[ Note: The function may terminate by throwing an exception. @MartinJames, your question looks like a dupe of, @MartinJames noreturn allows the compiler to do some optimisations. One example of a function with similar behavior is exit() which terminates the program. functions that exit the application, loop forever or throw exception). returns, the behavior is undefined. What if date on recommendation letter is wrong? returns, the behavior is undefined. Also, given what I've read about this specifier it seems that in order to make sure the function won't return to its point of invocation, one should call another noreturn function inside it and make sure that the latter is always run (in order to avoid undefined behavior) and doesn't cause UB itself. The reason the compiler doesn't treat it as a fatal error is historical. Go back to my original comment. What registers to save in the ARM C calling convention? Although the compiler compiles func in the usual manner (despite your noreturn), the noreturn affects calling functions. Making statements based on opinion; back them up with references or personal experience. Remember there is an implicit return at the closing brace. end note ]. @Random823 That would indeed be the most beneficial thing, along with a warning during compilation. @SteveSummit - I have never casually sat back in my reclining office chair, pulled up my spittoon, and said "Yeah, I know," (spit) "strickly speakin', it's underfined, but in prackiss, nuttin' too back never happint when we do it dat way. Diff Detail Repository rG LLVM Github Monorepo Build Status Buildable 36186 A function that's declared with _Noreturn (or noreturn) is not allowed to return to its caller. I created a new task. In the following sample,the else clause does not contain a return statement. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Why is this program erroneously rejected by three C++ compilers? Doesnt do very much. noreturn is a promise. Lets use a different function that the compiler doesn't know about. end note ], [ Note: Implementations are encouraged to issue a warning if a function marked [[noreturn]] might return. One example would be if it called exit() instead. The _Noreturn keyword appears in a function declaration and specifies that the function does not return by executing the return statement or by reaching the end of the function body (it may return by executing longjmp ). The [ [noreturn]] is an attribute which has whatever semantics it has. The noreturn attribute is supposed to be used for functions that don't return to the caller. Why doesn't GCC optimize a*a*a*a*a*a to (a*a*a)*(a*a*a)? What is the advantage of using two capacitors in the DC links rather just one? So no caller can make use of the fact that the function does not return because there is no caller. This allows optimizations, and is useful especially in cases where it's hard for the compiler to prove that a loop won't ever exit, or otherwise prove there's no path through a function that returns. Subsequent and other typed arguments are passed on the stack. Note: You should never do var = find_prisoner_with_knife(), since the return value is not meant to be caught. The bigger problem in the question is, with a piece of code involving UB, there is no guarantee that it'll build to produce a binary, at all. However, this is not a requirement, allowing the functions to be used in generic programming: The following standard library functions have this attribute: Get monthly updates about new articles, cheatsheets, and tricks. To make it clear, using noreturn function specifier does not stop a function form returning to its caller. How to make compatibility with IERC20? If we hit the person with a knife, we can just exit the function because we know there's only one knife and no reason the check rest of the prisoners. fastcall On the Intel 386, the fastcall attribute causes the compiler to pass the first argument (if of integral type) in the register ECX and the second argument (if of integral type) in the register EDX. That doesn't mean void functions (which do return to the caller - they just don't return a value), but functions where the control flow will not return to the calling function after the function finishes (e.g. The noreturn attribute is a promise that you make to the compiler about your function. What factors led to Disney retconning Star Wars Legends in favor of the new Disney Canon? So I agree, it's hard to imagine a sane case where it makes much of a performance difference. C++11 introduced the [[noreturn]] attribute. Given that the function is also declared to be noexcept it basically means that the corresponding function also can't thrown an exception. No-return functions. What else would your function do other than return after it had called printf()? Based on this promise, compiler can make certain decisions, perform some optimizations for the code generation. gcc7.2 output for bar() is interesting. . This specifier is typically used through the convenience macro noreturn, which is provided in the header stdnoreturn.h. [Archive Top] If the function won't return, you don't need to save a return address when the function is called, and the stack frame could be reused by the last function called in the non-returning function. All the call to this function is hello(); Flutter - Json.decode return incorrect json, error: The name 'Image' is defined in the libraries 'package:flutter/src/widgets/image.dart' and 'package:image/src/image.dart'. It's not a bug. It's valid because the function does not return. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. [FreeRTOS Home] If the programmer try to return any value from that function which is declared as _Noreturn type, then the compiler automatically generates a compile time error. Emit code that, when the function improperly returns, it crashes or behaves in arbitrarily unpredictable ways. The FreeRTOS support forum can be used for active support both from Amazon Web Services and the community. Hey, From http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2960.pdf: If control reaches the end of main without encountering a return statement, the effect is that of executing return 0; As for the rationale, I'm not sure, honestly. This is used for the same reason as break in loops. foo() returns None. How to tell compiler to not generate "retq" after inline asm in a _Noreturn function? This is transitive - a function that calls a noreturn function in all possible code paths can, itself, be assumed to be noreturn. @cmaster no attributes make code sane, only the knowledge of the coder. exit(1); } Does Calling the Son "Theos" prove his Prexistence and his Diety? the behaviour is undefined. According to chapter 6.7.4, C11, Paragraph 8. It is a promise made by the programmer to the compiler to allow it some more degree of freedom to generate optimized code. The routines are not "always noreturn", they're "noreturn in some circumstances" - specifically when exceptions are disabled. [[noreturn]] Explanation Indicates that the function will not return control flow to the calling function after it finishes (e.g. ret simply means that the function returns control back to the caller. I simply said although Sourav's answer has a lot of good info, it didnt (as he originally posted it, not as it appears now) technically answer the OP's question, unless you infer more from his original answer than he wrote. Again - very much undefined behavior. First of all, _Noreturnfunction specifier (or, noreturn, using <stdnoreturn.h>) is a hint to the compiler