Shared Library Exceptions & Vague Linkage

[akennis]

First of all, sorry for duplicating this post. I put it up in the
alt.comp.lang.l earn.c-c++ mistakenly.

I'm investigating a problem whereby exceptions thrown from functions in
a Shared Library which was dynamically loaded (dlopen) are not properly
caught by the caller. Specifically, when compiling with G++ version
4.0, the RTTI data associated with the exception types is not being
properly aligned between the Shared Library and its caller.

The online manual for GCC describes this as a problem due to its
implementation of Vague Linkage, but does not fully explain how the
problem can be resolved.

I've been able to resolve the problem by taking the typeid of the
exception type before entering the try/catch block in the driver. This
is illustrated in the following code:

// DynamicLink.h
// **************

#ifndef dynamiclink_h
#define dynamiclink_h

#include <typeinfo>

namespace DynamicLink
{
I'm investigating a problem whereby exceptions thrown from functions in
a Shared Library which was dynamically loaded (dlopen) are not properly
caught by the caller. Specifically, when compiling with G++ version
4.0, the RTTI data associated with the exception types is not being
properly aligned between the Shared Library and its caller.

The online manual for GCC describes this as a problem due to its
implementation of Vague Linkage, but does not fully explain how the
problem can be resolved.

I've been able to resolve the problem by taking the typeid of the
exception type before entering the try/catch block in the driver. This
is illustrated in the following code:

// DynamicLink.h
// **************

#ifndef dynamiclink_h
#define dynamiclink_h

#include <typeinfo>

namespace DynamicLink
{

enum ExceptionType1
{
a = 0,
b = 1

};

extern "C" void throwExceptionT ype1();
typedef void (* throwExceptionT ype1_Fn)();

#ifdef FIX_VAGUE_LINKA GE

extern "C" void dumpRTTI();
typedef void (* dumpRTTI_Fn)();

char const * ExceptionType1_ _TypeId =
typeid(DynamicL ink::ExceptionT ype1).name();

#endif

}

#endif

// DynamicLink.cpp
// ***************

// build: g++ -DFIX_VAGUE_LINK AGE -g -shared -o libDynamicLink. so
DynamicLink.cpp

#include "DynamicLin k.h"

#include <stdio.h>

namespace DynamicLink
{

extern "C" void throwExceptionT ype1()
{
throw a;

}

#ifdef FIX_VAGUE_LINKA GE

extern "C" void dumpRTTI()
{
printf("Shared Library side RTTI:\n");
printf("typeid( ExceptionType1) : %s %p\n", ExceptionType1_ _TypeId,
ExceptionType1_ _TypeId);

}

#endif

}

// Driver.cpp
// *********

// build: g++ -DFIX_VAGUE_LINK AGE -g -fpic -o DynamicLinkDriv er -ldl
Driver.cpp

#include "DynamicLin k.h"
using namespace DynamicLink;

#include <stdio.h>
#include <dlfcn.h>

int main(int argv, char const * * arc)
{
void * lib = dlopen("./libDynamicLink. so", RTLD_NOW | RTLD_GLOBAL);

#ifdef FIX_VAGUE_LINKA GE

dumpRTTI_Fn dumpRTTI_fn = (dumpRTTI_Fn) dlsym(lib, "dumpRTTI") ;
dumpRTTI_fn();
printf("Driver side RTTI:\n");
printf("typeid( ExceptionType1) : %s %p\n", ExceptionType1_ _TypeId,
ExceptionType1_ _TypeId);

#endif

try
{
throwExceptionT ype1_Fn throwExceptionT ype1_fn =
(throwException Type1_Fn) dlsym(lib, "throwException Type1");
throwExceptionT ype1_fn();
}
catch (ExceptionType1 & e)
{
printf("caught ExceptionType1: %s %u - %u\n", __FILE__, __LINE__,
(int) e);
}
catch (...)
{
printf("caught unknown exception: %s %u\n", __FILE__, __LINE__);
}

dlclose(lib);

return 0;

}

// END CODE

If you compile the Shared Library and its associated executable driver
using the g++ commands given, then the proper exception handlers will
be executed. If you remove the -DFIX_VAGUE_LINK AGE directive from the
compile commands, then the catch (...) handler is erroniously executed.

The trick is putting the typeid(...) calls in the common header file.
Somehow this seems to resolve the typeid misalignment.

I would like to understand why this method is solving the problem, and
also if there is a better way to go about doing this. I suspect that
there is some combination of compiler / linker commands which can be
used, but I've yet to figure out which ones.

