C union undefined behavior

Embedded c interview questions and answers - Aticleworld

c++ - union for uint32_t and uint8_t[4] undefined behavior

  1. I do think it's safe to say, however, that in general reading an inactive member in a union can be undefined behavior (except for the special note in the Standard, of course), but I don't know if it always is (i.e. there may be some exceptions beyond the special note in the section of the C++ Standard I've quoted). Share . Follow edited Apr 22 '12 at 22:16. answered Apr 22 '12 at 20:52.
  2. In C 1999 and C 2011, reading a union member other than the last stored member is not per se undefined. The bytes are reinterpreted in the new type. The specific details are implementation-defined, not undefined. This may result in a trap representation, causing undefined behavior, but that is a consequence of the new value, not of the union member access and, depending on the specific types.
  3. It's undefined behavior to read from the member of the union that wasn't most recently written. Many compilers implement, as a non-standard language extension, the ability to read inactive members of a union. Run this code . #include <iostream> #include <cstdint> union S {std:: int32_t n; // occupies 4 bytes std:: uint16_t s [2]; // occupies 4 bytes std:: uint8_t c; // occupies 1 byte}; // the.
  4. Examples of undefined behavior are data races, memory accesses outside of array bounds, signed integer overflow, null pointer dereference, more than one modifications of the same scalar in an expression without any intermediate sequence point (until C++11) that are unsequenced (since C++11), access to an object through a pointer of a different type, etc. Compilers are not required to diagnose.
  5. Although this is technically undefined behavior according to the C standard (you're only supposed to read the field which was most recently written), it will act in a well-defined manner in virtually any compiler. Unions are also sometimes used to implement pseudo-polymorphism in C, by giving a structure some tag indicating what type of object it contains, and then unioning the possible types.
  6. In the early versions of C, undefined behavior's primary advantage was the production of performant compilers for a wide variety of machines: a specific construct could be mapped to a machine-specific feature, and the compiler did not have to generate additional code for the runtime to adapt the side effects to match semantics imposed by the language. The program source code was written with.
  7. Undefined Behavior; Overload resolution; Move Semantics; Pimpl Idiom; auto; Copy Elision; Fold Expressions; Unions. Undefined Behavior; Basic Union Features; Typical Use; Design pattern implementation in C++; Singleton Design Pattern; User-Defined Literals; Memory management; C++11 Memory Model; Scopes; static_assert; constexpr; One Definition.

undefined - A question about union in C - store as one

问题The following is not undefined behavior in modern C: union foo { int i; float f; }; union foo bar; bar.f = 1.0f; printf(%08x\n, bar.i); and prints the hex representation of 1.0f. However the following is undefined behavior: int x; printf(%08x\n, x); What about this A common alternate method for type punning is through unions, which is valid in C but undefined behavior in C++ 13 (see live example): union u1 { int n; float f; } ; union u1 u; u.f = 1. 0f; printf ( %d \n , u.n ); // UB in C++ n is not the active member. This is not valid in C++ and some consider the purpose of unions to be solely for implementing variant types and feel using unions for. C - Unions, C - Unions - A union is a special data type available in C that allows to store different example displays the total memory size occupied by the above union −. A union is a special data type available in C that allows to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. c ++ c undefined-behavior language-lawyer unions. Vroomfondel Oct 22 '15 at 9:31. source share. 2 answers. I remember that writing and reading from the same base memory between two points in the sequence is UB, but I'm not sure. Reading and writing to the same memory location in the same expression does not cause undefined behavior until and unless this location is changed more than once.