about a theoretical promisemade by the programmer that this function will never return. end It inlines func(), and eliminates the foo=3 dead store, leaving just: Gcc still assumes that ext() is going to return, otherwise it could have just tail-called ext() with jmp ext. Sign in to comment Assignees No one assigned Labels None yet One example of a function with similar behavior is exit () which terminates the program. So, when func actually does return, main is going to start executing whatever crap follows the main function - be it padding, immediate constants, or a sea of 00 bytes. "Should" and "encouraged" are not requirements. It once took me some time when debugging a boot loader that had its own printf. It doesn't change, however, how the function is declared: all normal functions in C++ (i.e., all functions except the constructors, destructors, and conversion operators) have a declared return type. :- mov %gs:0x14,%eax. and, the paragraph 12, (Note the comments!! [ Note: The function may terminate by throwing an exception. Essentially, the optimizer didn't bother sticking a "ret" statement after the call to startApplication. ), Implicit return in function which does not return for "NoReturn" hint. Why function func() return after providing noreturn attribute? This is the function: Why could it be happening? Thanks for contributing an answer to Stack Overflow! Someone else might have made decisions based on your promise, and bad things can happen if you then break your promise. The purpose of the [ [noreturn]] attribute is probably to indicate that the function never returns in a normal way. ", @phonetagger no doubt, this is a very targeted answer and perfectly to the point. That's why I suggested "something like LIBCPP_NORETURN_IF_NO_EXCEPTIONS ", and I still think that's a good idea. The _Noreturn specifier may appear more than once in the same function declaration, the behavior is the same as if it appeared once. It is possible to tell gcc that a particular function never returns. _Noreturn function specifier is deprecated. It's basically the same as return in void functions in languages such as C++ or Java. return. Can I cover an outlet with printed plates? The _Noreturn keyword was introduced in C11. There are also situations in which a function call never ends, and this is usually done intentionally. Thanks for contributing an answer to Stack Overflow! [ [noreturn]] Explanation Indicates that the function does not return. Another Capital puzzle (Initially Capitals). functions that terminate the application, throw exceptions, loop indefinitely, etc.) @Ray - This back-and-forth between me and the four of you has gotten out-of-hand; I'm sorry it all appears under Andrew Henle's answer. It basically has three possibilities: The compiler might do 1, 2, 3, or some combination. Of course, if your function is actually called hello(), the culprit is almost certainly somewhere within your code base. wasm contract? Without the, Though Sourav's answer provides a more complete answer from a technical standpoint, this answer captures the real issue more succinctly. Note Adding __declspec (noreturn) to a function that is expected to return can result in undefined behavior. It could help for code-size if you have helper functions that throw exceptions. @phonetagger: Here's a real example of surprising UB: @phonetagger We don't use the fanciful (and, By contrast, Andrew answered the OP's question directly, in his opening sentence. Asking for help, clarification, or responding to other answers. First of all, _Noreturn function specifier (or, noreturn, using ) is a hint to the compiler about a theoretical promise made by the programmer that this function will never return. This is a Microchip project.. Leaving out those mechanisms might allow the compiler to generate more efficient code. Leaving out those mechanisms might allow the compiler to generate more efficient code. According to chapter 6.7.4, C11, Paragraph 8. I am Mahendra. Gcc warns when it's sure a noreturn function does return.) A compiler diagnostic is recommended if this can be detected. Therefore, it literally deleted all of the code after the call func (see for yourself at https://godbolt.org/g/8hW6ZR). It tells the compiler that the function it's applied to doesn't return to the caller. This permits certain optimizations and helps avoid spurious warnings of uninitialized variables. For example: void fatal(void) [ [noreturn]]; void fatal(void) { // . In any case, the [[noreturn]] tells the system that the corresponding function will never normally return, i.e., falling off the function ("after }") will probably result in undefined behavior. It's not a great example since printf is a standard C function that is known to return normally (unless you setvbuf to give stdout a buffer that will segfault?). [ [noreturn]] . It's pointless because the function is never called from C++ code. This permits certain optimizations and helps avoid spurious warnings of uninitialized variables. How the compiler optimizes noreturn functions is internal implementation detail, not reliable. More importantly, it helps avoid spurious warnings of uninitialized variables. The noreturn attribute is a promise that you make to the compiler about your function. By using mocks, we can assert that given objects behave in expected ways. The purpose of the [[noreturn]] attribute is probably to indicate that the function never returns in a normal way. Based on this promise, compiler can make certain decisions, perform some optimizations for the code generation. Why function func() return after providing noreturn attribute? All the call to this function is hello(); It is possible to tell gcc that a particular function never returns. Omitting the return type is not permitted by the C++ standard. Why are Linux kernel packages priority set to optional? The programmer has explicitly said > noreturn so if the function does return it's his problem. If the control path cannot be reached due to a function that never returns, you can use __declspec (noreturn) to prevent this warning or error. See your article appearing on the GeeksforGeeks main page and help other Geeks. Similarly the compiler will know not to warn you about missing return statements after calls to f(). (For example, because an exception is thrown unconditionally or sys.exit() is called. Our static analysis tools currently provide support for @return noreturn docblocks to denote functions that always throw or exit. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. But gcc doesn't tailcall noreturn functions, because that loses backtrace info for things like abort(). Passing Parameters in 64 bit Assembly Function from C language. A function call can end either by using the return keyword or by reaching the end of the function's block. In other words, the compiler does "flow analysis" and can determine whether the function actually might return. intercept.c:14: warning: `noreturn' function does return extern void _exit (int __status) __attribute__ ( (__noreturn__)); Or gcc knows what _exit is supposed to do. What I said is: If the code is at least half sane, that implies that there is zero performance impact of, @MartinJames - This feature is of immense benefit to static analysis tools, especially ones that perform worst-case stack depth analysis. Can LEGO City Powered Up trains be automated? On the contrary, I am saying that outlandish examples of what might result from UB may be funny, but they do not help confused new programmers better understand the machine within which their programs run. [[noreturn]] attribute should be used instead. Great for the scheduler routine for example. Normally, we usually test and assert that a method returns a certain response, but what about functions that don't return any response? end note ]. So a noreturn function that does actually return on ARM32 will break the ABI and cause hard-to-debug problems in the caller or later. You get to pick up the pieces when that breaks. /lib/libmatrix.a: file not recognized: File format not recognized collect2: ld returned 1 exit status, warning: useless type name in empty declaration, Excess elements in scalar initializer code compiles with gcc but not g++, compilation error on clock_gettime and CLOCK_MONOTONIC, My linux's gcc compiler not supporting boolean values. Users of these tools have found that syntax useful to describe the behaviour of their own code, but we think it's even . The text was updated successfully, but these errors were encountered: I suppose its a bug rather than a question. The program is ill-formed. noreturn function does return. Now, in case, you made a promise earlier and later, choose to violate this, the result is UB. Do inheritances break Piketty's r>g model's conclusions? C program help: Insufficient memory allocation but still workswhy? Please forgive me for asking, but, how is this 'noreturn' feature remotely useful to anyone? The C Standard in 6.7.4 Function specifiers, paragraph 12 specifically includes an example of a noreturn function that can actually return - and labels the behavior as undefined: In short, noreturn is a restriction that you place on your code - it tells the compiler "MY code won't ever return". The board-specific return routine simulates the rtc . . Without the, @MartinJames: Think about it this way: figuring out whether a function will return or not is, @phonetagger It is perfectly legal for a program containing undefined behavior to send a signal back in time that kills the compiler before it can produce the program. Compilers are encouraged, but not required, to produce warnings when a _Noreturn function appears to be capable of returning to its caller. How to tell GCC to generate 16-bit code for real mode. Give you a warning or error message pointing out that you broke your promise. This is done using the noreturn attribute: If the function does return despite the noreturn attribute, the compiler emits the warning you're seeing (which in your case gets converted into an error). Why do American universities cost so much? Now, in case, you made a promise earlier and later, choose to violate this, the result is UB. When I compile the C program below, I get this warning: If this sounds like undefined behavior, that's because it is. Do not assume that registers saved by the calling function are restored before calling the noreturn function. The _Noreturn keyword appears in a function declaration and specifies that the function does not return by executing the return statement or by reaching the end of the function body (it may return by executing longjmp). What you can do: patch the return address of a simple function on a stack and return. noreturn is a promise to the compiler that the function won't return. Most important is that error handling that ends in, @MartinJames - This feature is of immense benefit to static analysis tools, especially ones that perform worst-case stack depth analysis. That way, the compiler can avoid setting up the mechanisms that would allow the function to return properly. Is passing additional parameters through function pointer legal/defined in C? CGAC2022 Day 6: Shuffles with specific "magic number". :), @SouravGhosh I disagree about no guarantee that itll produce a binary at all, unless (as you suggest) the compiler is set to treat all warnings as errors. You signed in with another tab or window. How to test Flutter app where there is an async call in initState()? The attached is all the source code. I read this question about noreturn attribute, which is used for functions that don't return to the caller. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. Which Register Receive These Parameter? What factors led to Disney retconning Star Wars Legends in favor of the new Disney Canon? 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Jumping to the next "instruction" using gdb, gtk+ crashes on calling any cairo drawing function. If you don't want your function to return, call exit() or abort() or similar so it doesn't return. to your account, gives a warning Implicit return in function which does not return. How can a function not return? The compiler knows that the code following a call to a _Noreturn function is unreachable. With arrays, why is it the case that a[5] == 5[a]? It is also worth noting that such a function can return by executing longjmp. Report an error on this page (with anti-spam). A compiler diagnostic is recommended if this can be detected. So, main does call func, the CPU executes the function, and then, with ret, the CPU continues execution of main. If you do return from such a function, behavior is undefined, but this doesn't mean a sane compiler will allow you to mess the state of the application completely by removing the ret statement, especially since the compiler will often even be able to deduce that a return is indeed possible. "Because you wrote code that told it to. I know how to work around it but mypy should also support this special situation by checking if the hint of return value is NoReturn. Treat all warnings as errors and you won't be having a return, at all. @Random832 Because the compiler decided to be "nice": without, About messing with the state of the application, at some point I made a program where the main function had no return statement, and the. @cmaster no attributes make code sane, only the knowledge of the coder. The noreturn attribute is supposed to be used for functions that don't return to the caller. If the compiler finds a function with a control path that does not return a value, it generates a warning (C4715) or error message (C2202). If there's a possibility for control flow to return to the caller, the . In the following example, we return person's mother if the person given is a human. Is playing an illegal Wild Draw 4 considered cheating or a bluff? (Code + x86-64 asm on the Godbolt compiler explorer.). This makes slightly better code. . You're telling the compiler, "It may or may not be obvious, but I know, based on the way I wrote the code, that this function will never return." rev2022.12.7.43084. noreturn function does return. If ext is declared noreturn, that's exactly what we get. It basically has three possibilities: The compiler might do 1, 2, 3, or some combination. When I compile the C program below, I get this warning: 'noreturn' function does return cgcccompiler-errorswarningscompiler-warnings 20,776 It is possible to tell gccthat a particular function never returns. How to return uint256 datatype from ink! This tells that the function is indeed meant to return a value for later use, and in this case it returns None. then it's perfectly reasonable for the compiler to remove the some_other_func completely, it if feels like it. The bottom line, in programming as in real life, is: Don't make promises you can't keep. How can a function not return? Data Structures & Algorithms- Self Paced Course, Difference between %d and %i format specifier in C language, How to call function within function in C or C++, Returning a function pointer from a function in C/C++, Write a one line C function to round floating point numbers. A-143, 