Thank you,

Albert Kennis
enum ExceptionType1
{
a = 0,
b = 1

};

extern "C" void throwExceptionT ype1();
typedef void (* throwExceptionT ype1_Fn)();

#ifdef FIX_VAGUE_LINKA GE

extern "C" void dumpRTTI();
typedef void (* dumpRTTI_Fn)();

char const * ExceptionType1_ _TypeId =
typeid(DynamicL ink::ExceptionT ype1).name();

#endif

}

#endif

// DynamicLink.cpp
// ***************

// build: g++ -DFIX_VAGUE_LINK AGE -g -shared -o libDynamicLink. so
DynamicLink.cpp

#include "DynamicLin k.h"

#include <stdio.h>

namespace DynamicLink
{

extern "C" void throwExceptionT ype1()
{
throw a;

}

#ifdef FIX_VAGUE_LINKA GE

extern "C" void dumpRTTI()
{
printf("Shared Library side RTTI:\n");
printf("typeid( ExceptionType1) : %s %p\n", ExceptionType1_ _TypeId,
ExceptionType1_ _TypeId);

}

#endif

}

// Driver.cpp
// *********

// build: g++ -DFIX_VAGUE_LINK AGE -g -fpic -o DynamicLinkDriv er -ldl
Driver.cpp

#include "DynamicLin k.h"
using namespace DynamicLink;

#include <stdio.h>
#include <dlfcn.h>

int main(int argv, char const * * arc)
{
void * lib = dlopen("./libDynamicLink. so", RTLD_NOW | RTLD_GLOBAL);

#ifdef FIX_VAGUE_LINKA GE

dumpRTTI_Fn dumpRTTI_fn = (dumpRTTI_Fn) dlsym(lib, "dumpRTTI") ;
dumpRTTI_fn();
printf("Driver side RTTI:\n");
printf("typeid( ExceptionType1) : %s %p\n", ExceptionType1_ _TypeId,
ExceptionType1_ _TypeId);

#endif

try
{
throwExceptionT ype1_Fn throwExceptionT ype1_fn =
(throwException Type1_Fn) dlsym(lib, "throwException Type1");
throwExceptionT ype1_fn();
}
catch (ExceptionType1 & e)
{
printf("caught ExceptionType1: %s %u - %u\n", __FILE__, __LINE__,
(int) e);
}
catch (...)
{
printf("caught unknown exception: %s %u\n", __FILE__, __LINE__);
}

dlclose(lib);

return 0;

}

// END CODE

If you compile the Shared Library and its associated executable driver
using the g++ commands given, then the proper exception handlers will
be executed. If you remove the -DFIX_VAGUE_LINK AGE directive from the
compile commands, then the catch (...) handler is erroniously executed.

The trick is putting the typeid(...) calls in the common header file.
Somehow this seems to resolve the typeid misalignment.

I would like to understand why this method is solving the problem, and
also if there is a better way to go about doing this. I suspect that
there is some combination of compiler / linker commands which can be
used, but I've yet to figure out which ones.

Thank you,

Albert Kennis

[akennis]

I mixed up the message text - sorry again:

I'm investigating a problem whereby exceptions thrown from functions in
a Shared Library which was dynamically loaded (dlopen) are not properly
caught by the caller. Specifically, when compiling with G++ version
4.0, the RTTI data associated with the exception types is not being
properly aligned between the Shared Library and its caller.

The online manual for GCC describes this as a problem due to its
implementation of Vague Linkage, but does not fully explain how the
problem can be resolved.

I've been able to resolve the problem by taking the typeid of the
exception type before entering the try/catch block in the driver. This
is illustrated in the following code:

// DynamicLink.h
// **************

#ifndef dynamiclink_h
#define dynamiclink_h

#include <typeinfo>

namespace DynamicLink
{

enum ExceptionType1
{
a = 0,
b = 1

};

extern "C" void throwExceptionT ype1();
typedef void (* throwExceptionT ype1_Fn)();

#ifdef FIX_VAGUE_LINKA GE

extern "C" void dumpRTTI();
typedef void (* dumpRTTI_Fn)();

char const * ExceptionType1_ _TypeId =
typeid(DynamicL ink::ExceptionT ype1).name();

#endif

}

#endif

// DynamicLink.cpp
// ***************

// build: g++ -DFIX_VAGUE_LINK AGE -g -shared -o libDynamicLink. so
DynamicLink.cpp

#include "DynamicLin k.h"

#include <stdio.h>

namespace DynamicLink
{

extern "C" void throwExceptionT ype1()
{
throw a;

}

#ifdef FIX_VAGUE_LINKA GE

extern "C" void dumpRTTI()
{
printf("Shared Library side RTTI:\n");
printf("typeid( ExceptionType1) : %s %p\n", ExceptionType1_ _TypeId,
ExceptionType1_ _TypeId);

}

#endif

}

// Driver.cpp
// *********

// build: g++ -DFIX_VAGUE_LINK AGE -g -fpic -o DynamicLinkDriv er -ldl
Driver.cpp

