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Date: Thu, 20 Feb 2014 14:16:44 +0100
Message-ID: <CAMzR4E6eO=m_s4+js=AA_EGntU=LRj9ZURmUqrwYqdVJ31hQWw@mail.gmail.com>
Subject: Re: Bug with dlopen() and fork()
From: Jaime Fabregas Fernandez <jaime DOT fabregas AT tarlogic DOT com>
To: cygwin AT cygwin DOT com
Content-Type: text/plain; charset=ISO-8859-1

Hello Corinna,

As you've checked, this behaviour doesn't appear with dll's created by
gcc, but it does with Visual Studio C++.
That minimal example compiled with VS will result in the freeze of the
child process.

================ testlib.h ======================
#ifdef TESTLIB_EXPORTS
#define TEST_API extern "C" __declspec(dllexport)
#else
#define TEST_API extern "C" __declspec(dllimport)
#endif

TEST_API  int  mylib_open (const char *foo);
=============================================

=============== testlib.cpp ======================
#include "stdafx.h"
#include "testLib.h"

  int
  mylib_open (const char *foo)
  {
    return 1;
  }
==============================================

I've tested several compiler and linker options with same result.

Any ideas?

> On Feb 19 14:38, Jaime Fabregas Fernandez wrote:
>> Library references loaded by a process using dlopen() and dlsym() are
>> no more valid in child processes after running a fork(). Calls from
>> child process will never return.
>>
>> I've searched for a similar problem in the mailing lists and found
>> this unanswered thread from 2001:
>>
>> http://cygwin.com/ml/cygwin/2001-02/msg01225.html
>>
>>
>> I'm running cygwin64. This behaviour can be checked with the following
>> test program:
>>
>> ========================================================
>> #include <stdio.h>
>> #include <dlfcn.h>
>>
>> int (*myopen)(const char *);
>>
>> main(){
>>         void *handle;
>>         int ret;
>>         handle = dlopen("my_lib.dll", RTLD_LAZY);
>>         myopen = dlsym(handle, "mylib_open");
>>
>>         if( ! fork() ){
>>                 ret = myopen("");
>>                 printf("This printf never shows, call to myopen will
>> block for ever\n");
>>         }
>>         else{
>>                 ret = myopen("");
>>                 printf("%i\n", ret);
>>                 sleep(1);
>>         }
>> }
>> ========================================================
>>
>> Same program runs correctly (showing the two printf's) in a Linux environment.
>
> Works for me with 64 bit Cygwin.  I used this as DLL:
>
>   $ cat > my_lib.c <<EOF
>   int
>   mylib_open (const char *foo)
>   {
>     return 1;
>   }
>   $ gcc -shared -o my_lib.dll my_lib.c
>   $ gcc -g -o my_tst my_tst.c <- That's your above testcase
>   $ uname -a
>   CYGWIN_NT-6.3 vmbert8164 1.7.28(0.271/5/3) 2014-02-04 16:01 x86_64 Cygwin
>   $ ./my_tst
>   1
>   This printf never shows, call to myopen will block for ever
>   $
>
>
> Corinna
>
> --
> Corinna Vinschen                  Please, send mails regarding Cygwin to
> Cygwin Maintainer                 cygwin AT cygwin DOT com
> Red Hat

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