9th Floor, Sovereign Corporate Tower, We use cookies to ensure you have the best browsing experience on our website. Emit code that, when the function improperly returns, it crashes or behaves in arbitrarily unpredictable ways. And if you're going to optimize away the ret, optimizing away everything that's only needed if the function will return makes sense. This modified text is an extract of the original, [[deprecated]] and [[deprecated("reason")]], C++ Debugging and Debug-prevention Tools & Techniques, C++ function "call by value" vs. "call by reference", Curiously Recurring Template Pattern (CRTP), RAII: Resource Acquisition Is Initialization, SFINAE (Substitution Failure Is Not An Error), Side by Side Comparisons of classic C++ examples solved via C++ vs C++11 vs C++14 vs C++17, std::function: To wrap any element that is callable. Well occasionally send you account related emails. The following functions are noreturn in the standard library: https://en.cppreference.com/mwiki/index.php?title=c/language/_Noreturn&oldid=139635. This permits certain optimizations and helps avoid spurious warnings of uninitialized variables. So, what you should do here is to make sure that this function actually doesn't return control back to the callee. UV Project modifier : is there a way to combine two UV maps in a same material? -- ambiguous_import, Flutter, which folder not to commit to svn. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. However, there is a time and place for all of these.The following instructions are basically how the different methods should be used (or at least how I was taught they should be used), but they are not absolute rules so you can mix them up if you feel necessary to. Note: You should never do var = set_mother(my_person, my_mother), since the return value is not meant to be caught. Can GCC not complain about undefined references? First of all, _Noreturn function specifier (or, noreturn, using ) is a hint to the compiler about a theoretical promise made by the programmer that this function will never return. when I click on the warning, it takes me to the last line in the file(the last open paren). The assembly listing isn't truncated, it literally just ends after the call func because the compiler assumes any code after that would be unreachable. The assembly listing isn't truncated, it literally just ends after the call func because the compiler assumes any code after that would be unreachable. calling non-static member function without instantiation using `reinterpret_cast`. return None is never used if there are no other possible return values from the function. Designed by Colorlib. This is a read only archive of threads posted to the FreeRTOS support forum. But if you promise the compiler that your function won't return, and the compiler doesn't arrange for it to be possible for the function to return properly, and then you go and write a function that does return, what's the compiler supposed to do? It can then optimize without regard to what would happen if fatal ever did return. The main function is handled differently, as specified in the standard. Normally it is not allowed for the control flow to reach the end of a non-void function without returning something. The noreturn attribute is useful for a few library functions that cannot return, such as abort and exit. If you do return from such a function, behavior is undefined, but this doesn't mean a sane compiler will allow you to mess the state of the application completely by removing the ret statement, especially since the compiler will often even be able to deduce that a return is indeed possible. Forewarned that "startApplication ()" will never return, the compiler doesn't bother generating any code after the call to invoke it. It knows a lot about lot's of functions. You can see this in the assembly listing: the compiler has assumed, in main, that func won't return. Any basic block that ends with a return could be assumed to never be reached. For example, you could call exit inside it. note ], [ Note: Implementations are encouraged to issue a warning if a function marked [[noreturn]] might If you violate that restriction, that's all on you. The first declaration of the function must specify this attribute if any declaration specifies it. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. So, when func actually does return, main is going to start executing whatever crap follows the main function - be it padding, immediate constants, or a sea of 00 bytes. Am I mistaken in assuming this would be helpful with tail-call optimization or continuation-passing style? This is done using the noreturn attribute: If the function does return despite the noreturn attribute, the compiler emits the warning you're seeing (which in your case gets converted into an error). never return type is similar to the existing void return type, but the never type guarantees that the program will terminate or throw. To make it clear, using noreturn function specifier does not stop a function form returning to its caller. I am working in C, C++ and embedded. That way, the compiler can avoid setting up the mechanisms that would allow the function to return properly. portTASK. Past examples are: scalar typehints, return types, union types, mixed types, and static types. @phonetagger Actually, as time goes on, the number of times you can say "yeah, I know, strictly speaking it's undefined, but in practice nothing too bad will happen" keeps getting smaller and smaller. How to fight an unemployment tax bill that I do not owe in NY? Note. You get to pick up the pieces when that breaks. Declaring fatal as __declspec(noreturn) avoids an error or warning message. ), For C++, the behaviour is quite similar. Why does the C preprocessor interpret the word "linux" as the constant "1"? rev2022.12.7.43084. gcc noreturn attribute : void func() __attribute__ ( (noreturn)); noreturn () noreturn noreturn : #include <stdlib.h> void exit(int) { } // warning: 'noreturn' function does return [enabled by default] When does money become money? end The return value doesn't matter and you only want to exit the whole function. @Random832 Because if I'm reading other comments correctly the use of. Now, in case, you made a promise earlier and later, choose to violate this, the result is UB. Gcc could have optimized by omitting the mov store after the call as well. If the function declared _Noreturn returns, the behavior is undefined. Example In the following sample,the else clause does not contain a return statement. In the following example, we set person's mother's name and then the function exits after completing successfully. @phonetagger: Here's a real example of surprising UB: @cmaster: this answer's example of a kernel's, Here's a Godbolt compiler explorer link using the diff-view feature (using a CPP macro and, @PeterCordes - I don't find that surprising UB; it seems like a good proactive diagnostic tool for people who haven't learned to use, @phonetagger We don't use the fanciful (and. func itself is more interesting, and a bigger missed optimization. By clicking Sign up for GitHub, you agree to our terms of service and By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. It's extremely useful in some places, even though you don't need it that often. "Must" is a requirement. How to return uint256 datatype from ink! Based on this promise, compiler can make certain decisions, perform some optimizations for the code generation. Prior to the 1999 standard, C did permit the return type of a function to be omitted; it would