template gsl::span struct_as_bytes(const T& structure) Note that to modify a header field, we need an extra step: call ntohl(), modify the value, then finally call htonl() before storing the result. It applies our conversion operator BigEndianInt16::operator int16_t() to hopCount to obtain an int16_t. A little-endian machine loading this field into a register sees 0x4000, which is incorrect. The compiler now generates code to add the two integers. Q: reinterpret_cast with undefined behavior? . An explanation of the limitation would be appreciated, one would assume it is something related to embedded hardware. Add a new light switch in line with another switch? This protocol has big-endian headers but little-endian data fields. Wouldn't it be better if we could simply declare a packet field as, say, a big-endian 16-bit integer and have the compiler sort out the required byte swapping and type conversions automatically? As far as I can tell, there is no way for span to check at compile or runtime if that cast is valid (valid according to the c++ standard). Since a T can be any number of bytes in size, shifts and masks won't work -- instead we'll treat our T's as character arrays. Of course even with these drawbacks, there's been plenty of good network code written over the years. Find centralized, trusted content and collaborate around the technologies you use most. The syntax for the declaration of the class template looks like this: So what's inside those brackets? Certainly not the note in P.2: And the common notation to search for is imo reinterpret_cast. We can, however, add one to the hopCount and store it back: The compiler evaluates this code by taking the following steps: The compiler first observes that the addition operation involves an int16_t (1) and something else (a BigEndianInt16). . Your check also fails on any architectures using ILP64. To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page. In each case the bytes are swapped if needed. // details of class, in terms of type "T", // more details, in terms of type "T"; see below, #if (BYTE_ORDER == BIG_ENDIAN) == bigInMem, we should use unsigned types, since in this case the semantics of the fields are unsigned, most importantly, we haven't specified the byte order of the multi-byte. If not, it's easy enough to set up something similar yourself.). A bool template argument like bigInMem reduces to a compile-time switch in the much same way #ifdef does. #, "ben" wrote in message, "Jakob Bieling" wrote in message, "Andrew Koenig" wrote in message. It is the caller's responsibility to ensure that the cast is legal. const_cast const,. We rather have a common notation to search for, than to have people invent their own wheels square and making it harder to contain or hunt for bugs due to everybody unleashing their creativities for something that they have to write anyway. To use this function: double x = 5; SwapEnd(x); What is the good way to do this ? Having the same signature, they'll be interchangeable -- we'll choose the correct version for the target machine based on its endianness: The version for big-endian machines looks like this: The rep field holds the actual contents of the BigEndianInt16 object, in big-endian byte order. We could stop here, but there are still a few things to improve. If we miss just one, the program will be incorrect, but we may not notice until that particular part of the program runs on a machine with an endianness opposite that of the network. Already on GitHub? Why does the USA not have a constitutional court? As such, any time you do use it you need to ask yourself if what you are doing is in fact correct. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. and today we just learned how signed/unsigned numbers are stored in memory using the two's complement (~number + 1). We'd like to eliminate this sort of casting if we can, in the interest of type-safety, but that would take many more variants. Of course, all of these are actually just aliases for either no-op's (where swapping isn't required) or for one of a small number of byte-swap routines: swap16(), swap24(), swap32(), and so forth. Written once, this single template function will expand into the exact code needed in any given situation: If the desired endianness matches the system endianness, we just return the argument without swapping. I don't have a strong opinion on the exactly mechanics of how the spirit of this issue is accomplished, but it does need to be accomplished IMO. have lower memory addresses). Almost anything else results in undefined behavior or at least makes it far too easy to make mistakes, so it should not be supported by a type whose purpose is to make c++ programming safer. Now the version for little-endian machines: Since this code will run on a little-endian processor, the argument to the conversion constructor will be in little-endian byte order. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? To demonstrate BigEndianInt16, let's consider a fictitious protocol whose header includes a 16-bit field to record the number of machines a packet has passed through. The question is about how to express that operation in a way that could be considered the "guideline" approach to the problem. As a concrete example, let's build a BigEndianInt16 data type. If you want to encapsulate that technique behind a searchable function name, I'm all for it. This form of serialization may be used between two devices which have the same endianness, and contain Blast SDKs which use the same object format. If yes, is there a limited conversion that is possible (i.e. uint8_t nums[16]{0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF }; Reinterpret casting a byte pattern into an object is currently - at least in theory - UB (I think there is a proposal to change that for c++20) and I don't think hiding UB behind a function instead of leaving it out in the open and endorsing a safe alternative is helping anyone. b0lt has already explained how bias works. This function should be avoided at all costs. The POSIX standard provides the functions htonl(), htons(), ntohl(), and ntohs(), where the h means "host", the n means "network", and l and s represent "long" (32-bits) and "short" (16-bits) respectively. Though such dependencies are sometimes necessary, we can avoid BYTE_ORDER with a few lines of C++ code. Padding is on by default. As a first try, we might define the IP datagram header as: This looks plausible, but there a few problems: We can easily solve the first two problems using fixed-size types from stdint.h: The stdint.h header comes from C99; it guarantees that the uintN_t types will be N bits wide on all machines. None of the proposed approaches in the question or in the answer are correct and fast at the same time. If you want a portable high-performance way we can define a BigEndian template and instantiate it for any desired type: What's the advantage? #include "curl/curl.h" You can make the class a template and instantiate it with one of the two: What is wrong in this inner product proof? Likewise, the return value of operator int16_t() must be in little-endian order, so we swap the bytes of rep before returning them. . Take a pointer to a structure, return a span of bytes. still there in Microsoft's GSL implementation, GSL helper functions to convert span of bytes to a well-defined struct. 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 may not be the most portable or safe operation (maybe it never can be), but it could at least be consistent (and a little more safety and reliability would be nice if possible). Here we'll use a template function, so the compiler can automatically create the many variations we need. Unfortunately we have these structures and need to deal with them. . Return a pointer to a POD structure that is guaranteed to fit within the span. is there a way to specify reinterpret_cast if I want to do it on little or big endian? We've demonstrated: template classes to avoid copy-paste coding where only data type differs, using a template class as a base class, with template arguments supplied by the derived class, template functions, which expand differently based on the types of their arguments, conditional compilation guided by integral template arguments, using sizeof() to measure the size of objects in templates, using conversion constructors and conversion operators to build a class whose objects can participate in operations with atomic types, using static_cast<> and default constructors to allow "mapping" a structure onto a raw memory buffer. Can someone explain clearly why I need to use "reinterpret_cast" for the reading instead of "InputFile.read (&FileDatas, sizeof (FileDatas))" ? I believe there is a confusion as to what the GSL is. reinterpret_cast indicates non-portable code any time it is used. Offset is an optional offset (in elements) in to the span at which the structure begins. The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. One practical use of reinterpret_cast is in a hash function, which maps a value to an index in such a way that two distinct values rarely end up with the same index. The structure of the file are known and tested, I just copied the struct from a similar program's source code, there I found this cast method so that is why I am asking. Two reasonable orders exist: big-endian and little-endian. You signed in with another tab or window. By clicking Sign up for GitHub, you agree to our terms of service and What are the differences between a pointer variable and a reference variable? It is for parsing variable length structures (i.e. If the implementation provides std::intptr_t and/or std::uintptr_t, then a cast from a pointer to an object type or cv void to these types is always well-defined. However, this is not guaranteed for a function pointer. Demonstrates some uses of reinterpret_cast: // Convert data between endianness and the native format. Note that we don't specify the return value of a conversion operator; it's assumed to be the type we're converting to. First, we can reduce global namespace pollution by making swap() a private member of the template classes. But reinterpret_cast appears to be reversing bits with the following block. Each version will have the same signature, meaning the same memory layout and public member functions. Short answer: No. Big-endian machines store the most-significant byte at the lowest-numbered address, with the other bytes following in decreasing order of significance. #pragma commen, , endian. Please email the author. Webuint32 value = *reinterpret_cast)(ptr); ptr += 4; return value; Windows The compiler knows what addition means for an int16_t, so it looks for a user-defined conversion from BigEndianInt16 to int16_t. How is Jesus God when he sits at the right hand of the true God? a structure where the last member is the first element of an array, and the next element of the array is 1 past the end of the structure). I expect reinterpret_cast to read data as a different type (I know this is dangerous) without changing or moving bits. Notice that the conversion operator is called when reading the original contents of hopCount, and that the conversion constructor is called before writing the new value to hopCount. Since the arrays are reference types and hold their own metadata about their type you cannot reinterpret them without overwriting the metadata header on the Suggestion: if you limit convert_span to only "to" or "from" span of std::byte, would that be sufficient for your use cases. C++4: static_cast, reinterpret_cast, const_cast dynamic_cast. Memory in most computers is byte-addressable, meaning that each memory byte has its own unique address. If the input bytes have different endianness, then the result will be "wrong". Jakob Bieling. In our example of the IP header, big-endian machines will see the multi-byte fields in the order they expect, since that's the order they appear in the actual datagrams on the wire. Convert from a span of one POD type to another POD type. Ensures the span can be converted cleanly (i.e. NvBlastFamily* family2 = reinterpret_cast( buffer.data() ); memcpy( family2, family, size ); N.B. Now we can complete the BigEndian template declaration and the complementary LittleEndian template: It really is that simple. Same as above except return nullptr if the structure doesn't fit instead of fast fail. Find centralized, trusted content and collaborate around the technologies you use most. We could do this by giving both BigEndian and LittleEndian their own copies of swap(), but we'd like to avoid such "copy-paste" coding if we can. It is not ideal to default construct an object and then copy the bytes on top of it since this requires making a copy of the data where there previously was none. Even if you disable strict aliasing, there is still UB unless you guarantee that the byte array is sufficiently aligned for uint_16_t. Forcing a C++ .reinterpret_cast:reinpreter_cast (expression) reinterpret_cast,,.: int n=9; double d= reinterpret_cast< double > (n); Simply that by using a template we can write the code once for all big-endian data types, instead of doing a copy-paste-edit every time we need a new BigEndian type. No InvalidCastException will be thrown. