In the book More Effective C++ (Number 15), I read that code becomes significantly slower if exceptions are enabled, even if they are not used. In my opinion, exceptions are of limited use, and I try to avoid them, but this is another topic.
I do not entirely understand his statement:
What does enabling/disabling exceptions mean? It it the difference between having zero or more than zero try/catch blocks? Is it a compiler flag? What happens if I use a DLL in which exceptions can occur?
Suppose no exception is ever thrown:
Does the code become slower as a whole or are only the parts where the program enters/exits try/catch blocks become slower? According to the author, both is true.
How can I compile without exceptions? Can I do this even if I have try/catch blocks? Can I do this if DLLs I use might throw exceptions?
What does enabling/disabling exceptions mean?
Passing a flag to the compiler which disables standard conformance in relation to exceptions and makes it not generate any exception support.
What happens if I use a DLL in which exceptions can occur?
If some library handles exception internally, nothing. If it lets it escape to the caller (I never saw any library which does that, because of ABI issues, but whatever), your program crashes (in best case) as it cannot handle it. If there is a wrapper for DLL which your code include and which converts error codes to exceptions (common occurence), then it is the same as if you used exceptions in your code.
Does the code become slower as a whole or are only the parts where the program enters/exits try/catch blocks become slower? According to the author, both is true.
Notice that the book you are citing is old. Compilers are evolving. Modern compilers use zero-cost exceptions which do not incure perfomance cost if exception is not thrown. Exception handling does make executable larger as it should generate all data and code needed to process exceptions, but it should not make it slower on non-exceptional path.
How can I compile without exceptions? Can I do this even if I have try/catch blocks?
You do that in compiler-specific way. Consult your compiler documentation. Usually doing so makes compiler reject code containing any exception-related facilities, e.g. point out try as unrecognised identifier.
Related
I would like your help in understanding what are the possible approaches to using/disabling exceptions in C++.
My question is not about what is the best choice but just about what are the possible options and what these options imply.
Currently, the options I can think of are:
Compiling with -fno-exceptions and giving up most std containers (possibly defining internal containers which do not throw, such as suggested in SpiderMonkey Coding_Style)
Just avoiding to throw and catch in own code, but still using std containers which may throw exceptions. Being happy with the fact that, in the case of exceptions, the program may terminate without stack unwinding, and that even RAII handled external resources may be left hanging. (This seems to be Google C++ approach according to answers to this SO question)
No using exceptions but wrapping all in a catch all std::exception try block just to make sure stack is unwound and RAII handles to external resources are released before program is terminated, as for this Cert C++ rule
As above, but also throwing exceptions which will ultimately result in program termination.
Also using catched exceptions and recovering from exceptions.
I would like to know if my understanding of options is correct, and what I might be missing out or understanding wrong.
I would also like to know whether constraints on granting basic exception safety make sense for options 2-4 (where exceptions always ultimately lead to program termination) or if/how exception safety requirement can be relaxed/restricted to specific cases (e.g. handling external resources, files).
Update
Strictly speaking, when you want to disable exceptions only compiling with no exception support is true way to go, so Option 1. Because when you want to disable exceptions you should also not use or even handle them. Raising an exception would terminate or go to a hard faulty on most implementations instantly, and only by this you would avoid overhead implications like space or performance overhead even how small (see below) they are.
However if you just want to know which paradigms about exception usage are out there, your options are almost right, with the general idea you haven't mentioned of keeping exceptions exceptional and doing things that are likely or even just could throw, at program startup.
And even more in general, in comes to error handling in general: Exceptions are there to handle errors which you come across at runtime and if done correctly errors you can ONLY detect at runtime. You can avoid exceptions by simply making sure to resolve all problems you can detect before runtime, like proper coding and review (code time), using strict types and checks (templates , compile time), and rechecking them again (static analyser).
Update 2
If you understand you wrong about caring about exception safety I would say:
Basically at first it depends whenever you enable exceptions in general: If they are disabled you can't and shouldn't care about exception safety since there are none (or if they try to come into existence, you will terminate/crash/hardfault anyway).
