Transporting Exceptions Between Threads

Article
02/01/2013

Visual C++ supports transporting an exception from one thread to another. Transporting exceptions enables you to catch an exception in one thread and then make the exception appear to be thrown in a different thread. For example, you can use this feature to write a multithreaded application where the primary thread handles all the exceptions thrown by its secondary threads. Transporting exceptions is useful mostly to developers who create parallel programming libraries or systems. To implement transporting exceptions, Visual C++ provides the exception_ptr type and the current_exception, rethrow_exception, and copy_exception functions.

namespace std 
{
   typedef unspecified exception_ptr; 
   exception_ptr current_exception();
   void rethrow_exception(exception_ptr p);
   template<class E> 
       exception_ptr copy_exception(E e);
}

Parameters

Parameter	Description
unspecified	An unspecified internal class that is used to implement the exception_ptr type.
p	An exception_ptr object that references an exception.
E	A class that represents an exception.
e	An instance of the parameter E class.

Return Value

The current_exception function returns an exception_ptr object that references the exception that is currently in progress. If no exception is in progress, the function returns an exception_ptr object that is not associated with any exception.

The copy_exception function returns an exception_ptr object that references the exception specified by the e parameter.

Remarks

Scenario

Imagine that you want to create an application that can scale to handle a variable amount of work. To achieve this objective, you design a multithreaded application where an initial, primary thread creates as many secondary threads as it needs in order to do the job. The secondary threads help the primary thread to manage resources, to balance loads, and to improve throughput. By distributing the work, the multithreaded application performs better than a single-threaded application.

However, if a secondary thread throws an exception, you want the primary thread to handle it. This is because you want your application to handle exceptions in a consistent, unified manner regardless of the number of secondary threads.

Solution

To handle the previous scenario, the C++ Standard supports transporting an exception between threads. If a secondary thread throws an exception, that exception becomes the current exception. By analogy to the real world, the current exception is said to be in flight. The current exception is in flight from the time it is thrown until the exception handler that catches it returns.

The secondary thread can catch the current exception in a catch block, and then call the current_exception function to store the exception in an exception_ptr object. The exception_ptr object must be available to the secondary thread and to the primary thread. For example, the exception_ptr object can be a global variable whose access is controlled by a mutex. The term transport an exception means an exception in one thread can be converted to a form that can be accessed by another thread.

Next, the primary thread calls the rethrow_exception function, which extracts and then throws the exception from the exception_ptr object. When the exception is thrown, it becomes the current exception in the primary thread. That is, the exception appears to originate in the primary thread.

Finally, the primary thread can catch the current exception in a catch block and then process it or throw it to a higher level exception handler. Or, the primary thread can ignore the exception and allow the process to end.

Most applications do not have to transport exceptions between threads. However, this feature is useful in a parallel computing system because the system can divide work among secondary threads, processors, or cores. In a parallel computing environment, a single, dedicated thread can handle all the exceptions from the secondary threads and can present a consistent exception-handling model to any application.

For more information about the C++ Standards committee proposal, search the Internet for document number N2179, titled "Language Support for Transporting Exceptions between Threads".

Exception-Handling Models and Compiler Options

Your application's exception-handling model determines whether it can catch and transport an exception. Visual C++ supports three models that can handle C++ exceptions, structured exception handling (SEH) exceptions, and common language runtime (CLR) exceptions. Use the /EH and /clr compiler options to specify your application's exception-handling model.

Only the following combination of compiler options and programming statements can transport an exception. Other combinations either cannot catch exceptions, or can catch but cannot transport exceptions.

The /EHa compiler option and the catch statement can transport SEH and C++ exceptions.
The /EHa, /EHs, and /EHsc compiler options and the catch statement can transport C++ exceptions.

The /CLR or /CLR:pure compiler option and the catch statement can transport C++ exceptions. The /CLR compiler options imply specification of the /EHa option. Note that the compiler does not support transporting managed exceptions. This is because managed exceptions, which are derived from the System.Exception class, are already objects that you can move between threads by using the facilities of the common languange runtime.

