Hints and tips on debugging cygwin

1 files changed, 73 insertions, 0 deletions

diff --git a/winsup/cygwin/how-to-debug-cygwin.txt b/winsup/cygwin/how-to-debug-cygwin.txt
new file mode 100644
index 000000000..e5c1425e6
--- /dev/null
+++ b/winsup/cygwin/how-to-debug-cygwin.txt
@@ -0,0 +1,73 @@
+Copyright 2001 Red Hat Inc., Egor Duda
+
+So, your favourite program has crashed? And did you say something about
+'stackdump'? Or it just prints its output from left to right and upside-down?
+Well, you can file an angry bug report and wait until some of the core
+developers try to reproduce your problem, try to find what's the matter
+with your program and cygwin and fix the bug, if any. But you can do something
+better than that. You can debug the problem yourself, and even if you can't
+fix it, your analysis may be very helpful. Here's the (incoplete) howto on 
+cygwin debugging.
+
+1. The first thing you'll need to do is to build cygwin1.dll and crashed your
+application from sources. To debug them you'll need debug information, which
+is normally stripped from executables. 
+
+2. Create known-working cygwin debugging environment.
+- create a separate directory, say, c:\cygdeb, and put known-working
+  cygwin1.dll, gdb.exe in it.
+- create a wrapper c:\cygdeb\debug_wrapper.cmd:
+
+========= debug_wrapper.cmd =========
+rem setting CYGWIN_TESTING environement variable makes cygwin application
+rem not to interfere with other already running cygwin applications.
+set CYGWIN_TESTING=1
+c:\cygdeb\gdb.exe -nw %1 %2
+===================================
+
+3. Try to use cygwin's JIT debugging facility:
+- add 'error_start=c:\cygdeb\debug_wrapper.cmd' to CYGWIN environment
+  variable. When some application encounters critical error, cygwin will stop
+  it and execute debug_wrapper.cmd, which will run gdb and make it to attach to
+  the crashed application.
+
+4. Strace.
+  You can run your program under 'strace' utility, described if user's manual.
+  If you know, where the problem approximately is, you can add a bunch of 
+  additional debug_printf()s in the source code and see what they print in 
+  strace log. There's one common problem with this method, that some bugs
+  may misteriously disappear once the program is run under strace. Then the
+  bug is likely a race condition. strace has two useful options to deal with
+  such situation: -b enables buffering of output and reduces additional
+  timeouts introduced by strace, and -m option allows you to mask certain
+  classes of *_printf() functions, reducing timeouts even more.
+
+5. Problems at early startup. 
+  Sometimes, something crashes at the very early stages of application
+  initialization, when JIT debugging facility is not yet active. Ok, there's
+  another environment variable that may help. Create program_wrapper.cmd:
+
+========= program_wrapper.cmd =========
+rem setting CYGWIN_SLEEP environement variable makes cygwin application
+rem to sleep for x milliseconds at startup
+set CYGWIN_SLEEP=20000
+c:\some\path\bad_program.exe some parameters
+===================================
+  
+  Now, run program_wrapper.cmd. It should print running program pid.
+  After starting program_wrapper.cmd you've got 20 seconds to open another
+  window, cd to c:\cygdeb in it, run gdb there and in gdb prompt type
+
+  (gdb) attach <pid>
+
+  where <pid> is the pid that program_wrapper.cmd have printed.
+  After that you can normally step through the code in cygwin1.dll and
+  bad_program.exe
+
+6. Heap corruption.
+  If your program crashes at malloc() or free() or when it references some
+  malloc()'ed memory, it looks like heap corruption. You can configure and
+  build special version of cygwin1.dll which includes heap sanity checking.
+  To do it, just add --enable-malloc-debugging option to configure. Be warned,
+  however, that this version of dll is _very_ slow (10-100 times slower than
+  normal), so use it only when absolutely neccessary.