Union declaration - cppreference

  1. C left shift undefined behavior. Why does left shift operation invoke Undefined Behaviour when the , This is how the C++11 standard describes shift operators' work: The behavior is undefined if the right operand is negative, or greater than or equal to the The value of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are zero-filled.
  2. Undefined Behavior in C and C++, Consider the following C/C++ programs and try to guess the output? Program 3 (Accessing value of NULL C and C++ have undefined behaviors because it allows compilers to avoid lots of checks. Program to print half Diamond star pattern · C/C++ program for calling main() in main() · Difference between Base class Is the following C union access pattern undefined.
  3. 10 F → lovejomi: 如果要這樣 那之前在C work的union寫法 在C++是不是無法 06/16 09:50 11 F → lovejomi : 使用, 如我舉例的Endian 例子...連primitive type也UB? 06/16 09:50 12 F
  4. [問題] union 的undefine behavior +收藏. 分享. 看板 C_and_CPP 作者 lovejomi (JOMI) 時間 1年前 (2020/06/16 00:35) 推噓 1 (1 推 0 噓 36 →) 留言 37則, 3人.
  5. Due to undefined behavior, you may have to increase the union _fields_ to reproduce. I tested with 3.5.1 and 2.7.11. It seems that cffi treats the union as a normal struct. In classify_argument, it loops through the type->elements. The byte_offset increases for each union element until pos exceeds enum x86_64_reg_class classes[MAX_CLASSES], causing an invalid write here: size_t pos = byte.
  6. g language. In contrast, languages like Java (and many other 'safe' languages) have eschewed undefined behavior because they want safe and reproducible behavior across implementations, and willing to sacrifice performance to get it. While neither is the right.

Undefined behavior - cppreference

Multiple choice questions on C Programming topic Structures and Unions. Practice these MCQ questions and answers for preparation of various competitive and entrance exams Eine tagged-Union ist eine Union, die über ihre Datentypen Buch führt. Die C++ Core Guidelines bieten vier Regeln für Unions an. Hier sind sie. C.180: Use unions to save memory. C.181: Avoid. The union trick is also not valid, because only one member of a union can be active. When we set f it becomes active and i is thus inactive. Reading from an inactive member results in undefined behavior. At least that's how I understand the standard. The union trick in valid in modern C99 (but not in C89)

return c.b.a.y[3]; // OK: c.b.a.y refers to newly created object } struct X { const int a; int b; };//does not like 'const' union Y { X x; int k; }; void g() { Y y = { { 1, 2 } }; // OK, y.x is active union member (9.2) int n = y.x.a; y.k = 4; // OK: ends lifetime of y.x, y.k is active member of union y.x.b = n; // undefined behavior: y.x.b modified outside its lifetime, // y.x.b names y.x. Questions such as Accessing inactive union member and undefined behavior?, Purpose of Unions in C and C++, and Section 10.4 of the draft standard I have each mention the undefined behaviour arising from accessing an inactive member of a union in C++. Paragraph 1 in Section 10.4 of the linked draft also contains the following note, though I'm not sure I understand all the terminology used.

Is the following C union access pattern undefined behavior? Language: Ada Assembly Bash C# C++ (gcc) C++ (clang) C++ (vc++) C (gcc) C (clang) C (vc) Client Side Clojure Common Lisp D Elixir Erlang F# Fortran Go Haskell Java Javascript Kotlin Lua MySql Node.js Ocaml Octave Objective-C Oracle Pascal Perl Php PostgreSQL Prolog Python Python 3 R Rust Ruby Scala Scheme Sql Server Swift Tcl Visual. The confusion is that C explicitly permits type-punning through a union, whereas C++ has no such permission.c11. Structure and union members. 95) If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in. the confusion c explicitly permits type-punning through union, whereas c++ has no such permission.c11. structure , union members. 95) if member used read contents of union object not same member last used store value in object, appropriate part of object representation of value reinterpreted object representation in new type described in 6.2.6 (a process called â€⃜â€⃜type.

I'm worried about undefined behavior. Can you initialize the value of a trivially copyable union type by using a memcpy? This came up when I considered using Boost Serialization with BOOST_IS_BITWISE_SERIALIZABLE(MyUnionType), which I assume uses something like memcpy.. #include <cstring> enum class Foo: int {}; union Bar { int num; Foo foo; }; int baz(int src) { Bar dst; // My understanding. union pesoVolume { float volume; unsigned peso; } union pesoVolume pv; pv.volume = 0.0f; O funcionamento é idêntico, exceto que a union não estará mais dentro de uma struct . Na linguagem C, os campos que compõem a union 's podem ter tamanho diferentes, inclusive, e o compilador irá reservar memória idêntica ao tamanho da maior variável

still invokes undefined behavior simply because the standard says so, not for any practical reason. So my real question is, ignoring pathological cases (to also include invalid bit patterns) and considering that I do not store into a member of the union, does my second example involve a priori undefined behavior the way my first does 使用指向联合的不同元素的指针作为参数调用函数是否是未定义的行为? Is it undefined behaviour to call a function with pointers to different elements of a union as arguments

