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Date:	Mon, 7 Nov 2005 23:13:17 -0800 (PST)
From:	mario rossi <mario30468 AT yahoo DOT com>
Subject:	running gdb and attaching back: why doesn't this work?
To:	cygwin AT cygwin DOT com
MIME-Version:	1.0
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Hi, 
I've been trying to run gdb from a thread to get a
stack trace of such thread...but it doesn't seem to
work as expected under cygwin. The code I tried is
attached below.

The output I get from the code is:

[Switching to thread 3420.0xab0]
* 4 thread 3420.0xab0  0x77f75a59 in
ntdll!DbgUiConnectToDbg () from
/c/WINDOWS/System32/ntdll.dll
  3 thread 3420.0xe24  0x7ffe0304 in ?? ()
  2 thread 3420.0xfa4  0x7ffe0304 in ?? ()
  1 thread 3420.0xffc  0x7ffe0304 in ?? ()
[Switching to thread 1 (thread 3420.0xffc)]#0 
0x7ffe0304 in ?? ()
#0  0x7ffe0304 in ?? ()
#1  0x77f5c524 in ntdll!ZwWaitForMultipleObjects ()
from
/c/WINDOWS/System32/ntdll.dll
#2  0x77e75f0b in WaitForMultipleObjectsEx () from
/c/WINDOWS/system32/kernel32.dll
#3  0x00000002 in ?? ()
#4  0x0022dc1c in ?? ()
#5  0x00000001 in ?? ()
#6  0x00000000 in ?? () from

Now, thread 1 is in fact the one running main() and
the called functions, but it doesn't show the proper
stack trace. I can verifty that thread 1 is the right
one by attaching one more gdb started manually to the
very same app. If I do that and type "info threads" I
get:
* 4 thread 3420.0xfc0  0x77f75a59 in
ntdll!DbgUiConnectToDbg () from
/c/WINDOWS/System32/ntdll.dll
  3 thread 3420.0xe24  0x7ffe0304 in ?? ()
  2 thread 3420.0xfa4  0x7ffe0304 in ?? ()
  1 thread 3420.0xffc  f () at test.c:31

Any Idea what might be wrong, and how to solve it?

      Thanks,

             Mario

-----------------------------------8<---------------------------------------------------------------------
#include "stdio.h"

static char program_name[256];

#define CMD_FILE_NAME "./.teja_gdb_cmds"

void backtrace() {
  char hugebuf[512];
  FILE *cmds;

  cmds = fopen(CMD_FILE_NAME, "w");
  if (!cmds) {
    printf("unable to obtain stack trace (couldn't
open cmd file)\n");
    return;
  }
  fprintf(cmds, "attach %d\n", getpid());
  fprintf(cmds, "info threads\n");
  fprintf(cmds, "thread 1\n");
  fprintf(cmds, "bt\n");
  fprintf(cmds, "detach\n");
  fprintf(cmds, "quit\n");
  fclose(cmds);
  snprintf( hugebuf, sizeof hugebuf, "gdb -batch -x %s
%s",
CMD_FILE_NAME, program_name);

  system(hugebuf);

}

void f() {
  backtrace();
  while(1);

}

void g() {
  f();

}

int main(int argc, char **argv) {
  strcpy(program_name, argv[0]);
  g();
  return 0; 


	
		
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