FYI I just went through this w/ an application of ours. There is no easy way
to handle this. You can do this:
Display* disp = XOpenDisplay( 0 );
if ( ! disp )
{
// no DISPLAY variable is set - no x connection
}
However, if the DISPLAY variable is set but doesn't point to a usable x-server
(or it can't connect to the x-server), this will actually call halt/exit inside
the X library. The X library has this coded as the standard error behavior.
You can set a custom X11 IO error handler but it's kind of worthless because if
the error handler ever returns, the X11 library will exit. The work around is
to do a setjmp/longjump call in the error handler.
Here's the resulting code we use to check for a valid x-server connection. We
actually trap 3 different conditions (success, no display, display but can't
connect). You modify this a little bit if you just wanted to check for a valid
display.
====================================
#include <X11/Xlib.h>
#include <setjmp.h>
static jmp_buf s_displayJumpBuf;
static int s_xIOhandler( Display* disp )
{
// Jump back to the status function.
longjmp( s_displayJumpBuf, 1 );
return 0;
}
int serverStatus()
{
typedef int (*ErrorHandler)( Display* display );
// Initial execution path. setjmp() returns 0 to begin with. If
// XOpenDisplay fails and calls the error handler, then execution
// will resume here but with a non-zero return value.
if ( setjmp( s_displayJumpBuf ) == 0 )
{
// Set our handler and try to open the display.
ErrorHandler prevHandler = XSetIOErrorHandler( s_xIOhandler );
Display* disp = XOpenDisplay( 0 );
// Restore the previous handler.
XSetIOErrorHandler( prevHandler );
// Success - valid X11 display.
if ( disp )
{
return 0;
}
// DISPLAY env variable not set.
return -1;
}
else
{
// X11 IO error indicates that a DISPLAY variable was set but
// that it's invalid.
return -2;
}
}
> -----Original Message-----
> From: Christopher Barker [mailto:[email protected]]
> Sent: Monday, June 01, 2009 11:45 AM
> To: [email protected]
> Subject: Re: [Matplotlib-users] Non-X11 fallback on import
>
> Alexander Lamaison wrote:
> > I would like it to work
> > is to use the default backend if X is available and, otherwise, fall
> > back to using the Agg backend. How can I do this?
>
> I had a similar need a good while back with wxPython. I found some C
> code on the net that tries to connect to an X server and fails
> gracefully if it can't. I also wrote a Python wrapper around it. I
> haven't used it for years, and can't even find it on my machine, but
> thanks to the glory of archives and google, here it is:
>
> http://osdir.com/ml/python.wxpython/2004-03/msg00294.html
>
> You'd use it like:
>
> import matplotlib
> import isX
>
> if isX.isX:
> matplotlib.use("wxGTK") or whatever..
> else:
> matplotlib.use("AGG")
>
>
> note that you can also just check and see if the "DISPLAY" environment
> variable is set, but that won't be as reliable.
>
> However, as the whole user interaction has to be totally different, I
> wonder if you can't just have a different start-up script depending on
> whether they want to run the GUI version or not.
>
>
> -Chris
>
>
>
>
> --
> Christopher Barker, Ph.D.
> Oceanographer
>
> Emergency Response Division
> NOAA/NOS/OR&R (206) 526-6959 voice
> 7600 Sand Point Way NE (206) 526-6329 fax
> Seattle, WA 98115 (206) 526-6317 main reception
>
> [email protected]
>
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