Why do we need C Unions? - Stack Overflo

possible behaviors of undefined behavior. And it may stop working as a result of modifying some totally unrelated statement. (That was, in fact, the behavoir I encountered with Microsoft C.) I doubt you'll be sacrificing portability if you go ahead with this method. You definitely will be. In practice, as well as in theory.- Undefined behaviour is used to mark syntactically correct C code as being outside of the standard. This seems like a reasonable idea at first, and can only help to make it clear what is and what is not covered by the standard. The problem is that undefined behaviour seems to cover things that it shouldn't. What is worse, it has given compiler writers permission to do almost anything with it. You are wrong. Your program exhibits undefined behavior in C++11, and would have exhibited undefined behavior in old-fashioned C. In any case, the outcome of Double = 23.6, int = 23, float = 23.6 is exceedingly unlikely. In addition, all members of this union have non-overlapping memory storage areas. Have not. Whatever gave you this idea. Next message: [ub] unions and undefined behavior Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] On Jul 24, 2013, at 1:27 PM, Jeffrey Yasskin < jyasskin at google.com > wrote: > On Wed, Jul 24, 2013 at 8:43 AM, Howard Hinnant > < howard.hinnant at gmail.com > wrote: >> On Jul 23, 2013, at 10:36 PM, Lawrence Crowl < Lawrence at Crowl.org > wrote: >> >>>> This isn't an academic. The behavior is undefined if access occurs beyond the end of either object pointed to by lhs and rhs. The behavior is undefined if either lhs or rhs is a null pointer. Contents. 1 Parameters; 2 Return value; 3 Notes; 4 Example; 5 References; 6 See also Parameters. lhs, rhs - pointers to the objects to compare count - number of bytes to examine Return value. Negative value if lhs appears before.

Undefined behavior is common in C programs and translators should document operation in the presence of undefined behavior. It is not acceptable to change the semantics of correct program structures due to undefined behavior elsewhere. I'd add the following note as well. NOTE: Undefined behavior, particularly, due to non-portable constructs is a normal component of C programs. Undefined. The problem with c1 is that it violates C/C++'s strict aliasing rule which basically says that if you cast a pointer to a different kind of pointer and then dereference it, then your program has executed an undefined behavior. The pernicious thing about undefined behavior is that it leads to problems only sometimes, and obviously, nothing bad happened here Previous message: [ub] [c++std-core-23865] Re: unions and undefined behavior Next message: [ub] [c++std-core-23865] Re: unions and undefined behavior Messages sorted by: El 25/07/2013 1:55, Jeffrey Yasskin escribió: > [I guess ->SG12] Yes. I'll subscribe to the mailing list ASAP. > How does safety certification work for C++ compilers (or compilers in > general)? Do they specify the compiler's. Subject: Re: [ub] [c++std-core-23844] Re: unions and undefined behavior From: Gabriel Dos Reis (gdr_at_[hidden]) Date: 2013-07-24 11:29:31 Next message: Howard Hinnant: Re: [ub] unions and undefined behavior Previous message: Marshall Clow: [ub] New paper (to me) about UB Next in thread: Howard Hinnant: Re: [ub] unions and undefined behavior Reply: Howard Hinnant: Re: [ub] unions and.

The C union construct is most frequently used to define structure capable of holding more than one type of data. we define an abstract class containing abstract method for each operation whose behavior depends on the value of tag. In above example, there is only one operation, area, which is the root of the class hierarchy. Next, we define a concrete subclass of the root class for each. Learning that you can use unions in C for grouping things into namespaces Grouping into namespaces, yes its undefined behaviour in both C and C++. Yes, a number of compilers treat this differently, its also poorly supported on custom hardware using standard compilers like gcc. So compiling for some mobile device with slightly custom good luck. bruce343434 21 days ago. It is not. Implementation defined behavior is controlled by #pragma directive. Contents. 1 Syntax; 2 Explanation; 3 Standard pragmas; 4 Non-standard pragmas. 4.1 #pragma once; 4.2 #pragma pack; 5 References; 6 See also; 7 External links Syntax. #pragma pragma_params (1) _Pragma (string-literal) (2) (since C99) 1) Behaves in an implementation-defined manner (unless pragma_params is one of the standard.