If you enable exceptions but don't use them in your code, like in case 2, 4 and 3, no problem you want to terminate anyway, so missing cleanup code is not relevant (and the stuff in 3. gets still run in any case). But then should make it clear to everybody that you don't want to use them, so they won't try to recover from exceptions.
And if you use them in your code, you should care about exception safety in the way, that when an exception gets thrown, you clean up after your self too, and then its up the main handler or future code changes, when ever you still terminate or can recover. Not cleaning up but still using exception wouldn't make sense. Then you can just stick to option 1.
This should be to my knowledge exhaustive. For more information see below.
Recommendation
I recommend option 4. and I tell you why:
Exceptions are a very misunderstood and misused pattern. They are misused for program flow like the languages Java does in a excessive. They are often forbidden in embedded or safety code because of their nature that is hard to tell which handler will catch it, if there is one, and how long this will take, for which the C++ std just says basically "implementation magic".
Background
However in my reasoning the hate for exceptions is basically a big XY problem:
When people are complaining that their recovery is broken and hard to tell, then the usual problem is that the don't see you can't or should do much about the most exceptions, that's what they are for. However things like a timeout or a closed tcp connection are hardly non normal, but many people use exceptions for that, that would be wrong of course. But if your whole OS tells you that there is no network adapter or no more memory what could you do? The only thing you probably want is trying to log the reason somewhere and this should be done in one try/catch around the main block.
The same is for safety/real-time programs: For the example of running out of memory, when that happens you are **** anyway, the strategy then is to do this stuff at an unsafe initialisation time, in which exceptions are no problem too.
When using containers with throwing members its a similar scenario, what would you be able to do when you would get their error code instead? Not much, thats the reason why you would make sure at code time that there is no reason for errors, like making sure that an element is really in there, or reserving capacity for your vectors.
This has the nice benefit of cleaner code, not forgetting to check for errors and no performance penalty, at least using the common C++ implementations like gcc etc.
The reason of 3. is debatable, you could do this in my opinion too, but then my question is: What do you have to do in the destructors which wouldn't cleanup your operating system anyway? The OS would cleanup memory, sockets etc.. If have a freestanding scenario, the question remains, whenever different: You plan to halt because your UART broke for example, what would you like to do before halting? And why can't you do it in the catch block before rethrow?
To sum up
Has the problem of not using any throwing code, and still be left with the problem of how to handle rare error codes. (Thats why so many C programmers still use goto or long jumps)
Not viable IMHO, worst of both
as mentioned ok, but what do you need to do in your static DTors, what you even un-normal termination wouldn't do?
My favourite
Only if you really have rare conditions that you are actual able to recover from
What I mean as a general advice: Raising and exception should mean that something happened that should never happen in normal operation, but happened due to a unforeseeable fault, like hardware fault, detached cables, non found shared libraries, a programming fault you have done, like trying to .at() at an index not in the container, which you could have known of, but the container can not. Then it is only logical that throw an exception should almost every time lead to program termination/hard fault
As a consequence of compiling with exception support, is that for example for a freestanding ARM program, your program size will increase by 27kB, probably none for hosted (Linux/Windows etc.), and 2kB RAM for the freestanding case (see C++ exception handler on gnu arm cortex m4 with freertos )
However you are paying no performance penalty for using exceptions when using common compilers like clang or gcc, when you code runs normal i.e. when your branches/ifs which would trow an exception, are not triggered.
As a reference i.e. proof to my statements I refer to ISO/IEC TR 18015:2006 Technical Report on C++ Performance, with this little excerpt from 7.2.2.3:
Enable exception handling for real-time critical programs only if
exceptions are actually used. A complete analysis must always include
the throwing of an exception, and this analysis will always be
implementation dependent. On the other hand, the requirement to act
within a deterministic time might loosen in the case of an exception
(e.g. there is no need to handle any more input from a device when a
connection has broken down). An overview of alternatives for exception
handling is given in §5.4. But as shown there, all options have their
run-time costs, and throwing exceptions might still be the best way to
deal with exceptional cases. As long as no exceptions are thrown a
long way (i.e. there are only a few nested function calls between the
throw-expression and the handler), it might even reduce run-time
costs.