default to int. Notice that the "next" instruction after the call at 7e0a belongs to an entirely different routine. Reference: http://en.cppreference.com/w/c/language/_Noreturn. This can be used by compilers to make some optimizations and generate better warnings. How to test Flutter app where there is an async call in initState()? First of all, _Noreturn function specifier (or, noreturn, using <stdnoreturn.h>) is a hint to the compiler about a theoretical promise made by the programmer that this function will never return. And where do I get it? accessing command-line arguments with gdb, what does this instruction do? However, it does in fact return (when control reaches the end of irs body, since it doesn't enter an infinite loop, it doesn't call other "noreturn" functions, etc.). (When is a debt "realized"?). This is done using the noreturnattribute: void func() __attribute__ ((noreturn)); It is a promise made by the programmer to the compiler to allow it some more degree of freedom to generate optimized code. Not the answer you're looking for? A function is "no-return" if we never reach a return instruction. ), For C++, the behaviour is quite similar. gcc and clang both emit nearly the same thing: This function could assume that it doesn't return, and use rbx and rbp without saving/restoring them. This function attribute is a GNU compiler extension that the ARM compiler supports. This attribute applies to the name of the function being declared in function declarations only. Accepted answer. acknowledge that you have read and understood our, Data Structure & Algorithm Classes (Live), Full Stack Development with React & Node JS (Live), Fundamentals of Java Collection Framework, Full Stack Development with React & Node JS(Live), GATE CS Original Papers and Official Keys, ISRO CS Original Papers and Official Keys, ISRO CS Syllabus for Scientist/Engineer Exam, Dynamic Memory Allocation in C using malloc(), calloc(), free() and realloc(), Left Shift and Right Shift Operators in C/C++, Different Methods to Reverse a String in C++, INT_MAX and INT_MIN in C/C++ and Applications, Taking String input with space in C (4 Different Methods), Modulo Operator (%) in C/C++ with Examples, http://en.cppreference.com/w/c/language/_Noreturn. Be "nice" to you and figure out a way to have the function return properly anyway. Any idea to export this circuitikz to PDF? It is a promise made by the programmer to the compiler to allow it some more degree of freedom to generate optimized code. . return. By printing a warning, the compiler has done its job as far as the standard is concerned. A function declared with a _Noreturn function specifier shall not return to its caller. Adding any attribute doesn't change this rule. Already on GitHub? Any idea to export this circuitikz to PDF? Why does the C preprocessor interpret the word "linux" as the constant "1"? Not the answer you're looking for? This is a useful optimization in cases where gcc can't prove whether a function does or doesn't return. You could either change the function prototype to remove the portTASK_FUNCTION, so it would be: static void vFlashVersionTask ( void *pvParameters ) { } Or let the compiler know the function won't return by adding the redundant code: for ( ;; ); after the call to vTaskDelete (). the behaviour is undefined. Use these archive pages to search previous posts. Clarifying this answer: for a function with __attribute__((noreturn)), if the function body does not end with an infinite loop, or a call to another noreturn function, you get this warning. On the actual behavior, there is no difference. What is evaluation order of function parameters in C? This could be done in many different ways and using return is probably not even the best way, but it's just an example to show how to use return for exiting a function. By contrast By contrast, Andrew answered the OP's question directly, in his opening sentence. After the removal of noreturn keyword, C11 standard (known as final draft) of C programming language introduce a new _Noreturn function specifier that specify that the function does not return to the function that it was called from. 516), Help us identify new roles for community members, Help needed: a call for volunteer reviewers for the Staging Ground beta test, 2022 Community Moderator Election Results, error: function declared 'noreturn' should not return, function might be possible candidate for attribute noreturn. The compiler is required to diagnose such an error, but it's not required to treat it as fatal. C++ is derived from C, so early (pre-standard) versions of C++ had the same rule. It gets the above warnting. I have other tasks that seem to compile fine. That doesn't mean void functions (which do return to the caller - they just don't return a value), but functions where the control flow will not return to the calling function after the function finishes (e.g. But if you promise the compiler that your function won't return, and the compiler doesn't arrange for it to be possible for the function to return properly, and then you go and write a function that does return, what's the compiler supposed to do? A function declared with a _Noreturn function specifier shall not return to its caller. IIRC, if a function specified with noreturn function specifier eventually returns to its caller, either. The contract cannot be broken. Specific word that describes the "average cost of something". Real UB always has reasonable explanations if you dive into the resultant assembly code. The function specifiers in C are a hint to the compiler, the degree of acceptance is implementation defined. GCC already optimizes main to fall off the end of the function if func() returns, even with the default -O0 (minimum optimization level) that it looks like you used. 2022 ITCodar.com. What is this symbol in LaTeX? How does Sildar Hallwinter regain HP in Lost Mine of Phandelver adventure? All Rights Reserved. wasm contract? Again - very much undefined behavior. @user694733 I believe the core of the question is that the questioner expected, Actually, the only thing that can be optimized by. no return function does not save the registers on the entry as it is not necessary. The noreturn keyword tells the compiler to assume that fatal cannot return. Based on this promise, compiler can make certain decisions, perform some optimizations for the code generation. Can someone explain why I can send 127.0.0.1 to 127.0.0.0 on my network. 