Security Note
We recommend that you specify the /EHsc compiler option and catch only C++ exceptions. You expose yourself to a security threat if you use the /EHa or /CLR compiler option and a catch statement with an ellipsis exception-declaration (catch(...)). You probably intend to use the catch statement to capture a few specific exceptions. However, the catch(...) statement captures all C++ and SEH exceptions, including unexpected ones that should be fatal. If you ignore or mishandle an unexpected exception, malicious code can use that opportunity to undermine the security of your program.

We recommend that you specify the /EHsc compiler option and catch only C++ exceptions. You expose yourself to a security threat if you use the /EHa or /CLR compiler option and a catch statement with an ellipsis exception-declaration (catch(...)). You probably intend to use the catch statement to capture a few specific exceptions. However, the catch(...) statement captures all C++ and SEH exceptions, including unexpected ones that should be fatal. If you ignore or mishandle an unexpected exception, malicious code can use that opportunity to undermine the security of your program.

Usage

The following sections describe how to transport exceptions by using the exception_ptr type, and the current_exception, rethrow_exception, and copy_exception functions.

exception_ptr Type

Use an exception_ptr object to reference the current exception or an instance of a user-specified exception. In the Microsoft implementation, an exception is represented by an EXCEPTION_RECORD structure. Each exception_ptr object includes an exception reference field that points to a copy of the EXCEPTION_RECORD structure that represents the exception.

When you declare an exception_ptr variable, the variable is not associated with any exception. That is, its exception reference field is NULL. Such an exception_ptr object is called a null exception_ptr.

Use the current_exception or copy_exception function to assign an exception to an exception_ptr object. When you assign an exception to an exception_ptr variable, the variable's exception reference field points to a copy of the exception. If there is insufficient memory to copy the exception, the exception reference field points to a copy of a std::bad_alloc exception. If the current_exception or copy_exception function cannot copy the exception for any other reason, the function calls the terminate (CRT) function to exit the current process.

Despite its name, an exception_ptr object is not itself a pointer. It does not obey pointer semantics and cannot be used with the pointer member access (->) or indirection (*) operators. The exception_ptr object has no public data members or member functions.

Comparisons:

You can use the equal (==) and not-equal (!=) operators to compare two exception_ptr objects. The operators do not compare the binary value (bit pattern) of the EXCEPTION_RECORD structures that represent the exceptions. Instead, the operators compare the addresses in the exception reference field of the exception_ptr objects. Consequently, a null exception_ptr and the NULL value compare as equal.

current_exception Function

Call the current_exception function in a catch block. If an exception is in flight and the catch block can catch the exception, the current_exception function returns an exception_ptr object that references the exception. Otherwise, the function returns a null exception_ptr object.

Details:

The current_exception function captures the exception that is in flight regardless of whether the catch statement specifies an exception-declaration statement.

The destructor for the current exception is called at the end of the catch block if you do not rethrow the exception. However, even if you call the current_exception function in the destructor, the function returns an exception_ptr object that references the current exception.

Successive calls to the current_exception function return exception_ptr objects that refer to different copies of the current exception. Consequently, the objects compare as unequal because they refer to different copies, even though the copies have the same binary value.

SEH Exceptions:

If you use the /EHa compiler option, you can catch an SEH exception in a C++ catch block. The current_exception function returns an exception_ptr object that references the SEH exception. And the rethrow_exception function throws the SEH exception if you call it with the transported exception_ptr object as its argument.

The current_exception function returns a null exception_ptr if you call it in an SEH __finally termination handler, an __except exception handler, or the __except filter expression.

A transported exception does not support nested exceptions. A nested exception occurs if another exception is thrown while an exception is being handled. If you catch a nested exception, the EXCEPTION_RECORD.ExceptionRecord data member points to a chain of EXCEPTION_RECORD structures that describe the associated exceptions. The current_exception function does not support nested exceptions because it returns an exception_ptr object whose ExceptionRecord data member is zeroed out.

If you catch an SEH exception, you must manage the memory referenced by any pointer in the EXCEPTION_RECORD.ExceptionInformation data member array. You must guarantee that the memory is valid during the lifetime of the corresponding exception_ptr object, and that the memory is freed when the exception_ptr object is deleted.