I thought this would sound a general simple question but I got up this when reading C++ exception specification. that said in one of the book, C++11 now have a keyword 'noexcept' that means no exception will be thrown from a function when it is declared with the function header and that said the reason for this keyword came into existence is C++ exception specifications are checked at run time rather than at compile time, so they offer no programmer guarantees that all exceptions have been handled. and hence they conclude two case a function would throw exception or if we are clear if it will never throw, then use noexcept for optimization(hopefully)
void foo() noexcept();
Here is the main question. Which system software perform those run time checking(I hope not compiler/linker/loader) and also which system software is responsible for allocating memory at run time(dynamic memory allocation) when this are all not taken care by compiler and others?
There is no active "system software" checking for exceptions, as you phrase it; rather, throwing an exception is an action taken by the program itself. The program passes the exception back up the stack until the exception matches an exception handler.
If no exception handler matches, then the exception is caught by the bootstrap code (main is not the actual entry point for a typical program, but is where the runtime hands control to the programmer) and the program terminates.
AFAIK this is done by the C++ runtime (libstdc++ for example). In case of exceptions, there are some guards added around the functions by the compiler (this is necessary anyway to call destructors in case exception is thrown), and in the case of noexcept, if the function throws (or if it throws other exception than advertised by the throw() specification), terminate() is called by the C++ runtime and the application is shut down.
Memory heap allocations are also (by default) done by the C++ runtime libraries.
Typically the responsible software isn't one clearly identifiable piece of code, but small fragments of code sprinkled through the executable. The compiler translates your code into binary instructions, and noexcept is no exception ;).
Indeed, you would not say that the "standard library" handles this. Exceptions and exception specifications are rather a core language feature, more fundamental than the standard library.
You could similarly ask, what piece of software ensures that when I call a function in C++, that the caller actually receives the values that I pass in? What piece of software manipulates the stack frame pointers while my program is running?
From the point of view of the standard I would say "the implementation" is responsible for these details. In some languages, like Java for instance, there is a "Java Runtime Environment" which is very clearly responsible for these things, and you could try to study exactly how it does them. In C++ there is no universal runtime environment -- like others have said, the compiler is responsible to generate code that ensures that these things happen, and that code ends up sprinkled throughout your resulting executable. How exactly the compiler achieves its task is implementation-specific, you can't give a general answer beyond what the standard says, and generally it specifies the expected behavior, not the details under the hood.
When you ask
also which system software is responsible for allocating memory at run time(dynamic memory allocation
this is again an implementation detail, it will differ from compiler to compiler.
This C++ program is a CGI script, I have no desire to deal with exceptions. I'd rather get a marginal performance boost and let the OS (Linux) handle cleanup after the process dies.
I am using the Standard C++ Library, and want any function to die like in Perl: Whenever it throws an exception. Without unwinding, or running any further code in my process.
How does -fno-exceptions work? If I have no catch at all in my code, and basically pretend like exceptions do no exist. but I do use std:: c++ library which can throw()?
Option #1: Simply never catch exceptions.
Exceptions don't have much overhead when they're not thrown or caught; if you're throwing and not prepared to catch, well, you're doing to die anyway, so the performance impact at that point is trivial. Note also that stack unwinding will not be performed if an exception is not handled; the program will simply terminate without performing stack unwinding.
It's important to note that, in G++, exceptions have almost no overhead when not actually thrown. G++ generates extra information sufficient to trace back the execution of the program through the stack, and some extra code to invoke destructors, etc - however none of this extra code or data is ever used until an exception is actually thrown. So you should not see a performance difference between code with exceptions enabled but not used and code with exceptions disabled (through whatever mechanism).
Option #2: Pass -fno-exceptions.
This flag instructs G++ to do two things:
All exception handling in STL libraries are removed; throws are replaced with abort() calls
Stack unwind data and code is removed. This saves some code space, and may make register allocation marginally easier for the compiler (but I doubt it'll have much performance impact). Notably, however, if an exception is thrown, and the library tries to unwind through -fno-exceptions code, it will abort at that point, as there is no unwind data.
This will, effectively, turn all exceptions into abort()s, as you would like. Note, however, that you will not be allowed to throw - any actual throws or catchs in your code will result in a compile-time error.
Option #3: (Nonportable and not recommended!) Hook __cxa_allocate_exception.