#include "DynamicLin k.h"
using namespace DynamicLink;

#include <stdio.h>
#include <dlfcn.h>

int main(int argv, char const * * arc)
{
void * lib = dlopen("./libDynamicLink. so", RTLD_NOW | RTLD_GLOBAL);

#ifdef FIX_VAGUE_LINKA GE

dumpRTTI_Fn dumpRTTI_fn = (dumpRTTI_Fn) dlsym(lib, "dumpRTTI") ;
dumpRTTI_fn();
printf("Driver side RTTI:\n");
printf("typeid( ExceptionType1) : %s %p\n", ExceptionType1_ _TypeId,
ExceptionType1_ _TypeId);

#endif

try
{
throwExceptionT ype1_Fn throwExceptionT ype1_fn =
(throwException Type1_Fn) dlsym(lib, "throwException Type1");
throwExceptionT ype1_fn();
}
catch (ExceptionType1 & e)
{
printf("caught ExceptionType1: %s %u - %u\n", __FILE__, __LINE__,
(int) e);
}
catch (...)
{
printf("caught unknown exception: %s %u\n", __FILE__, __LINE__);
}

dlclose(lib);

return 0;

}

// END CODE

If you compile the Shared Library and its associated executable driver
using the g++ commands given, then the proper exception handlers will
be executed. If you remove the -DFIX_VAGUE_LINK AGE directive from the
compile commands, then the catch (...) handler is erroniously executed.

The trick is putting the typeid(...) calls in the common header file.
Somehow this seems to resolve the typeid misalignment.

I would like to understand why this method is solving the problem, and
also if there is a better way to go about doing this. I suspect that
there is some combination of compiler / linker commands which can be
used, but I've yet to figure out which ones.

Thank you,

Albert Kennis

[Jack Klein]

On 24 Jul 2006 19:23:05 -0700, "akennis" <a_******@yahoo .comwrote in
comp.lang.c++:

First of all, sorry for duplicating this post. I put it up in the
alt.comp.lang.l earn.c-c++ mistakenly.

I'm investigating a problem whereby exceptions thrown from functions in
a Shared Library which was dynamically loaded (dlopen) are not properly
caught by the caller. Specifically, when compiling with G++ version
4.0, the RTTI data associated with the exception types is not being
properly aligned between the Shared Library and its caller.

[snip]

You're still in the wrong place. C++ does not have or define shared
libraries, dynamic loading, or dlopen().

news:comp.os.li nux.development .apps

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html

[Peter]

akennis wrote:

First of all, sorry for duplicating this post. I put it up in the
alt.comp.lang.l earn.c-c++ mistakenly.

I'm investigating a problem whereby exceptions thrown from functions in
a Shared Library which was dynamically loaded (dlopen) are not properly
caught by the caller. Specifically, when compiling with G++ version
4.0, the RTTI data associated with the exception types is not being
properly aligned between the Shared Library and its caller.

make certain that you have only one definition of the matching symbols
in all of your exceutable and shared libraries.
I guess this is being caused by having multiple copies of the matching
code inside the excecutable and again inside the shared library

[akennis]

Peter wrote:

make certain that you have only one definition of the matching symbols
in all of your exceutable and shared libraries.
I guess this is being caused by having multiple copies of the matching
code inside the excecutable and again inside the shared library

If the -DFIX_VAGUE_LINK AGE command is removed, then the only code
common between the shared library and the executable are the type &
function declarations in the header file. These are required inorder
to establish the shared library API.

I think that the type_info objects are being compiled into the binaries
automatically, and so these ARE present in both. How can I prevent the
type_info stuff from being compiled into the executable?

Thanks.

[Peter]

akennis wrote:

void * lib = dlopen("./libDynamicLink. so", RTLD_NOW | RTLD_GLOBAL);
....
dlclose(lib);

dlopen belongs into a constructor and dlclose into the matching
destructor.
If dlopen fails throw an exception containing the error information.

[Peter]

akennis wrote:

try
{
throwExceptionT ype1_Fn throwExceptionT ype1_fn =
(throwException Type1_Fn) dlsym(lib, "throwException Type1");
throwExceptionT ype1_fn();
}
catch (ExceptionType1 & e)

it is unlikely that you want to modify the exception inside the catch
block.
Thus catch a const reference of this type.

[Peter]

akennis wrote:

I mixed up the message text - sorry again:

I did not know that it is possible to get the typeid for an
enumeration.
Try again with a class which has at least one virtual function
-- e.g. derived from the standard exception class.

And also -- the call to dlsym() belongs into a C++ wrapper which checks
for success and throws an exception otherwise.
The assumption being that you write such a wrapper only once and reuse
it.
And another assumption being that at the place where you call dlopen()
you don't want to deal with the possibility that it may fail,
but you want to delegate this task to some piece of code up the stack.