How Do Exceptions Operate Internally in C ?

How Exceptions Work Behind the Scenes in C

Introduction:

Exceptions are often questioned for their efficiency. This article explores the intricate workings of exceptions in C to provide a comprehensive understanding of their execution process.

Exception Handling Mechanism:

Exceptions are not directly akin to repeatedly executing returns; they involve an additional layer of exception tables. When an exception is thrown, the compiler generates a call to __cxa_throw, which utilizes these tables to find the handler and unwind the stack until it reaches the appropriate handler.

Throwing an Exception:

Throwing an exception requires significant runtime overhead because:

• __cxa_throw must locate the handler and unwind the stack.
• The allocated exception must be destroyed after the handler is executed.

Exceptional Returns:

In contrast, returning a value incurs minimal overhead. Exceptions should be utilized sparingly, only for exceptional scenarios.

Example Code:

The following simplistic C code illustrates the instruction generation for exception handling:

<code class="cpp">class MyException
{
public:
    MyException() { }
    ~MyException() { }
};

void my_throwing_function(bool throwit)
{
    if (throwit)
        throw MyException();
}

void another_function();
void log(unsigned count);

void my_catching_function()
{
    log(0);
    try
    {
        log(1);
        another_function();
        log(2);
    }
    catch (const MyException& e)
    {
        log(3);
    }
    log(4);
}</code>

Generated Assembly:

Compiling this code with GCC on Linux resulted in the following assembly output:

1. Throwing an Exception:

   _Z20my_throwing_functionb:
    ...
    movl    , (%esp)
    call    __cxa_allocate_exception
    ...
    movl    %eax, (%esp)
    call    __cxa_throw
    ...

2. Exception Handling Table:

   .section    .gcc_except_table,"a",@progbits
   .LLSDATT9:
    ...
    .uleb128 .LEHB1-.LFB9
    .uleb128 .LEHE1-.LEHB1
    .uleb128 .L12-.LFB9
    .uleb128 0x1
    ...

Conclusion:

Exceptions require substantial overhead for handling and unwinding, while return values incur minimal expense. Exceptions are optimal for exceptional scenarios, but should be used judiciously to avoid performance degradation.

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