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Date: Thu, 23 Apr 2009 14:42:09 -0700
From: ncokwqc02 AT sneakemail DOT com
Subject: RE: detecting program invocation from a symbolic link
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The result is apparently compiler dependent.

Here's some simple test code:

        #include <iostream>
        int main(int, const char** argv)
        {
                using namespace std;
                cout << "Hello World, I was called from " << argv[0] << endl;
                return 0;
        }

I build this code to create an executable named
        test_sym_link.exe
using the MS Visual Studio C++ compiler, the Intel C++ compiler, the Metrowerks Codewarrior C++ compiler, and the g++ compiler that came with Cygwin.

Then I create a symbolic link like this from a Cygwin shell
        $ ln -s test_sym_link.exe new_file
which produces the following directory entry
        $ ls -l new_file
        lrwxrwxrwx 1 ottusch None 17 Apr 23 13:39 new_file -> test_sym_link.exe
as expected.

If I run the program as built using any of the compilers other than g++ I get
        $ ./new_file
        Hello World, I was called from C:\Documents and Settings\ottusch\test_sym_link.exe
which, of course, is also what I get if I run ./test_sym_link

Only in the case where the compiler is g++ do I get
        $ ./new_file
        Hello World, I was called from ./new_file
which is the desired, Unix-like, result.

These results were all obtained while working from a Cygwin console. The symbolic link 'new_file' shows up as 'new_file.lnk' in a Windows command prompt shell and I don't know how to execute it directly from the command line.

Nice that it works with g++ but I need this to work with the other compilers too. Any suggestions?

jjo

> -----Original Message-----
> From: Christopher Faylor
> cgf-use-the-mailinglist-please-at-cygwin.com |cygwin.com|
> [mailto:...]
> Sent: Thursday, April 23, 2009 12:59 PM
> To:
> Subject: Re: detecting program invocation from a symbolic link
>
> On Thu, Apr 23, 2009 at 07:18:40PM -0000,
> ncokwqc02 AT sneakemail DOT com wrote:
> >I have some C++ code that runs from the command line in a console
> >shell.  It is designed to behave differently depending on whether it
> >was called directly by name or by a differently named symbolic link.
> >This is easy to check under Unix because argv[0] contains the name of
> >the first command line argument, which is either the
> executable name or
> >the name of the link.
> >
> >This doesn't work under Windows, however, because argv[0] always
> >contains the name of the program being called whether it is
> actually on
> >the command line or via a link.
> >
> >Even symbolic links created using 'ln -s' under Cygwin
> behave this way.
> >I'm wondering if there is a way around this problem.  In other words,
> >any way for my C++ program to detect the name of the symbolic link if
> >one was used to invoke the executable.
>
> When a cygwin program B is run from a cygwin program A and cygwin
> program B is a symlink to program C, then when C is eventually run its
> argv[0] should be "B", like on UNIX.
>
> None of this works when running B from a non-cygwin program like the
> windows command prompt since the windows command prompt doesn't
> understand cygwin symlinks.
>
> So, AFAICT, Cygwin does like UNIX.  If you are not seeing
> this behavior
> we need more details and a simple test case to reproduce the problem.
>
>

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