C++ exceptions are implemented using (among others) the __cxa_allocate_exception and __cxa_throw internal library functions. You can implement a LD_PRELOAD library that hooks these functions to abort():
void __cxa_allocate_exception() { abort(); }
void __cxa_throw() { abort(); }
WARNING: This is a horrible hack. It should work on x86 and x86-64, but I strongly recommend against this. Notably, it won't actually improve performance or save code space, as -fno-exceptions might. However, it will allow the throw syntax, while turning throws into abort()s.
-fno-exceptions turns all standard library throw's into a call to std::abort(). That handles the part you can't modify directly, the rest is to not use them at all in your code.
Of course, I really doubt your justification in doing this. You only "lose" performance when you actually throw, and you're throwing out a significant and helpful bit of the language.
In case anyone stumbles upon this question, I'd like to correct what #GManNickG and (https://stackoverflow.com/a/7249460/157344) and #bdonlan (https://stackoverflow.com/a/7249442/157344) said in their answers. Unfortunately the part about "-fno-exception" removing all exception handling code and turning all throws into aborts is wrong. Well - partially wrong. This is true when you compile the library in question (libstdc++v3) with this flag, but not true if you use this library (as an .a or .so or .dll or whatever) in your own code compiled with this flag. In the latter case the exception handling code in YOUR code is forbidden, but all the calls to exception handling inside the library remain (because the library was compiled WITHOUT this flag, with exceptions enabled), so if you use new then your executable WILL have exception handling code - the only difference is that you cannot anything about these exceptions with a catch() (which is forbidden in your code), so all throws effectively end up as abort(), but only because no one catches them.
Quote:
This C++ program is a CGI script, I have no desire to deal with exceptions.
Then don't. Simple. The exception will get to the top of the stack very quickly.
But I would urge you to do so. To do so means that you are thinking of the things that can go wrong.
Just don't catch them anywhere in your code. In that case, a termination handler will be called and your program will "crash".
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Will C++ exceptions safely propagate through C code?
If you have c code, for example the png lib, with your own io handlers which are written in c++ and an exception gets thrown due to some io error. is it ok to let it go through the c code and catch it outside of the c code? i know that care has to be taken with memory leaks but typically all structures get pre-allocated.
It is entirely up to the compiler if this will work or not. None of the language standards can obviously say anything about what the other language should do.
In the best case, the exception will pass the C code and get back to the next C++ level while possibly leaking any dynamically allocated C structures. In the less good case, it will crash and burn!
One possibilty is of course to compile the C code as C++, while also checking that it is exception neutral.
The answer to your question is implementation-defined.
For GCC, as #Kerrek says, compiling the C code with -fexceptions will ensure the run-time remains consistent when an exception is thrown through C code. (Note that modern exception mechanisms are not just implemented via setjmp/longjmp; they actually unwind the stack frame by frame in order to handle destructors and try/catch blocks properly.)
However, the C code itself may not be expecting to have exceptions thrown through it. Lots of C code is written like this:
acquire a resource
do some stuff
release resource
Here "acquire a resource" could mean malloc, or pthread_mutex_lock, or fopen. If your C++ code is part of the "do some stuff", and it throws an exception, well... Whoops.
Resource leaks are not the only problem; so is correctness is general. Imagine:
subtract $100 from savings account
add $100 to checking account
Now imagine an exception gets thrown after the first step finishes but before the second one does.
In short, although your implementation may provide a way to let you throw exceptions through C code, it is a bad idea unless you also wrote that C code knowing exactly where an exception might be thrown.
It's okay for your C++ code if it's:
Compiled with exceptions enabled.
Exception safe.
It's okay for your C or C++ code if:
No resources need to be collected explicitly between the throwing of the exception, and its being caught.
Code compiled with C++ exception support adds special hooks to clean up when exceptions are thrown and not caught in that scope. C++ objects allocated on the stack will have their destructors invoked.
The only resource reclaimed in C (or C++ without exception support) when an exception is thrown is the space allocated in each stack frame.