516), Help us identify new roles for community members, Help needed: a call for volunteer reviewers for the Staging Ground beta test, 2022 Community Moderator Election Results, Why does this C++ snippet compile (non-void function does not return a value). (Undefined behaviour allows this, but it's at least a quality-of-implementation problem: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82158.). Elegant error handling in Dart like Scala's `Try`, Flutter Error: "Widget cannot build because is already in the process of building", Flutter: Calling startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK flag, Expanded() widget not working in listview. ), For C++, the behaviour is quite similar. Why does C++ code for testing the Collatz conjecture run faster than hand-written assembly? That's not what I said. If the function does return despite the noreturn attribute, the compiler emits the warning you're seeing (which in your case gets converted into an error). (It's obviously harmful when the function does simply return, though. The C Standard in 6.7.4 Function specifiers, paragraph 12 specifically includes an example of a noreturn function that can actually return - and labels the behavior as undefined: In short, noreturn is a restriction that you place on your code - it tells the compiler "MY code won't ever return". How to characterize the regularity of a polygon? and, the paragraph 12, (Note the comments!! If use of assembly is not an option, C does not have a notion of "jumping" to a function. How hard is it (really) to decompile assembly code? In return for using our software for free, we request you play fair and do your bit to help others! We've got 15 prisoners and we know one of them has a knife. Based on this promise, compiler can make certain decisions, perform some optimizations for the code generation. It simply means that the function ended successfully. Compilers are encouraged, but not required, to produce warnings when a _Noreturn function appears to be capable of returning to its caller. Asking for help, clarification, or responding to other answers. So using _Noreturn or noreturn from <stdnoreturn.h> is fine and equivalent. I don't see what your point is in 1. marking the function as non-returning, 2. writing a function that does nothing - probably you could just eliminate both? As others have mentioned, this is classic undefined behavior. Alternative idiom to "ploughing through something" that's more sad and struggling. What compiler are you using? Making statements based on opinion; back them up with references or personal experience. Although the compiler compiles func in the usual manner (despite your noreturn), the noreturn affects calling functions. #include <stdio.h> #include <stdlib.h> _Noreturn void view () { return 10; } int main (void) { printf("Ready to begin.\n"); view (); printf("NOT over till now\n"); return 0; } Output: Adding any attribute doesn't change this rule. To learn more, see our tips on writing great answers. end note ]. This is transitive - a function that calls a noreturn function in all possible code paths can, itself, be assumed to be noreturn. If someone knows, please add it to my answer or as a comment. NoReturn is used for situations, where the function does not return at all. Look at the following code example contained in Code Box 12-4: As you can see, the function main_loop performs the . Since you're unlikely to be using noreturn in your code, the likely explanation is that you have a function whose name clashes with a standard noreturn function, as in the below example: If it's not a human, we return None since the person doesn't have a mother (let's suppose it's not an animal or something). Copyright (C) Amazon Web Services, Inc. or its affiliates. Apparently inlining them is ok, though. You're telling the compiler, "It may or may not be obvious, but I know, based on the way I wrote the code, that this function will never return." It makes the optimisations easier. note ], [ Note: Implementations are encouraged to issue a warning if a function marked [[noreturn]] might [Live FreeRTOS Forum] Do sandcastles kill more people than sharks? Reference - What does this error mean in PHP? Adding __declspec(noreturn) to a function that is expected to return can result in undefined behavior. A function declared with a _Noreturn function specifier shall not return to its caller. If the control path cannot be reached due to a function that never returns, you can use __declspec(noreturn) to prevent this warning or error. In modern C++, omitting the return type is an error. Give you a warning or error message pointing out that you broke your promise. The smarter the compilers get, the more leeway they seem to grant themselves to do really crazy stuff if you give them any excuse. Quoting from chapter 7.6.4, C++14, paragraph 2 (emphasis mine), If a function f is called where f was previously declared with the noreturn attribute and f eventually In the following example, func () is declared with noreturn specifier but it returns to its caller. This is the function: Why could it be happening? Do school zone knife exclusions violate the 14th Amendment? Why is integer factoring hard while determining whether an integer is prime easy? If the function declared _Noreturn returns, the behavior is undefined. I could imagine that functions implementing some sort of application-loop could also qualify. I'm not going to insist on that name, but something more descriptive than _LIBCPP_ATTR_NORETURN, please. How can I return multiple values from a function? Find centralized, trusted content and collaborate around the technologies you use most. privacy statement. > > 2) Introduce a __builtin_unreached () builtin function . thanks! Such a function may end by calling std::terminate or std::exit, or by throwing an exception. (This may not be permitted by. Gcc for ARM32 actually does that, but still emits instructions to return otherwise cleanly. As others have mentioned, this is classic undefined behavior. I KNOW it has something to do with the protos that FreeRTOS needs.. This page was last modified on 12 May 2022, at 20:56. It can be used for a function to indicate that the function does not return to the caller by either executing a return statement, or by reaching the end if it's body (it is important to note that this does not apply to void functions, since they do return to the caller, they just do not return any value). Every other instruction present is a missed optimization. How to link a C object file with a Assembly Language object file? Quite a bit > for frequently called or very small functions. Why didn't Doc Brown send Marty to the future before sending him back to 1885? For example if f has the noreturn attribute, the compiler could warn you about g() being dead code when you write f(); g();. You can see this in the assembly listing: the compiler has assumed, in main, that func won't return. And generated assembly of the code using this: Why does function func() return after providing noreturn attribute? IIRC, if a function specified with noreturn function specifier eventually returns to its caller, either. [October 2018 Threads]. And at the point of definition of the function, the compiler should not require your assistance to determine that code following the while statement is dead, including a function postlude if any. The blockchain tech to build in a crypto winter (Ep. This frequently makes the > use of noreturn functions unattractive. [ Note: The function may terminate by throwing an exception. The function specifiers in C are a hint to the compiler, the degree of acceptance is implementation defined. The [[noreturn]] is an attribute which has whatever semantics it has. The smarter the compilers get, the more leeway they seem to grant themselves to do really crazy stuff if you give them any excuse. What compiler options did you use? You promised func wouldn't return, but you made it return anyway. How to check if a capacitor is soldered ok. Do sandcastles kill more people than sharks? Sign up for a free GitHub account to open an issue and contact its maintainers and the community. the behaviour is undefined. Specifies that the function does not return to its point of invocation. For instance, the function below will always either throw an exception or call std::terminate, so it is a good candidate for [[noreturn]]: This kind of functionality allows the compiler to end a function without a return statement if it knows the code will never be executed. Be "nice" to you and figure out a way to have the function return properly anyway. To make it clear, using noreturn function specifier does not stop a function form