Add strace debugging section.

winsup/cygwin/how-to-debug-cygwin.txt

@@ -71,3 +71,26 @@ c:\some\path\bad_program.exe some parameters
   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 necessary.
+
+7. Program dies when running under strace.
+  If your program crashes when you run it using strace but runs ok (or has a
+  different problem) otherwise, then there may be a problem in one of the
+  strace *_printf statements.  Usually this is caused by a change in arguments
+  resulting in a %s being used with something other than a pointer to a
+  string.
+
+  To debug this scenario, do something like this:
+
+   bash$ gdb -nw yourapp.exe
+   (gdb) dll cygwin1
+   (gdb) l dll_crt0_1
+   (gdb) bp <<first line in the function>>
+   (gdb) run
+   (gdb) set strace.active=1
+   (gdb) continue
+
+  The program will then run in "strace mode", calling each strace *_printf,
+  just like it does when run under the strace program.  Eventually, the
+  program will crash, probably in small_printf.  At that point, a 'bt'
+  command should show you the offending call to strace_printf with the
+  improper format string.