Exceptions

Exception handling is also a core feature of the conventional C++. However, this feature is considered to be too dangerous, because of unpredictable code execution time and too expensive (in terms of code size) for bare metal platforms. The usage of single throw statement in the source code will result in more than 120KB of extra binary code in the final binary image. Just try it yourself with your compiler and see the difference in size of the produced binary images.

It is possible to forbid usage of throw statements by providing certain options to the compiler. For GNU compiler (gcc) please use -fno-exceptions in conjunction with -fno-unwind-tables options. According to this page of gcc manual, all the throw statements are supposed to be replaced with call to abort(). Unfortunately this information seems to be outdated. The behaviour I see with my latest (at the moment of writing) gcc version 4.8 is a bit different.

When the compilation is performed with the options specified above and there is a throw statement in the code (for example throw std::runtime_error("Some error")), the compilation fails with error message:

main.cpp:34:42: error: exception handling disabled, use -fexceptions to enable
     throw std::runtime_error("Some error");

However, all the throw statements from standard library are compiled in and cause the whole exception handling support code overhead to be included in the final binary image, despite the compilation options forbidding the exceptions. The test application test_cpp_exceptions has simple code that causes the exceptions to be thrown:

std::vector<int> v;
v.at(100) = 0;

The generated code of the main function looks like this:

00015f60 <main>:
   15f60:    e92d4008     push    {r3, lr}
   15f64:    e59f0000     ldr    r0, [pc]    ; 15f6c <main+0xc>
   15f68:    eb0000a8     bl    16210 <_ZSt20__throw_out_of_rangePKc>
   15f6c:    00013868     andeq    r3, r1, r8, ror #16

We also can see there are multiple exception related functions in the produced listing, such as __cxa_allocate_exception, __cxa_throw, _ZSt20__throw_out_of_rangePKc, _ZSt21__throw_bad_exceptionv, etc... The size of the binary image will also be huge (about 125KB) due to exceptions handling.

If you would like to use STL classes that may throw exceptions, such as std::string, std::vector, but refuse to pay the expensive price of extra code space for exceptions handling, you'll have to do two things. First, make sure that exception conditions never occur in your code run, i.e. if throw statement is about to get executed, it means there is a bug in your code. Second, override the definition of all the "_throw*" functions the compiler tries to use. In order to identify all these functions you'll have to temporarily disable usage of standard library by passing -nostdlib compilation option to your gcc compiler. For the code example above the compilation without standard library will fail with error message:

main.cpp.o: In function `main':
main.cpp:(.text.startup+0x8): undefined reference to `std::__throw_out_of_range(char const*)'
collect2: error: ld returned 1 exit status

Let's try to override std::__throw_out_of_range(char const*):

namespace std
{

void __throw_out_of_range(char const*)
{
    while (true) {}
}

}

This time the compilation will succeed. Let's now compile the result code with standard library included (without using -nostdlib option) and check the binary image size. With my compiler the size is 1.3KB, which is much much better than 120KB when exception handling is used.

CONCLUSION: Excluding exception handling support is a well known and widely used practice in C++ bare metal development. Even when relevant compilation options are used (-fno-exceptions and -fno-unwind-tables in GNU compiler), there is still a need to override various __throw_* functions used by the compiler and provided by the standard library.

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