return result;
> }
You are using the & operator in a very strange manner here. It most
certainly doesn't work the way you might think it does.
You should also avoid using system, if possible.
> And here is the output:
> bash-2.05$ ./a.out
You're not running the executable as `root'. Since you are not the
superuser, you do not have permissions to operate on the pseudo-tty
that login attempts to work with, and this is why you get the
following error message:
> Cannot open your terminal '/dev/ttyp0' - please check.
Three possibilities that you might wish to investigate further are:
1. Write a shell script that does the equivalent of the system() call
you are using now. This should be fairly easy and will work fine
if you execute the script from a root shell.
2. Fix your program by removing the bad use of `&'.
3. Avoid using system() which I vaguely recall being described with a
lot of bad words in various places and use fork(), exec(), _exit(),
waitpid() and exit() instead.
- Giorgos
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