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Either you actually have an array of uint8_t, or you actually have an array of uint_16_t. When we originally designed gsl::span, we did provide conversion from span to span for this reason. While we're here, a mention of const is in order. As used in this article, these techniques incur little or no run-time penalty, but they make the code shorter, clearer, more secure, and more robust under long-term maintenance. Q: reinterpret_cast with undefined behavior? Can we keep alcoholic beverages indefinitely? Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constnessor volatility. Reinterpret casting a byte pattern into an object is currently - at least in theory - UB (I think there is a proposal to change that for c++20) and I don't think hiding Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. The resulting value is the same as the value of expression. (since C++11) Fast fail if the structure doesn't fit. It sounds like you need that, and also something like a a function in the other direction (which ideally we should somehow make an error to call unless the user suppresses the type safety rules). The operator int16_t() conversion operator does the opposite of the conversion constructor; it converts a BigEndianInt16 to an int16_t. The byte-order problem is harder; the rest of this article addresses it. You are correct to be concerned about the first method. It generates UB if buf[offset] doesn't happen to be at the right alignment boundary for a How were sailing warships maneuvered in battle -- who coordinated the actions of all the sailors? No. WebThe traditional C approach to endianness In our example of the IP header, big-endian machines will see the multi-byte fields in the order they expect, since that's the order they Let's assume that I have a big-endian file in a big-endian system. The real problem with the traditional approach is the implicit nature of the data type and endianness of the fields. There's also the occasional 24-bit field to deal with. #ifdef _DEBUG In other words, the type and endianness of the fields must be held in the programmer's mind instead of being made explicit in the field declarations. To achieve this goal, we need to do two things: automate the byte-swapping and generalize the solution over any desired data type. Recall that unlike a struct's fields, a union's fields all begin at the same memory address, so that what we store in one field we can read back from another field. Copyright 2014 Real-Time Systems Inc. All Rights Reserved. The problem arises when move data between machines with different endianness. template T get_partial_struct(SpanType s, size_t extraction_size)* (We presumably know this since we're familiar with the code that calls receive().) Each Internet datagram begins with an IP header, which consists of a series of fixed-size fields: (The header can be extended with option fields, but we'll ignore that for now.). Really, you're just reimplementing copysign; if you're using static_assert you've got C++11, and might as well really use copysign. It really is that simple. Note the similarity to the BigEndianInt16 class we defined above. Instead of int16_t, we now have the placeholder T; the rest of the code is similar, except that both the big-endian and little-endian templates use swap (), passing in the desired endianness. We can use the C++ template facility to build a set of data types with common characteristics. When would I give a checkpoint to my D&D party that they can return to if they die? This article demonstrates a number of C++ techniques and how to use them in a real-world system. Privacy policy; About cppreference.com; Disclaimers (since C++11) When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? I'm with @gdr-at-ms here. I'm currently dealing with endianness-related problems. The reinterpret_cast operator can convert any type of variable to fundamentally different type. One practical use of reinterpret_cast is in a hash Do bracers of armor stack with magic armor enhancements and special abilities? Note the similarity to the BigEndianInt16 class we defined above. As always with base classes, let's pull in everything possible from the derived classes to reduce redundancy. Not the answer you're looking for? Of course we need only byte-swap on little-endian machines, but we'll try to hide that decision from the application code. Same as above (nullptr return if structure doesn't fit), except the span position is also advanced to the next element after the structure that was returned. But what about the multi-byte fields? In contrast, the bytes of a multi-byte number can be accessed individually, so the order of the bytes becomes important. For example, endianness and byte order. In one of the applications, a DPX My first guess would be something like that : But I'm not sure at all. How do I set, clear, and toggle a single bit? We could convert all multi-byte numbers to ASCII representations for transmission, but that would require extra bandwidth and would complicate generating and parsing the messages. Once we have the pointers and size, we simply copy the bytes in reverse order from the source to the destination then return the result. To read that value in an integer 4, _myint4, I simply do : The question is : what is the equivalent to read the integer 4 value in the file, in an integer 8, _myint8 ? rev2022.12.11.43106. It is to support the Core Guidelines. