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Subject: Re: Is it possible to define the root directory in a cross compiled
 program
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From: Roger Kaufman <rogerkaufman AT interocitors DOT com>
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Date: Tue, 5 Jan 2021 11:49:01 -0500
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Hi All,

> On 1/5/2021 10:02 AM, Bill Stewart wrote:
> > On Tue, Jan 5, 2021 at 6:34 AM Eliot Moss wrote:
> >
> >> Is there a Windows equivalent to chroot (either the program or the 
> library/system call)?
> >
> > See: https://cygwin.com/cygwin-ug-net/highlights.html
> >
> > Quoting:
> >
> > "Chroot is supported. Kind of. Chroot is not a concept known by
> > Windows. This implies some serious restrictions. First of all, the
> > chroot call isn't a privileged call. Any user may call it. Second, the
> > chroot environment isn't safe against native windows processes. Given
> > that, chroot in Cygwin is only a hack which pretends security where
> > there is none. For that reason the usage of chroot is discouraged.
> > Don't use it unless you really, really know what you're doing."
> >
> > What I have found is that the cygwin chroot is not a security boundary
>
> Right.  My impression was that the OP was more interested in having the
> functionality of where / is, though I could be wrong, of course.
>
> I also saw web posts about Windows' RUNAS command, which deals with 
> some of
> the security implications, but does not re-root your file hierarchy.
>
> Best - Eliot 

This is the OP. We can close this out. Brian Inglis mentioned the 
Windows /dev/null is "nul"
and so that solved the problem in this case. In the code below, both 
fopen's succeed
when compiled with gcc and the "nul" fopen succeeds when cross compiled with
x86_64-w64-mingw32-g++

The backstory is, I cross compile because I have code only compatible 
when cross
compiled. However, I run the code in the bash shell. Now in the bash 
shell, I can't change
directory higher than / which is expected (I know of cygdrive/c of course)

However since it is now technically a windows  program it can "see out" 
into the
file system. I have for that program, an environment variable path I 
also have to prefix e.g.
/cygwin64/home... its just something I have to live with. But it does 
make portability
imperfect as the same code compiles in Linux.

#include <cstdio>

int main(int argc, char **argv)
{
   FILE *errfile1 = fopen("/dev/null", "w");
   if (!errfile1) // must be a valid pointer
     errfile1 = stderr;

   FILE *errfile2 = fopen("nul", "w");
   if (!errfile2) // must be a valid pointer
     errfile2 = stderr;

   fprintf(errfile1, "/dev/null did not succeed\n");
   fprintf(errfile2, "nul did not succeed\n");

   return 0;
}

Roger

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