returning to its caller. Quoting from chapter 7.6.4, C++14, paragraph 2 (emphasis mine), If a function f is called where f was previously declared with the noreturn attribute and f eventually returns, the behavior is undefined. But if there's an exception throw/catch in a hot loop, you're doing something wrong. noreturn is a promise. . More info about Internet Explorer and Microsoft Edge. [FAQ] Since you're unlikely to be using noreturn in your code, the likely explanation is that you have a function whose name clashes with a standard noreturn function, as in the below example: Another obvious candidate for such a clash is abort(3). This value None can then be used elsewhere. Most probably, the function is marked with __attribute__((noreturn)). At one place in the code there was a call All rights reserved. What is the purpose of a function prototype? Clang does make that optimization in bar() / main(). If you violate that restriction, that's all on you. I read this question about noreturn attribute, which is used for functions that don't return to the caller. Is it safe to enter the consulate/embassy of the country I escaped from as a refugee? then it's perfectly reasonable for the compiler to remove the some_other_func completely, it if feels like it. Were CD-ROM-based games able to "hide" audio tracks inside the "data track"? How to make compatibility with IERC20? CGAC2022 Day 5: Preparing an advent calendar. As a consequence, the compiler knows that the code following a call to a __declspec(noreturn) function is unreachable. What else would your function do other than return after it had called printf()? Have a question about this project? How do I best silence a warning about unused variables? If this sounds like undefined behavior, that's because it is. IIRC, if a function specified with noreturn function specifier eventually returns to its caller, either. We loop through each prisoner one by one to check if they have a knife. -- ambiguous_import, Flutter, which folder not to commit to svn. The function specifiers in C are a hint to the compiler, the degree of acceptance is implementation defined. The behavior is undefined if the function with this attribute actually returns. Connect and share knowledge within a single location that is structured and easy to search. Since you're unlikely to be using noreturn in your code, the likely explanation is that you have a function whose name clashes with a standard noreturn function, as in the below example: Another obvious candidate for such a clash is abort(3). You can also define your own functions that never return by using the noreturn attribute. Quoting from chapter 7.6.4, C++14, paragraph 2 (emphasis mine), If a function f is called where f was previously declared with the noreturn attribute and f eventually That's not what I said. You MUST NOT return from the function. I am passionate about programming. Why did NASA need to observationally confirm whether DART successfully redirected Dimorphos? People on SO are wont to make up wild examples of what might result from UB, from. This article is contributed by Bishal Kumar Dubey. exit(1) at the end of the function. How can I compile without warnings being treated as errors? See the example here: This works when the AAReturnedValues abstract attribute uses AAIsDead ( D65243) and because AAIsDead relies on AANoReturn. You MUST NOT return from the function. We can assert many different things such as: Elegant error handling in Dart like Scala's `Try`, Flutter Error: "Widget cannot build because is already in the process of building", Flutter: Calling startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK flag, Expanded() widget not working in listview, warning: data definition has no type or storage class, Errors in compiling Linux kernel module: "CONFIG_X86_X32 enabled but no binutils support" and unwanted "n" characters. To learn more, see our tips on writing great answers. In my opinion THIS is the best answer! The answer isnt Since its UB it could do, little demons coming out of your nostrils, we can predict most of the likely results of UB, it's only UB in C99/C11 if the caller uses the result, gcc.gnu.org/bugzilla/show_bug.cgi?id=55747#c4, Flutter - Json.decode return incorrect json, error: The name 'Image' is defined in the libraries 'package:flutter/src/widgets/image.dart' and 'package:image/src/image.dart'. It is the programmer's responsibility to make sure that this function never returns, e.g. It can be used for a function to indicate that the function does not return to the caller by either executing a return statement, or by reaching the end if it's body (it is important to note that this does not apply to void functions, since they do return to the caller, they just do not return any value). An example of a function that doesn't return is abort. So, it turns out, noreturn does not make the function not return at all, it's just a specifier that tells the compiler that the code of this function is written in such a way that the function won't return. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. It doesn't change, however, how the function is declared: all normal functions in C++ (i.e., all functions except the constructors, destructors, and conversion operators) have a declared return type. Thank you. According to chapter 6.7.4, C11, Paragraph 8. The noreturn keyword does not affect the exceptional path when that applies: a noreturn -marked function may still return to the caller by throwing an exception. We use this logic to identify and mark "no-return" functions. warning: 'noreturn' function does return [enabled by default] Already have an account? Therefore, it literally deleted all of the code after the call func (see for yourself at https://godbolt.org/g/8hW6ZR). It has the __declspec equivalent __declspec(noreturn).However, __attribute((noreturn)) and __declspec(noreturn) differ in that when compiling a function definition, if the function reaches an explicit or implicit return, __attribute((noreturn)) is ignored and the compiler generates a warning. Later use, and this is a promise earlier and later, choose to violate this the. ; next & quot ; no-return & quot ; if we never Reach a statement! This logic to identify and mark & quot ; functions usual manner ( despite your noreturn ) ) service! ; next & quot ; next & quot ; next & quot ; next & quot ; functions (... A assembly language object file 5 [ a ] hide '' audio tracks inside the average! Optimized code, security updates, and a bigger missed optimization, ( Note the comments! user licensed... Not assume that fatal can not return because there is an Implicit return void... But it 's at least a quality-of-implementation problem: https: //en.cppreference.com/mwiki/index.php? title=c/language/_Noreturn &.... Op 's question directly, in programming as in real life, is do! Software for free, we use this logic to identify and mark & ;. Promise, compiler can make use of when a _Noreturn function type is similar to the existing void return,... Disney retconning Star Wars Legends in favor of the new Disney Canon C++ or Java might!, perform some optimizations for the code after the call func ( see yourself. A way to have the function to be caught or exit function do other than after. How is this 'noreturn ' feature remotely useful to anyone gt ; noreturn so if function. Provided in the usual manner ( despite your noreturn ) to a form. You violate that restriction, that 's more sad and struggling was last modified on 12 may 2022, all. Freertos needs as well there was a call all rights reserved compile fine URL into your RSS reader is.! Entirely different routine specifies it me to the compiler to remove the some_other_func,. The assembly listing: the function declared with a _Noreturn function is never called from C++ code testing! That doesn & # x27 ; function does return it & # x27 ; t to! If ext is declared noreturn, which is used for functions that &. Typically used through the convenience macro noreturn, that 's more sad struggling! Returns to its caller as C++ or Java violate that restriction, that func wo n't.! Thing, along with a return statement function specifiers in C are a hint to the caller noreturn. Beneficial thing, along with a warning about unused variables not owe in NY noting that such a form... An error or warning message violate that restriction, that func wo n't return..! '' and can determine whether the function does not stop a function returning... The Stack ; no-return & quot ; next & quot ; next & quot ; no-return & ;... Do I best noreturn' function does return a warning during compilation std::terminate or std:terminate. 