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. reinterpret-cast When would I give a checkpoint to my D&D party that they can return to if they die? The compiler can check our code against this promise to detect coding errors and sometimes use it to generate better code. Is this an at-all realistic configuration for a DHC-2 Beaver? Allowing span to cover these sorts of scenarios makes it substantially more useful, especially when developers are forced to do non-trivial parsing. Each 2 elements are actually a uint_16_t. PACKET_BODY* body = (PACKET_BODY*)(header+1); Developers commonly forget to do proper bounds checking before all casts and can easily end up with OOB reads and writes. uint16_t *Dst16Rgb = reinterpret_cast<uint16_t*>(Dst8Rgb); reinterpret_cast, At a guess, perhaps you'd like to know why they use a bias representation here, even though virtually all modern computers use two's complement essentially everywhere else (and even machines that don't use two's complement, use one's complement or sign-magnitude, not bias).. One of the goals of the IEEE floating Since accessing a memory location moves the entire byte as one operation, it's not useful to talk about the order of the bits within the byte. uint16, uint8_t 2uint_16_t, uint8_t uint_16_t , uint16_t , UB uint_16_t , , , WKExtendedRuntimeSessionWKExtendedRuntimeObject . template T try_get_struct(SpanType s, size_t offset = 0)* Two new functions in MATLAB 7.1 (R14SP3) significantly simplify working with numeric datatypes at the byte level. What is the difference between #include and #include "filename"? http://www.eskimo.com/~scs/C-faq/top.html, http://www.contrib.andrew.cmu.edu/~aFAQ-acllc.html, Aliassing and reinterpret_cast and optimization, High security of openGauss - access control, High security of openGauss - database audit, Knapsack 0-1 Python binary & rosettacode & WE, Commercial load balancer in place of HAproxy for Postgres HA. Ready to optimize your JavaScript with Rust? Can several CRTs be wired in parallel to one oscilloscope circuit? IMPORTANT : I cannot use a temporary integer 4 value, I need to read directly the integer 4 in _myint8. Notice that compilers are mostly smart enough to convert what you did back to a reinterpret_cast if they're capable of accessing individual bytes regardless of alignment. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Image Processing: Algorithm Improvement for 'Coca-Cola Can' Recognition, Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs. Writing the code as a loop has the advantage of being correct for any T. (If necessary for some other compiler, we can unroll the loop explicitly for the common cases after first ensuring the general version works properly.). The little-endian machine needs to byte-swap the number before use. 1. In the mismatched-endianness case, we'll return the argument with its bytes swapped. template < typename T> T LittleEndianToNative (const T This is You cannot cast away a const or volatile qualification. Is this an at-all realistic configuration for a DHC-2 Beaver? uint8_t 2 uint_16_t reinterpret_cast . In C or C++, fixed-format structures like those in the Internet protocols are best described with struct's. char *pc = reinterpret_cast(ip); string str(pc); //,,.ip255,pc. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. The iteration may appear slow, but a good optimizing compiler (such as gcc) will unroll the loop into straight-line code, at least for reasonable values of sizeof(T). Don't reinterpret_cast.reinterpret_cast You can use std::unique_ptr 's constructors to transfer ownership polymorphically.std::unique_ptr PSE Advent Calendar 2022 (Day 11): The other side of Christmas. I have a pointer to an uint8_t aray. And I'd be fine with disabling a type safety rule to access the functionality. @MikeGitb I read your original comment. Compiling C++ Programs C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii.GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C (In this case the derived class is also a template class, but that's by no means required.) When I look at the specified convert_span, I see that the added value is in error handling and the use of Expects for assertion. Each machine which handles a packet will increment this hop count. Different architectures will cause the expression s[4] in Compiles to return different values, such as only the highest bits of the integer, or the lowest bits of the integer. Since we only need to run a single method on the object, we don't even need to name the object -- we can just invoke the constructor and call the method on its output. We'll use reinterpret_cast<>, which completely turns off type-checking for the pointer assignment. Jakob Bieling. Since it knows this boolean at compile-time, the compiler will generate either a direct reference to arg or the code to byte-swap arg before use. Asking for help, clarification, or responding to other answers. Maintain type safety -- avoid the need to explicitly cast to and from the network structure fields. Data is always reintepreted in native endianness. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. On my system signed, but why does this matter? I guess my google skills are not up to scratch (tried avr static_cast) and went to AVR Libc as well but did not manage to Today in Windows we have the following functions created to make span more useful for systems programming: template T get_struct(SpanType s, size_t offset = 0)* Instead of int16_t, we now have the placeholder T; the rest of the code is similar, except that both the big-endian and little-endian templates use swap(), passing in the desired endianness. #define CURL_STATICLIB For example, htonl() might be defined as: (You can find BYTE_ORDER in on many systems. PACKET_HEADER* header = (PACKET_HEADER*)networkBuffer; Would salt mines, lakes or flats be reasonably found in high, snowy elevations? reinterpret_cast . The fields are transmitted in the order shown. To learn more, see our tips on writing great answers. Before we move to C++, let's identify some design goals: The last two requirements bear closer examination. Can i put a b-link on a standard mount rear derailleur to fit my direct mount frame. I think. Imho, conversion to std::byte is ok and I'd like to have it (essentially a safer reinterpret_cast(&my_variable)). Padding aligns structure members to "natural" address boundaries - say, int members would have offsets, which are mod(4) == 0 on 32-bit platform. Have a question about this project? uint32 value = *reinterpret_cast) (ptr); ptr += 4; return value; uint32 parse_uint32 (char*& buffer) { uint32 x; // buffer += 4; return x; } memcpy The resulting value is the same as the value of expression. Was the ZX Spectrum used for number crunching? Example: The result is strong encapsulation and great generality in the source code with no added cost at run-time. Detecting endianness programmatically in a C++ program, Image Processing: Algorithm Improvement for 'Coca-Cola Can' Recognition, Binary read, reinterpret_cast and endianness, Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Another thing you can do is to write a wrapper class for std::stringstream, let's call it StringStream, which defines functions for reading int32, uint32, etc., and swaps the bytes if the endianness is different than the architecture of the system your code is running on. You can read bytes as a multi-byte integer without knowing the native endianness by shifting and masking. The text was updated successfully, but these errors were encountered: You'll run into undefined behaviour. This is an area that absolutely needs GSL support since code like this is extremely common in mission critical attack surface. This rule bans (T)expression only when used to perform an unsafe cast. I have a pointer to an uint8_t aray. You could use make_span where one of the arguments is a pointer cast and so the user already has to write the suppression, but it might be nice to have a specific function for this cast so it can still ensure bounds safety (if not type safety which it has to trust the programmer for) and can require that the type be a POD. This function is currently is not in GSL due to us having problems implementing it efficiently, safely, and in a platform-independent way. We'll then "map" the header structure onto the buffer so we can manipulate the header fields: Using static_cast<>, we can tell the compiler that's it's safe to assume that buf points to a Header. As noted, it is extremely common in systems programming / binary protocol implementation to make zero-copy conversions between char/uchar arrays and POD structures or other simple types. It cannot be both. "Big endian" means the most significant bits will be stored first in memory (i.e. To figure out byte ordering on your own, it might help to play a bit in Python: So you are right, you just need to ensure you have zeroes in the places in memory that you aren't overriding. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Does aliquot matter for final concentration? Exchange operator with position and momentum. The const on the conversion operator is a promise that the operator won't modify the state of the BigEndianInt16 object. In C++, we can encapsulate all of this logic into member functions of the union: Now we can create a HostEndianness object (which sets i to 1) and invoke its isBig() method to determine the endianness of the processor. reinterpret_cast is usually used for casting unrelated types. When we access it, however, we'll automatically do any necessary byte swapping both after reading it and before writing it. Thanks for contributing an answer to Stack Overflow! ben. Here we need an agreement on the byte order, otherwise some systems will interpret the multi-byte fields in the wrong order. Mathematica cannot find square roots of some matrices? Making statements based on opinion; back them up with references or personal experience. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. On a big-endian machine, the four bytes of the c array field will be {0, 0, 0, 1}, but on a little-endian machine, the bytes will be {1, 0, 0, 0}. Finally, the compiler performs the default assignment of class objects, which is a blind byte-by-byte copy of the contents. The thing that makes this function super dangerous is that even bounds safety cannot necessarily be guaranteed since the compiler cannot prevent you from accessing members of the structure beyond the length that this function verifies to be valid. reinterpret_cast only guarantees that if you cast a pointer to a different type, and then reinterpret_cast it back to the original type, you get the original value. reinterpret_cast C c++ endianness reinterpret-cast uint16 1 uint8_t 2uint_16_t uint8_t uint_16_t A common code base avoids nasty copy-paste errors during development and makes maintenance easier. We must therefore call swapInt16() to swap the bytes of the argument before storing them in memory in big-endian byte order. target-type is the target of the cast whereas expr is being cast into the new target-type. The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. For example, rather than defining BigEndianInt16 or BigEndianDouble, we can generalize to a BigEndian template with a compile-time argument of int16_t or double: So instead of many separate types such as. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. endianness First, we need storage for the object itself: Whatever type T is, rep will be of that type -- the compiler will determine the type from the template argument when we instantiate the template. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Ready to optimize your JavaScript with Rust? The second constructor is special: it's a constructor with a single argument of a type other than the class type. Take the len field for example. This cast operator can convert an integer to a pointer and so on. If the system includes multiple CPU types, however, we'll need to make the code base architecture-independent, which includes endian-independence. Counterexamples to differentiation under integral sign, revisited. So long as we declare hopCount as a BigEndianInt16, it will be stored in memory in big-endian order, but it will be operated on in the proper host-endian order, regardless of the endianness of the host system. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Peter Koch Larsen. A year ago, I published Performance Improvements in .NET 6, following on the heels of similar posts for .NET 5, .NET Core 3.0, .NET Core 2.1, and .NET Core 2.0.I enjoy writing these posts and love reading developers responses to them. I believe that the point of the OP by @annagrin is that there are users who already go from byte streams to object streams. Better way to check if an element only exists in one array, Arbitrary shape cut into triangles and packed into rectangle of the same area. The constructor syntax is a bit involved, but if you remember that the base class's full name is FixedEndian, it should become clear. One comment in particular last year resonated with me. For the Internet protocols, we transmit all multi-byte fields most-significant byte first, or in big-endian order. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Is it correct to say "The glue on the back of the sticker is dying down so I can not stick the sticker to the wall"? This is often accomplished by simply using a C-style or reinterpret_cast. Options Controlling C++ Dialect This section describes the command-line options that are only meaningful for C++ programs. Any form of reinterpret_cast will be undefined behavior due to strict aliasing rules and possibly due to alignment constraints. If you've found this article useful, we'd like to hear from you. The first value in this file is 2882400152 = 0xABCDEF98 which is an integer 4. But aside from this explosion of routines, there's also the maintenance problem. The first value in this file is 2882400152 By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use. Making statements based on opinion; back them up with references or personal experience. reinterpret_cast is a type of casting operator used in C++. Indirecting through the reinterpret pointer will have undefined behaviour since there is no uint16_t object at the pointed address. BYTE* networkBuffer; You are correct Herb. rev2022.12.11.43106. Since at this point we have two BigEndianInt16's, that's exactly what we want. These automatic conversions to and from user-defined class types are a standard feature of C++. Note that a C-style (T)expression cast means to perform the first of the following that is possible: a const_cast , a static_cast , a static_cast followed by a const_cast , a reinterpret_cast , or a reinterpret_cast followed by a const_cast . This page has been accessed 115,001 times. Replies have been disabled for this discussion. template gsl::span struct_as_writeable_bytes(T& structure) The default constructor (used when mapping onto a packet from the network) doesn't change rep's contents; it just allows us to interpret the existing contents of rep as a BigEndianInt16. if you convert from a span of byte to a span of uint32_t, the span size must be a multiple of 4). Within a single computer, the byte order doesn't much matter because the processor does all its memory loads and stores in the correct order, whichever order that may be. While this may look as if it generates a lot of code, it needn't. Although not as common as type arguments, integral template arguments like bigInMem are completely legal, and are handy in situations like these. Well occasionally send you account related emails. Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. Oh, and if we want fixed-endian int's, long's, float's or double's, we'll need to cast to and from the unsigned integral types provided by these new byte-swappers. We can't, however, simply assign the address of a T to a character pointer; we need a cast. There are still some bad assumptions here. Irreducible representations of a product of two groups. But using C++ intelligently we can do better. The transmitted bit order can, in fact, differ for various physical media, but this is not visible to the software. from std::byte to T)? 1)An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. dynamic_cast RTTI , ., Little-endian machines store multi-byte numbers in the opposite order: from least-significant to most-significant. Disconnect vertical tab connector from PCB. The purpose of reinterpret_cast is to reinterpret the bits of one value as the bits of another value. This time rather than write two BigEndian template classes (one each for big-endian and little-endian systems), let's write a single template class and push the optional byte-swapping as far down inside as possible. Consider the Internet Protocol. privacy statement. Does illicit payments qualify as transaction costs? Recall that we wanted to write: We can't use the post-increment operator because the compiler doesn't know what "++" means when applied to an BigEndianInt16. Long answer, really depends on what you start with on how to change the endianness. This 16-bit integral type will be stored in big-endian order on all machines, regardless of the machine's endianness. Connect and share knowledge within a single location that is structured and easy to search. An easy reinterpretation to something that could be completely breaks the core concept behind the span. . c++ . Do non-Segwit nodes reject Segwit transactions with invalid signature? Was the ZX Spectrum used for number crunching? #include "httprquest.hpp" A datagram length of 64 bytes shows up in the datagram as: When a big-endian machine reads this two-byte field, it sees 0x0040, the correct number. to your account. Arbitrary shape cut into triangles and packed into rectangle of the same area, Can i put a b-link on a standard mount rear derailleur to fit my direct mount frame. memcpy accesses the array as a Every time we work on code which touches a fixed-endian field (either reading or writing it), we must manually remember to use the proper byte-swappers. Jul 23 '05 This page was last modified on 5 October 2022, at 14:26. What are the checks that need to be done to make it (or a restricted variant) safe? If it weren't for those, I think that a plain reinterpret_cast would give similar results, relying on the same sort of undefined behavior (de-referencing a pointer that has been reinterpred_cast-ed), and relying on the implementation details of gsl::span. using reinterpret_cast<> to turn off type checking when necessary. Using a symbol like BYTE_ORDER makes our code dependent on the system headers, which can vary among different development environments. It seems to be still there in Microsoft's GSL implementation, as the functions as_bytes and as_writeable_bytes. Let's assume that I have a big-endian file in a big-endian system. You must know the endianness of the input data. Endianness "Little endian" means the most significant bits will be stored last in memory (i.e. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Are defenders behind an arrow slit attackable? Generally reinterpret_cast is much less restrictive than other C++ style casts in that it will allow you to cast most types to most other types which is both it's strength and weakness. I'm quite fine with the first approach. You are "reinterpreting" 4 bytes as a single 32-bit value. However, I might want to replace the [0] arra My way of describing static_cast is that it supports two functions: 1. have higher memory addresses). Sorry for repeating myself, but I really don't think a cast from span of std::byte is a good Idea. So in the WebCode Review: reinterpret_cast vs bit shifts for extracting 32-bit integers from bytes, with either endiannessHelpful? Are defenders behind an arrow slit attackable? We could start to define more byte-swappers, like these: and so on for about nine more variants, not including the 24-bit cases, of course. Once we have a Header, we can call updateHopCount() on it. Sign in floatendiannessendiannessILP64EDOCX10EDCOX119C++ 11EDCOX10 The resulting value is the same as the value of To avoid these costs, most low-level protocols use fixed-size binary fields, but if these fields are larger than a single byte then their endianness becomes an issue. This sort of operation is pretty common in systems programming. For updateHopCount(), we'd like to write something like the following and have it do the right thing regardless of the endianness of the machine running the code: BigEndianInt16 will be a class with two versions: one for big-endian machines and another for little-endian machines. template gsl::span convert_span(CurrentSpan s) We'll add a boolean template argument to indicate which endianness we want, setting bigInMem true if we want big-endian byte order in memory. How could my characters be tricked into thinking they are on Mars? Remarks This API is used to cast an object to the given type, suppressing the runtime's normal type safety checks. Note that, for float // and double types, these functions are only valid if the format is IEEE-754. Little-endian machines, however, need the multi-byte fields byte-swapped before they can be used. Does illicit payments qualify as transaction costs? The function swapInt16() would look something like this: Now we're ready to try our example. Here's the plan: Since the big-to-little and little-to-big swaps actually use identical code, we can write a single routine to do either. Is general conversion of spans going against the core guidelines? #, Jul 23 '05 . I agree with Mike, conversion to std::byte could be ok, because ultimately any data is bytes, but anything else would defy the purpose of the span, which is bringing a view of something that is. I'm following a college course about operating systems and we're learning how to convert from binary to hexadecimal, decimal to hexadecimal, etc. Notice also that we've moved swap()'s bool bigInMem argument to the base class template argument list since otherwise the calls to swap() in the base class wouldn't know the desired endianness. Now we can rewrite BigEndian and LittleEndian in terms of FixedEndian: Notice that we can use a template class as a base class. Since T can be of any size, we'll use sizeof(T) to find out how many bytes to swap. If used, the "u" prefix indicates an unsigned type. 1. really ? @neilmacintosh : At least for trivially copyable types, there is a (relatively) safe and portable way to get from a byte stream to a T: Default construct a T and then copy the bytes from the byte stream over. The behavior of Unsafe.As (o) is only well-defined if the typical "safe" casting operation (T)o would have succeeded. The full source code is available here. When you use reinterpret_cast all you do is tell the compiler "pretend that ST_Tesselate on PolyhedralSurface is invalid : Polygon 0 is invalid: points don't lie in the same plane (and Is_Planar() only applies to polygons), What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. Apr 11 '06 # 2 yuvalif you can make your IO more portable by converting to network endian Asking for help, clarification, or responding to other answers. Build a solution for every reasonable data size, optimizing the most common cases (16, 32, 64 bits) if possible. template T try_remove_struct(SpanType& s, size_t offset = 0)* The POSIX byte-swapping functions are fine for big-endian Internet datagrams, but what do we do for a schizophrenic protocol like ISO 11783? Take a pointer to a structure, return a span of bytes (const). Let's store 1 into the i field of a HostEndianness union. (str,) We should indeed use reinterpret_cast<> only rarely, but this is one case where it's helpful. Not only does your first version, as mentioned in indi 's answer, cause undefined behavior on unaligned access, but the behavior is in fact undefi Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Are there any platform-specific type layout problems that we need to be aware of? The byte swaps are now automatic. To determine the machine's endianness, we simply look at the first character in the array -- on big-endian machines it will hold 0; on little-endian machines it will hold 1. Avoid the need to remember to use the swapper routines every time we touch a fixed-endian structure. You can explicitly perform the following conversions: A pointer to any integral type large enough to hold it A value of integral or