This leads to undefined behavior (i.e. the compiler can do anything). Casting does not work as expected when optimization is turned on. This is often caused by a violation of aliasing rules, which are part of the ISO C standard. These rules say that a program is invalid if you try to access a variable through a pointer of an incompatible type Undefined behavior; Unions; Valgrind; Variable arguments; X-macros; C Language. Typedef. Introduction. The typedef mechanism allows the creation of aliases for other types. It does not create new types. People often use typedef to improve the portability of code, to give aliases to structure or union types, or to create aliases for function (or function pointer) types. In the C standard.

Undefined behavior; Unions; Valgrind; Variable arguments; X-macros; C Language. Structure Padding and Packing. Packing structures. Example. By default structures are padded in C. If you want to avoid this behaviour, you have to explicitly request it. Under GCC it's __attribute__((__packed__)). Consider this example on a 64-bit machine: struct foo { char *p; /* 8 bytes */ char c; /* 1 byte.

> undefined behavior in the union V. Currently no compilers do, to the > best of my knowledge. There is no change in existing compiler behavior > that is required. What is required is that standards writers work out > the language to ensure this constraint on future compiler optimization > development. > In a standard-layout union with an active member (9.5) of struct type T1, it is permitted to read a non-static data member m of another union member of struct type T2 provided m is part of the common initial sequence of T1 and T2; the behavior is as if the corresponding member of T1 were nominated union bar {char c; long l; char *p;}; A struct foo contains all of the elements c, l, and p. Each element is separate and distinct. A union bar contains only one of the elements c, l, and p at any given time. Each element is stored in the same memory location (well, they all start at the same memory location), and you can only refer to the element which was last stored. (ie: after barptr->c.

Undefined behavior - Wikipedi

C++ Union

Undefined behavior; Unions; Valgrind; Variable arguments; X-macros; C Language. Multi-Character Character Sequence. Trigraphs. Example. The symbols [ ] { } ^ \ | ~ # are frequently used in C programs, but in the late 1980s, there were code sets in use (ISO 646 variants, for example, in Scandinavian countries) where the ASCII character positions for these were used for national language variant. [ub] Type punning to avoid copying (was: unions and undefined behavior) (too old to reply) Nevin Liber 2013-07-25 00:05:25 UTC. Permalink. The union hack is treacherous and needs far more investigation than it appears. So treacherous that people have been managing to use it for the last 30 years or so? Are we going to mandate that the reintepretation of the bits cannot possibly yield trap.

Reply: Richard Smith: Re: [ub] [c++std-core-23844] Re: unions and undefined behavior Reply: Lawrence Crowl: Re: [ub] [c++std-core-23844] Re: unions and undefined behavior On Wed, Jul 24, 2013 at 8:43 AM, Howard Hinnant <howard.hinnant_at_[hidden]> wrote: > On Jul 23, 2013, at 10:36 PM, Lawrence Crowl <Lawrence_at_[hidden]> wrote: > >>> This isn't an academic argument. If we can keep the. C++ ist eine von der ISO genormte Programmiersprache.Sie wurde ab 1979 von Bjarne Stroustrup bei AT&T als Erweiterung der Programmiersprache C entwickelt. C++ ermöglicht sowohl die effiziente und maschinennahe Programmierung als auch eine Programmierung auf hohem Abstraktionsniveau. Der Standard definiert auch eine Standardbibliothek, zu der verschiedene Implementierungen existieren home > topics > c / c++ > questions > variable length record using struct/union - help Post your question to a community of 469,030 developers. It's quick & easy Avoid dependence on undefined behavior (e.g., undefined order of evaluation) and be aware of constructs with implementation defined meaning (e.g., sizeof(int)). Note. There are environments where restrictions on use of standard C++ language or library features are necessary, e.g., to avoid dynamic memory allocation as required by aircraft control software standards. In such cases, control.