'S all on you or error message pointing out that you broke your promise compiler... Degree of freedom to generate more efficient code ; for frequently called or small... But something more descriptive than _LIBCPP_ATTR_NORETURN, please add it to my answer or a... Default to int that way, the else clause does not return. ) it could help for code-size you. Perform some optimizations for the code following a call to a __declspec ( noreturn avoids! The behavior is undefined s a possibility for control flow to return otherwise cleanly this about. Is concerned few library functions that do n't make promises you ca n't prove whether function! Next `` instruction '' using gdb, what you should never do var = find_prisoner_with_knife ( ) return it... Beneficial thing, along with a warning or error message pointing out you! It that often does the C preprocessor interpret the word `` linux '' as the constant `` ''. Case that a [ 5 ] == 5 [ a ] such a function may terminate by an! Example here: this works when the function does not return. ) so. Code for real mode compiler about your function and technical support providing noreturn attribute std::exit, you. M not going to insist on that name, but, how is this program erroneously rejected by three compilers. Question directly, in programming as in real life, is: do n't return the. An unemployment tax bill that I do not owe in NY calling function after finishes... ``, @ phonetagger no doubt, this is the function actually might return. ) app where there an... Could call exit inside it loop indefinitely, etc. ): void fatal ( void {...: //godbolt.org/g/8hW6ZR ) your function do other than return after providing noreturn attribute restriction that... Never called from C++ code combine two uv maps in a same noreturn' function does return know! Perfectly to the caller, the degree of freedom to generate optimized code warnings a... Am I mistaken in assuming this would be if it appeared once n't tailcall noreturn functions is internal detail! Returning something some time when debugging a boot loader that had its own printf my network it..., union types, union types, union types, mixed types, and bad things can happen if violate... Gt ; is fine and equivalent rejected by three C++ compilers that FreeRTOS needs very small functions function performs! 1999 standard, C did permit the return type is similar to compiler... Instructions to return properly 4 considered cheating or a bluff asm on Stack. Imagine a sane case where it makes much of a function declared with a _Noreturn is! Contrast by contrast by contrast, Andrew answered the OP 's question directly, in his opening sentence person. C program help: Insufficient memory allocation but still workswhy being treated as errors and you wo return. For yourself at https: //godbolt.org/g/8hW6ZR ) example contained in code Box 12-4: as you can also define own... Zone knife exclusions violate the 14th Amendment a C object file with warning. At one place in the following example, you 're doing something wrong what we get questions,... Must specify this attribute applies to the name of the code following a call rights... Your bit to help others find anything incorrect, or some combination compiler supports exit... Without warnings being treated as errors mistaken in assuming this would be helpful with tail-call optimization continuation-passing! The new Disney Canon 1 ''? ) given is a debt `` realized ''?.... Mother 's name and then the function must specify this attribute if any declaration it! 'S not required to diagnose such an error, because that loses backtrace for. _Libcpp_Attr_Noreturn, please add it to my answer or as a refugee read only archive of threads posted the. Services, Inc. or its affiliates yourself at https: //godbolt.org/g/8hW6ZR ) Disney retconning Star Legends! Said & gt ; & gt ; is fine and equivalent explicitly said & gt ; fine! My network the return type is an attribute which has whatever semantics has... Nice '' to you and figure out a way to have the function to return properly.! By clicking Post your answer, you could call exit inside it kernel! Function returns control back to the compiler can make use of the function does not return, as... You only want to share more information about the topic discussed above 's an exception in. It be happening that often C program help: Insufficient memory allocation but emits... Without returning something pointless because the function main_loop performs the values from a that! Or responding to other answers later use, and bad things can happen if fatal ever did return..! Use, and bad things can happen if fatal ever did return. ) once in file. A ] that registers saved by the programmer to the callee and exit actually called hello ( ) Implicit. The usual manner ( despite your noreturn ) ) looks like a dupe of, @ no! Even though you do n't return. ) the control flow to the point belongs to entirely! With this attribute if any declaration specifies it functions are noreturn in the C! ( ( noreturn ) avoids an error on this page ( with anti-spam ) worldwide. @ phonetagger no doubt, this is a read only archive of threads posted to the compiler required. In case, you made it return anyway reference - noreturn' function does return does this error mean in PHP your! Compiles func in the following sample, the noreturn attribute after providing noreturn attribute is probably to indicate the. It finishes ( e.g it returns None compiler diagnostic is recommended if this can be used for functions don! Did n't Doc Brown send Marty to the caller things can happen if fatal ever return... Recommended if this can be used for functions that exit the whole function a fatal error is historical ``... And generated assembly of the function: why does the C preprocessor interpret the ``... This can be used instead ( despite your noreturn ) to a __declspec ( noreturn ) the... Or throw _Noreturn function is marked with __attribute__ ( ( noreturn ) to a _Noreturn function specifier not! To allow it some more degree of freedom to generate 16-bit code for the... Do I best silence a warning during compilation is probably to indicate that the function terminate... Jumping to the compiler that the code following a call all rights reserved done its job far... Maintainers and the community design / logo 2022 Stack Exchange Inc ; user contributions licensed under CC BY-SA }. Not necessary flow to the name of the new Disney Canon its point of invocation and easy search...
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