# Environment

Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

    set DISPLAY=:0.0         # Set DISPLAY from cmd.exe window
    start xwin -multiwindow  # Start XWin

# Startup log for XWin
 
Welcome to the XWin X Server 
Vendor: The Cygwin/X Project 
Release: 1.18.3.0 
OS: CYGWIN_NT-6.1 Oceandrive 2.5.2(0.297/5/3) 2016-06-23 14:29 x86_64 
OS: Windows 7 Service Pack 1 [Windows NT 6.1 build 7601] (Win64) 
Package: version 1.18.3-1 built 2016-06-20 
 
XWin was started with the following command line: 
 
xwin -multiwindow 
 
xterm                    # Starts correctly 
 

# Compile code with following flags

    gfortran -g -fdollar-ok -c testxod.f
    gfortran -g -I/cygdrive/c/cygwin64/usr/include -I/cygdrive/c/cygwin64/usr/include/X11 -c isox11.c
    gfortran  testxod.o isox11.o defmap.o bmp_comms.o read_cmap_params.o isoflush.o -o ../ISOPARIX/testxod.exe -L/cygdrive/c/cygwin64/lib -lX11

#
# Try to open display from call to isox11.c inside testxod.f.  Run under gdb
#

(gdb) run  
Starting program: /cygdrive/e/ISOPERF/ISOPARIX/testxod  
[New Thread 7856.0x1de0]  
[New Thread 7856.0x1eb8]  
ISOX11.C: x11winope about to connect to display :0.0  
  
Program received signal SIGSEGV: Segmentation fault - invalid memory reference.  
  
Backtrace for this error:  
[Thread 7856.0x1eb8 exited with code 3]  
[Inferior 1 (process 7856) exited with code 03]  
(gdb) q  

# Detail from the code around the failure point

#
# Relevant code lines
#

    char               *isodisplayname;
    Display            *isodisplay
    isodisplayname=XDisplayName(NULL);
    (void)printf("ISOX11.C: x11winope about to connect to display %s\n",isodisplayname);

#
#      Failure point
#

    if((isodisplay=XOpenDisplay(isodisplayname))==NULL)
    {
    *rc=-1;
    *isocols=0;
    *maxcols=2;
    printf("ISOX11.C: x11winope: Failure in XopenDisplay etc etc");
    return;
    }