This section of the GCC manual is quite helpful:
-fexceptions
Enable exception handling. Generates extra code needed to
propagate exceptions. For some targets, this implies GCC will
generate frame unwind information for all functions, which can
produce significant data size overhead, although it does not affect
execution. If you do not specify this option, GCC will enable it
by default for languages like C++ which normally require exception
handling, and disable it for languages like C that do not normally
require it. However, you may need to enable this option when
compiling C code that needs to interoperate properly with exception
handlers written in C++. You may also wish to disable this option
if you are compiling older C++ programs that don't use exception
handling.
Exceptions aren't usually really thrown "through" code; as the action implies, they are thrown "over" code.
As a forinstance; at least old-school Objective-C exceptions are usually implemented with setjmp and longjmp; essentially storing the addresses of code for later use.
It makes sense for C++ exceptions to be implemented with similar mechanisms; and as such; the type of code the exception is thrown "through", or more accurately, "over" matters little, if at all. One could even imagine a setting where an Objective-C exception is thrown over a C++ catch, or vice versa.
Edit: As Bo Persson mentiones; this is not to say that a C++ exception interrupting C code wouldn't cause havoc and leaks; but exceptions being thrown is usually a Bad Thing™ all round; so it's not likely to matter much.
PS: Kudos on actually asking a question where using both C and C++ tags is relevant. ;)
C doesn't know anything about exceptions so it's not possible to say what would happen.
To write conforming code you'd need to catch exceptions prior to returning into the C library, translate to error codes, and then translate the C-library error back into an exception on the other end.
i am reading this page http://www.cplusplus.com/doc/tutorial/exceptions.html
it says if i write function() throw(); no exceptions can be thrown in that function. I tried in msvc 2005 writing throw(), throw(int), throw() and nothing at all. each had the exact same results. Nothing. I threw int, char*, another type and it was all caught the same way. It looks like throw doesnt affect it at all. What does function() throw() actually do?
See this article for details on C++ exception specifications and Microsoft's implementation:
Microsoft Visual C++ 7.1 ignores exception specifications unless they are empty. Empty exception specifications are equivalent to __declspec(nothrow), and they can help the compiler to reduce code size.
[...] If it sees an empty exception specification, it will assume you know what you are doing and optimize away the mechanics for dealing with exceptions. If your function throws anyway - well, shame on you. Use this feature only if you are 100% positive your function does not throw and never will.
What you're finding is that that version of VC++ didn't enforce specification exceptions. I believe that was documented as a variance from the standard.
However, exception specifications are usually not a good idea. If a program violates them in a standards-conforming implementation (which the VC++ from VS 2005 was not in this case), the system is supposed to catch it. This means that the specification is not a compiler optimization hint, but rather forces the compiler to go to extra lengths, and sometimes produce suboptimal code.
See the Boost rationale for reasons why the highly regarded Boost project does not use exception specifications. This is Boost, which is something of the poster child for doing weird and useful things with advanced parts of the language.
Quoting from A Pragmatic Look at Exception Specifications:
(Mis)understandings
The second issue has to do with
knowing what you’re getting. As many
notable persons, including the authors
of the Boost exception specification
rationale, have put it,
programmers tend to use exception
specifications as though they behaved
the way the programmer would like,
instead of the way they actually do
behave.
Here’s what many people think that
exception specifications do:
Guarantee that functions will only throw listed exceptions (possibly
none).
Enable compiler optimizations based on the knowledge that only listed
exceptions (possibly none) will be
thrown.
The above expectations are, again,
deceptively close to being correct.
See the link for the full details.
Throwing an exception is not enough, you need a try {} catch() block to catch exceptions. If you don't catch exceptions, std::terminate() is called and your program exits abruptly. Take some time out and have go at this.
throw specifications are designed for two purposes:
To serve as a contract between interface implemented and interface user - you state which exceptions can be throwned from your method, some people consider it part of an interface. (contract) Ala checked exceptions in Java.
As a way to signal the compiler that it can apply certain optimizations in case no exceptions can be thrown from a method/procedure (setting up exception handling costs something)
Throwing an exception not specified in throw() clause is a mistake, however at no point is the implementation required to verify it for you. In fact it's not even possible to verify, as it includes all the possible exceptions from subroutines your subroutine calls. (possibly from other modules) It is not even possible within a single module, as is easily reduced to a halting problem :)