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Christopher Faylor
2002-01-30 02:44:21 +00:00
parent 1c2b9fe563
commit 8e2deb48d6
1 changed files with 68 additions and 61 deletions
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129
winsup/cygwin/how-to-debug-cygwin.txt
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@@ -1,22 +1,25 @@
Copyright 2001 Red Hat Inc., Egor Duda Copyright 2001 Red Hat Inc., Egor Duda
So, your favorite program has crashed? And did you say something about So, your favorite program has crashed? And did you say something about
'stackdump'? Or it just prints its output from left to right and upside-down? 'stackdump'? Or it just prints its output from left to right and
Well, you can file an angry bug report and wait until some of the core upside-down? Well, you can file an angry bug report and wait until some
developers try to reproduce your problem, try to find what's the matter of the core developers try to reproduce your problem, try to find what's
with your program and cygwin and fix the bug, if any. But you can do something the matter with your program and cygwin and fix the bug, if any. But
better than that. You can debug the problem yourself, and even if you can't you can do something better than that. You can debug the problem
fix it, your analysis may be very helpful. Here's the (incomplete) howto on yourself, and even if you can't fix it, your analysis may be very
cygwin debugging. 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 1. The first thing you'll need to do is to build cygwin1.dll and your
application from sources. To debug them you'll need debug information, which crashed application from sources. To debug them you'll need debug
is normally stripped from executables. information, which is normally stripped from executables. You probably
also want to build a version of the dll with more debugging capabilities
by reconfiguring your build directory, specifying the --enable-debugging
option to configure.
2. Create known-working cygwin debugging environment. 2. Create known-working cygwin debugging environment.
- create a separate directory, say, c:\cygdeb, and put known-working - create a separate directory, say, c:\cygdeb, and put known-working
cygwin1.dll and gdb.exe in it. cygwin1.dll and gdb.exe in it.
- create a wrapper c:\cygdeb\debug_wrapper.cmd: - create a wrapper c:\cygdeb\debug_wrapper.cmd:
========= debug_wrapper.cmd ========= ========= debug_wrapper.cmd =========
rem setting CYGWIN_TESTING environment variable makes cygwin application rem setting CYGWIN_TESTING environment variable makes cygwin application
@@ -26,26 +29,30 @@ c:\cygdeb\gdb.exe -nw %1 %2
=================================== ===================================
3. Try to use cygwin's JIT debugging facility: 3. Try to use cygwin's JIT debugging facility:
- add 'error_start=c:\cygdeb\debug_wrapper.cmd' to CYGWIN environment add 'error_start=c:\cygdeb\debug_wrapper.cmd' to CYGWIN environment
variable. When some application encounters critical error, cygwin will stop 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 it and execute debug_wrapper.cmd, which will run gdb and make it to attach to
the crashed application. the crashed application.
4. Strace. 4. Strace.
You can run your program under 'strace' utility, described if user's manual. 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 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 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 bug is likely a race condition. strace has two useful options to deal with
such situation: -b enables buffering of output and reduces additional such situation: -b enables buffering of output and reduces additional
timeouts introduced by strace, and -m option allows you to mask certain timeouts introduced by strace, and -m option allows you to mask certain
classes of *_printf() functions, reducing timeouts even more. classes of *_printf() functions, reducing timeouts even more.
Note that strace does not use the cygwin DLL and so any process that it
starts does not inherit a cygwin environment. It is equivalent to starting
a program from the command prompt.
5. Problems at early startup. 5. Problems at early startup.
Sometimes, something crashes at the very early stages of application Sometimes, something crashes at the very early stages of application
initialization, when JIT debugging facility is not yet active. Ok, there's initialization, when JIT debugging facility is not yet active. Ok, there's
another environment variable that may help. Create program_wrapper.cmd: another environment variable that may help. Create program_wrapper.cmd:
========= program_wrapper.cmd ========= ========= program_wrapper.cmd =========
rem setting CYGWIN_SLEEP environment variable makes cygwin application rem setting CYGWIN_SLEEP environment variable makes cygwin application
@@ -54,43 +61,43 @@ set CYGWIN_SLEEP=20000
c:\some\path\bad_program.exe some parameters c:\some\path\bad_program.exe some parameters
=================================== ===================================
Now, run program_wrapper.cmd. It should print running program pid. Now, run program_wrapper.cmd. It should print running program pid.
After starting program_wrapper.cmd you've got 20 seconds to open another 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 window, cd to c:\cygdeb in it, run gdb there and in gdb prompt type
(gdb) attach <pid> (gdb) attach <pid>
where <pid> is the pid that program_wrapper.cmd have printed. where <pid> is the pid that program_wrapper.cmd have printed.
After that you can normally step through the code in cygwin1.dll and After that you can normally step through the code in cygwin1.dll and
bad_program.exe bad_program.exe
6. Heap corruption. 6. Heap corruption.
If your program crashes at malloc() or free() or when it references some 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 malloc()'ed memory, it looks like heap corruption. You can configure and
build special version of cygwin1.dll which includes heap sanity checking. build special version of cygwin1.dll which includes heap sanity checking.
To do it, just add --enable-malloc-debugging option to configure. Be warned, 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 however, that this version of dll is _very_ slow (10-100 times slower than
normal), so use it only when absolutely necessary. normal), so use it only when absolutely necessary.
7. Program dies when running under strace. 7. Program dies when running under strace.
If your program crashes when you run it using strace but runs ok (or has a 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 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 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 resulting in a %s being used with something other than a pointer to a
string. string.
To debug this scenario, do something like this: To debug this scenario, do something like this:
bash$ gdb -nw yourapp.exe bash$ gdb -nw yourapp.exe
(gdb) dll cygwin1 (gdb) dll cygwin1
(gdb) l dll_crt0_1 (gdb) l dll_crt0_1
(gdb) bp <<first line in the function>> (gdb) bp <<first line in the function>>
(gdb) run (gdb) run
(gdb) set strace.active=1 (gdb) set strace.active=1
(gdb) continue (gdb) continue
The program will then run in "strace mode", calling each strace *_printf, The program will then run in "strace mode", calling each strace *_printf,
just like it does when run under the strace program. Eventually, the just like it does when run under the strace program. Eventually, the
program will crash, probably in small_printf. At that point, a 'bt' program will crash, probably in small_printf. At that point, a 'bt'
command should show you the offending call to strace_printf with the command should show you the offending call to strace_printf with the
improper format string. improper format string.