Add strace debugging section.

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

@@ -6,12 +6,12 @@ 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 (incomplete) howto on 
+fix it, your analysis may be very helpful. Here's the (incomplete) howto on
 cygwin debugging.
 
 1. The first thing you'll need to do is to build cygwin1.dll and your crashed
 application from sources. To debug them you'll need debug information, which
-is normally stripped from executables. 
+is normally stripped from executables.
 
 2. Create known-working cygwin debugging environment.
 - create a separate directory, say, c:\cygdeb, and put known-working
@@ -33,8 +33,8 @@ c:\cygdeb\gdb.exe -nw %1 %2
 
 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 
+  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 
+  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 mysteriously disappear once the program is run under strace. Then the
   bug is likely a race condition. strace has two useful options to deal with
@@ -42,7 +42,7 @@ c:\cygdeb\gdb.exe -nw %1 %2
   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. 
+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:
@@ -53,7 +53,7 @@ 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
@@ -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.