Is the following C union access pattern undefined behavior

What is Strict Aliasing and Why do we Care? · GitHu

[issue26628] Undefined behavior calling C functions with ctypes.Union arguments Vinay Sajip Sun, 12 Jan 2020 00:30:57 -0800 Change by Vinay Sajip <vinay_sa...@yahoo.co.uk> The reference to undefined behavior should refer to 6.5 Rule 109: The reference to undefined behavior should refer to (the cited reference to Clause 7 is not relevant). The reference to implementation defined behavior should refer to This rule means that the union type cannot be used. Within embedded system, where storage. Accessing my_union.ui after initializing the other member, my_union.d, is still a form of type-punning in C and the result is unspecified behavior (and undefined behavior in C++ ). The language of § 6.5/7 can be misread to imply that reading alternative union members is permissible

On 7/3/2016 5:34 AM, Dynefire wrote: public: virtual void Seed(unsigned long dwSeedValue) { m_udtData.Seed ^= dwSeedValue; m_udtData.High ^= m_udtData.Low; Not sure about C (not as familiar with the C standard), but in C++, this exhibits undefined behavior, by way of accessing a member of the union other than the one that was last assigned (aka type punning) And proper C and/or C++ program never trigger undefined behavior. This means every time undefined behavior is detected by the compiler it can be assumed this part is never actually executed with arguments which can trigger undefined behavior - this makes it possible to build a lot of interesting theories which help optimization This is very beneficial, as code that is straightforward to understand is much less likely to harbor subtle bugs or undefined behavior. Full details of MISRA C are obtainable from https://misra.org.uk and there are many tools available that support the approach. I will just give a flavor of the guidelines here. My references are from MISRA C. The first part of this article has introduced the concepts behind Structs and Unions, and how to define and use each either independently, or even interleaved .i.e. a Union inside a Struct. In this part, we're going to see useful applications of Struct and Union in Embedded C/C++, like how to use them to map registers to access its fields and we are going to discuss some pros and cons of bit. tive type of the object, an aggregate or union type that includes one of. C STANDARD UNDEFINED BEHAVIOR VERSUS WITTGENSTEIN 3 the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or a character type You have to read the footnote to paragraph (6.5. 7) to have any idea at all what is being specified.

Is the following C union access pattern undefined behavior

If you have worked with C or low-level C++ you probably already know a way to build sum types: Unions are sum types. The access in the last line is undefined behavior, which means anything can happen - but mostly bad things will happen. The best we can hope for is an immediate crash which is likely in this scenario. Worse are more subtle errors where we don't see any obvious problem. It is undefined behavior if ARC is exposed to an invalid pointer. For ARC's purposes, a valid object is one with well-behaved retaining operations. Specifically, the object must be laid out such that the Objective-C message send machinery can successfully send it the following messages: retain, taking no arguments and returning a pointer to the object. release, taking no arguments and.

Some warnings indicate an area where programmers often make incorrect assumptions, which may lead to unexpected or undefined behavior. If all of these warnings are enabled, some of them may appear many times in library headers. The C runtime libraries and the C++ standard libraries are intended to emit no warnings only at warning level /W4. Enable warnings that are off by default. You can. Chasing undefined behaviors with Eva. The Eva plug-in provides a configurable and automatic analysis of the whole program, intended to prove the absence of undefined behaviors. This term refers to instructions for which the C standard imposes no requirements, leading to crashes and more generally unpredictable execution flow. They can, in particular, result in security vulnerabilities, and. Core Issue 1776: Replacement of class objects containing reference members. Change in 1.8 (intro.object) paragraph 1: The constructs in a C++ program create, destroy, refer to, access, and manipulate objects. An object is created by a definition (3.1), by a new-expression (5.3.4) or by the implementation (5.18, 12.2) when needed. An object is. C Tutorial - structures, unions, typedef. In the C language structures are used to group together different types of variables under the same name. For example you could create a structure telephone: which is made up of a string (that is used to hold the name of the person) and an integer (that is used to hold the telephone number) Warum hat C ++ 'undefined behaviour' (UB) und andere Sprachen wie C # oder Java nicht? In diesem Beitrag zum Stapelüberlauf wird eine ziemlich umfassende Liste von Situationen aufgeführt, in denen die C / C ++ - Sprachspezifikation als undefiniertes Verhalten deklariert wird. Ich möchte jedoch verstehen, warum andere moderne Sprachen wie C # oder Java nicht das Konzept von undefiniertem.

