When and Why Should You Utilize the __try Keyword in C ?

C Try/Catch/Finally Blocks and the __try Keyword

Structured Exception Handling (SEH) is the operating system-level exception handling mechanism supported by Windows. Compilers for Windows typically utilize SEH to implement C exceptions.

While the C keywords throw and catch exclusively handle C exceptions, Microsoft Visual C (MSVC) compiler provides non-standard __try and __except keywords to handle SEH exceptions.

__try Block:

The non-standard __try block defines the scope where an exception can occur, just like a try block in C . It's followed by __except and __finally blocks.

__except Block:

The __except block is similar to a catch block in C but offers more flexibility. It has an exception filter expression that evaluates whether an active exception should be handled.

__finally Block:

The __finally block contains code that runs after any exception handling. It's similar to a finally block in C#, but not directly equivalent in standard C .

Use Case for __try Blocks:

SEH exceptions can include those generated by the operating system, interoperating code using SEH, or managed code using the ".NET" exception code. To catch these exceptions in C , __try blocks must be used along with __except blocks.

Example Program:

The example program demonstrates SEH exceptions and how C destructors can be called during SEH exception unwinding.

#include <windows.h>
#include <iostream>

class Example {
public:
  ~Example() { std::cout << "destructed" << std::endl; }
};

int filterException(int code, PEXCEPTION_POINTERS ex) {
  std::cout << "Filtering " << std::hex << code << std::endl;
  return EXCEPTION_EXECUTE_HANDLER;
}

int main() {
  __try {
    Example e;
    int* p = 0;
    *p = 42;  // intentially generating a processor fault
  } __except (filterException(GetExceptionCode(), GetExceptionInformation())) {
    std::cout << "caught" << std::endl;
  }

  return 0;
}

Output:

Filtering c0000005
destructed
caught

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