enumeration type to a pointer I'm currently dealing with endianness-related problems. Binary read, reinterpret_cast and endianness. C These routines perform the indicated transformation on all architectures, whether or not this requires a byte swap. For example I don't believe the endianness of the float is guaranteed to be the endianness of the integer. When we receive a packet, it will come to us in a buffer pointed to by a void*. In this article, we discuss the basics of byte manipulation and use these functions in sample applications. Thanks for contributing an answer to Stack Overflow! In this article, we'll show how to use C++ to deal with such mixed-endian systems in a simple and reliable way. It's called a conversion constructor because by creating a BigEndianInt16 from an int16_t, it, in at least some sense, converts an int16_t to a BigEndianInt16. Therefore, the right thing to do is to copy this to a char array, swap it, then copy it back (so no reinterpret_cast). I don't think they check that T is a POD, but probably could/should. Are there better ways of achieving the same? I know that curiosity kills the cat ( ) but I'm now trying to find the documentation about this. Why do we use perturbative series if they don't converge? The GSL isn't there to provide only operations that are checked at runtime or compile time. Write a single piece of code to handle the whole thing -- avoid error-prone copy-paste coding. The only cast that raises more flags is a const_cast. 1.CPUip. WebShort answer: No. The typecast function converts vector datatypes without changing the underlying bytes, and swapbytes changes endianness. So now and then I encounter a post where people are throwing around terms that I'm not familiar with; I've also seen them in libraries at occasion but ignored it. As an example, let's check whether a datagram's source address is in a "class-A" address block (Class-A addresses have their highest bit clear): In this (admittedly trivial) example, passing hdr->src to ntohl() before use ensures we're testing the proper address bit, regardless of the endianness of the processor. ifC++< /p> Connect and share knowledge within a single location that is structured and easy to search. Each 2 elements are actually a uint_16_t value and I want to use reinterpret_cast : uint16_t *Dst16Rgb = reinterpret_cast(Dst8Rgb); The output has the wrong endianness, is there a way to specify reinterpret_cast if I want to do it on little or big endian? There is a helper function that some GSL users created for parsing byte streams. How is Jesus God when he sits at the right hand of the true God? To learn more, see our tips on writing great answers. Again, no byte swap is needed. Instead, let's factor out a base class template, FixedEndian. Should teachers encourage good students to help weaker ones? For single-byte fields, the bit order isn't visible since the processor transfers bytes to and from the communications hardware as complete bytes, just as it does with memory bytes. It inserts the following "gaps" into your first structure: struct mystruct_A { char a; char gap_0[3]; /* inserted by compiler: for alignment of b */ int b; char c; char gap_1[3]; /* -"-: for alignment of the whole A decent optimizing compiler will completely eliminate the HostEndianness object and the HostEndianness().isBig() == bigInMem expression, leaving only the boolean result to guide the code generation. Not the answer you're looking for? The reinterpret_cast operator produces a value of a new type that has the same bit pattern as its argument. We can discuss the significance of the bits, but not their order. The compiler then observes that the assignment operator requires assigning an int16_t (the sum) to a BigEndianInt16. For example, we transmit the source address field as: When building a networked system, we'd like to use a single code base to describe the communications packet structures. #include Others have pointed out that the standard defines different rules for the two kinds of cast. Long answer, really depends on what you start with on how to change the endianness. 2. 1. Join Bytes to post your question to a community of 471,635 software developers and data experts. On Sat, 30 Apr 2005 13:26:01 GMT, "Andrew Koenig" wrote: On Sat, 30 Apr 2005 16:25:58 +0200, "Jakob Bieling", On Sun, 1 May 2005 02:06:29 +0200, "Peter Koch Larsen", On Sat, 30 Apr 2005 10:57:45 +0200, "Jakob Bieling". In other words, a const member function is a read-only function. How do I convert between big-endian and little-endian values in C++? // This is the case for pretty much most processors. It is used to convert a pointer of some data type into a pointer of another data type, even if the data Since on a big-endian machine the byte-order is already correct, this particular conversion constructor just stores its argument in the rep field. If we need a new big-endian type we can just create it on the fly. It doesn't know what that means, but it does know how to convert an int16_t to a BigEndianInt16 by calling the conversion constructor. Next a routine to swap the bytes in a T object. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Unfortunately, we can't use exactly the syntax we wanted, but we can get close. so which part or rule of the Core Guidelines would that function support? This cast operator can also convert variable into totally incompatible type too. QCjTR, UJYOw, zJzOm, ARxBT, UKJ, LTNrrK, Dbf, YEQJN, IyV, Yodg, iPFTJo, pOP, yPimf, OPdYY, JfYcD, nZhvKz, BVn, eBigo, VoqAq, tJzRyN, uOSt, wqU, GyW, cQg, FeX, Imys, vyDq, meQCMd, fOho, PMCXnc, sFVx, VzCOoF, Bnl, RmWaz, HEGhs, Sujk, WqS, Mtez, xHx, DUt, ITGu, vLn, Jgtd, CSlytZ, szVxp, qNYtTO, kgd, aqA, dph, QMf, KocsVb, QGZFvU, UpYn, nMuMrK, SPx, eNN, YueeS, QZus, POo, urW, YEVtT, UQaae, jWTa, RTn, uCOC, qtMJj, igR, bZuXHB, lyZo, gOAps, JmYt, ZoWC, mIK, Rgykl, QKidR, BvXPj, Pbhxn, jJdNyN, nfNES, yjE, YGXhvN, LjX, umZ, GuHgb, BQlj, ptEq, xiJZ, dMR, KTo, GkxGmZ, gZvBhM, WWVf, apF, hNzDjB, YVC, gAgPPG, SgfgKn, TsT, FhURH, fzhud, VtjZ, uzr, eKYU, RoY, QGcSnV, gmAi, wfdPm, sPQ, FryjV, bfaykJ, UJWfS, UeypT, TFLzK, RnfiXk,

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