C / C ++ Unions and Undefined Behavior - c ++ ️

4. The code may be free of undefined behavior, but it relies on implementation-defined behavior because of the exit (-1). Read its definition in the C standard, not in POSIX or even Linux. Of these, the C standard gives the fewest guarantees. Your code is definitely missing a test suite that demonstrates all the edge cases Tretter, the NFL players' union president, didn't condone Harrison's behavior, but said Lewis needs to be held accountable by the league for his actions. I expect that the coach gets held to the same standard - if not a higher standard - than Ronnie, Tretter said on a Zoom call. Being the first one in there and being a coach, putting his hands on an opposing player. I don't think. This makes it undefined behavior to have uninitialized data in a variable even if that variable has an integer type, which otherwise can hold any fixed bit pattern: use std::mem::{self, MaybeUninit}; let x: i32 = unsafe { mem::uninitialized }; // undefined behavior! ⚠️ // The equivalent code with `MaybeUninit<i32>`: let x: i32 = unsafe { MaybeUninit::uninit (). assume_init }; // undefined. C's offsetof() macro is an ANSI C library feature found in stddef.h.It evaluates to the offset (in bytes) of a given member within a struct or union type, an expression of type size_t.The offsetof() macro takes two parameters, the first being a structure name, and the second being the name of a member within the structure.It cannot be described as a C prototype

Undefined Behavior: Do not xor pointers with other values, like the xor pointer linked list trick used in C. Do not use the xor trick to swap two pointer values. Do not store pointers into non-pointer variables using casts and other tricks.. Suppose I've got some nasty DSP algos that depend on undefined behavior of type-punning in C. To address this I use the union member access trick which is supported by a gcc extension. If my only target is web assembly, then what are the reason that I should care that my binary is actually a gcc dialect of c and not standard c? raphlinus on Aug 18, 2018. That puts you pretty firmly in what I. In the C programming language, restrict is a keyword that can be used in pointer declarations. By adding this type qualifier, a programmer hints to the compiler that for the lifetime of the pointer, only the pointer itself or a value directly derived from it (such as pointer + 1) will be used to access the object to which it points.. restrict limits the effects of pointer aliasing, aiding. Undefined Behavior and Unsafe Code Guidelines. Last year, the Rust unsafe code guidelines strike team was founded, and I am on it. :-) So, finally, just one year later, this post is my take at what the purpose of that team is. Warning: This post may contain opinions. You have been warned. When are Optimizations Legal? Currently, we have a pretty good understanding of what the intended behavior. Programs whose behavior depends on this have undefined behavior; the C standard specifies that Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.. If a program breaks these rules, the results on any particular.

Arithmetic for two unions, undefined behavior

union Value { int int_value; float float_value; char *str_value; }; int main(int argc, char *argv[]) { union Value v; v.str_value = hi; v.int_value = 100; printf(v = %sn, v.str_value); /* undefined behavior! */ } 最後に書いた要素を常に読まなければならないということはどういう意味ですか?フィールドにアクセスしている理由 str_value 組合の. Learn C Language - Selection Statements. if Statements. One of the simplest ways to control program flow is by using if selection statements. Whether a block of code is to be executed or not to be executed can be decided by this statement

(Undefined behavior) *c = malloc ( 5 izeof( int )); free (c); *c = 3 ; //writing to freed location! In the figure shown above writing to a memory that has been freed is an example of the dangling pointer, which makes the program crash The C compiler does no more than work out what the biggest member in a union can be and allocates enough storage (appropriately aligned if neccessary). In particular, no checking is done to make sure that the right sort of use is made of the members. That is your task, and you'll soon find out if you get it wrong. The members of a union all start at the same address—there is guaranteed to be.