You can use structured exception (SE) translator functions together with the transport exceptions feature. If an SEH exception is translated to a C++ exception, the current_exception function returns an exception_ptr that references the translated exception instead of the original SEH exception. The rethrow_exception function subsequently throws the translated exception, not the original exception. For more information about SE translator functions, see _set_se_translator.

rethrow_exception Function

After you store a caught exception in an exception_ptr object, the primary thread can process the object. In your primary thread, call the rethrow_exception function together with the exception_ptr object as its argument. The rethrow_exception function extracts the exception from the exception_ptr object and then throws the exception in the context of the primary thread. If the p parameter of the rethrow_exception function is a null exception_ptr, the function throws std::bad_exception.

The extracted exception is now the current exception in the primary thread, and you can handle it as you would any other exception. If you catch the exception, you can handle it immediately or use a throw statement to send it to a higher level exception handler. Otherwise, do nothing and let the default system exception handler terminate your process.

copy_exception Function

The copy_exception function takes an instance of a class as its argument and then returns an exception_ptr that references the instance. Usually, you specify an exception class object as the argument to the copy_exception function, although any class object can be the argument.

Calling the copy_exception function is equivalent to throwing a C++ exception, catching it in a catch block, and then calling the current_exception function to return an exception_ptr object that references the exception. The Microsoft implementation of the copy_exception function is more efficient than throwing and then catching an exception.

An application typically does not require the copy_exception function, and we discourage its use.

Example

The following example transports a standard C++ exception and a custom C++ exception from one thread to another.

// transport_exception.cpp
// compile with: /EHsc /MD
#include <windows.h>
#include <stdio.h> 
#include <exception>
#include <stdexcept>

using namespace std;

// Define thread-specific information.
#define THREADCOUNT 2
exception_ptr aException[THREADCOUNT]; 
int           aArg[THREADCOUNT];

DWORD WINAPI ThrowExceptions( LPVOID ); 

// Specify a user-defined, custom exception. 
// As a best practice, derive your exception 
// directly or indirectly from std::exception. 
class myException : public std::exception { 
};
int main()
{
    HANDLE aThread[THREADCOUNT];
    DWORD ThreadID;

    // Create secondary threads.
    for( int i=0; i < THREADCOUNT; i++ )
    {
        aArg[i] = i;
        aThread[i] = CreateThread( 
            NULL,       // Default security attributes.
            0,          // Default stack size.
            (LPTHREAD_START_ROUTINE) ThrowExceptions, 
            (LPVOID) &aArg[i], // Thread function argument.
            0,          // Default creation flags.
            &ThreadID); // Receives thread identifier.
        if( aThread[i] == NULL )
        {
            printf("CreateThread error: %d\n", GetLastError());
            return -1;
        }
    } 

    // Wait for all threads to terminate.
    WaitForMultipleObjects(THREADCOUNT, aThread, TRUE, INFINITE); 
    // Close thread handles.
    for( int i=0; i < THREADCOUNT; i++ ) {
        CloseHandle(aThread[i]); 
    }

    // Rethrow and catch the transported exceptions.
    for ( int i = 0; i < THREADCOUNT; i++ ) {
        try {
            if (aException[i] == NULL) {
                printf("exception_ptr %d: No exception was transported.\n", i);
            }
            else {
                rethrow_exception( aException[i] );
            }  
        }
        catch( const invalid_argument & ) {
            printf("exception_ptr %d: Caught an invalid_argument exception.\n", i);
        }
        catch( const myException & ) {
            printf("exception_ptr %d: Caught a  myException exception.\n", i);
        }
    }
} 
// Each thread throws an exception depending on its thread 
// function argument, and then ends. 
DWORD WINAPI ThrowExceptions( LPVOID lpParam ) 
{ 
    int x = *((int*)lpParam);
    if (x == 0) {
        try {
            // Standard C++ exception.
            // This example explicitly throws invalid_argument exception. 
            // In practice, your application performs an operation that 
            // implicitly throws an exception.
            throw invalid_argument("A C++ exception.");
        }  
        catch ( const invalid_argument & ) { 
            aException[x] = current_exception();
        } 
    }
    else {
        // User-defined exception.
        aException[x] = copy_exception( myException() ); 
    }
    return TRUE; 
}

exception_ptr 0: Caught an invalid_argument exception.
exception_ptr 1: Caught a  myException exception.

Requirements

Header: <exception>

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