https://devblogs.microsoft.com/oldnewthing/20230109-00/?p=107685 Skip to main content [RE1Mu3b] Microsoft The Old New Thing The Old New Thing The Old New Thing * Home * DevBlogs * Developer + Visual Studio + Visual Studio Code + Visual Studio for Mac + DevOps + Developer support + CSE Developer + Engineering@Microsoft + Azure SDK + IoT + Command Line + Perf and Diagnostics + Dr. International + Notification Hubs + Math in Office + React Native * Technology + DirectX + PIX + SurfaceDuo + Startups + Sustainable Engineering + Windows AI Platform * Languages + C++ + C# + F# + Visual Basic + TypeScript + PowerShell Community + PowerShell Team + Python + Q# + JavaScript + Java + Java Blog in Chinese * .NET + .NET + .NET MAUI + Blazor + ASP.NET + NuGet + Xamarin + .NET Blog in Chinese * Platform Development + #ifdef Windows + Apps for Windows + Azure Depth Platform + Azure Government + Bing Dev Center + Microsoft Edge Dev + Microsoft Azure + Microsoft 365 Developer + Old New Thing + Windows MIDI and Music dev + Windows Search Platform * Data Development + Azure Cosmos DB + Azure Data Studio + Azure SQL Database + OData + Revolutions R + SQL Server Data Tools * More [ ] Search Search * No results Cancel On leading underscores and names reserved by the C and C++ languages [png] Raymond Chen January 9th, 202313 3 The C and C++ languages reserve certain categories of names for the implementation, which means that you cannot use them in your own code. Some are reserved unconditionally, precluding their use for for variable names, parameter names, classes, methods, macros, whatever. Others are reserved only in certain contexts. The rules for C++ are collected in the [lex.name] chapter. The rules for C happen to match the C++ rules for the most part, (section 7.1.3 "Reserved identifiers"), so that makes things easier to remember. +-------------------------------------------------------------------+ | Pattern | Conditions | |---------------------------------+---------------------------------| | Begins with two underscores | Reserved | |---------------------------------+---------------------------------| | Begins with underscore and | Reserved | | uppercase letter | | |---------------------------------+---------------------------------| | Begins with underscore and | Reserved in global scope | | something else | (includes macros) | |---------------------------------+---------------------------------| | Contains two consecutive | Reserved in C++ (but okay in C) | | underscores | | +-------------------------------------------------------------------+ Note that a popular convention of prefixing private members with an underscore runs afoul of these rules if the member name begins with an uppercase letter. class Widget { public: Widget(); private: int _size; // okay void _Toggle(); // not okay }; The C language does not have namespaces, so it also must reserve names in the global namespace for future expansion. Some names may not be used by symbols with external linkage. You can use them for type names, enumeration members, local variables, and functions or global variables declared with static storage class, but not for extern functions or extern global variables. // Not allowed: Identifier with external linkage // beginning with "str" and a lowercase letter. int strategy; void strafe() { /* ... */ } // Allowed: Identifier with internal linkage beginning // with "str" and a lowercase letter. static int strawberry; static void stream_video() { /* ... */ } Furthermore, if you include the corresponding header file, the names are permitted to be shadowed by function-like macros. This means that if you intend to use a reserved name for someting without external linkage, you must first #undef it or enclose the name in parentheses to prevent it from being treated as a macro. // Including this header may result in the definition // of a function-like macro named "strategy". #include // Must enclose in parentheses to prevent misinterpretation // as function-like macro. static void (strategy)(); As of C11, identifiers matching the following regular expressions may not be used for symbols with external linkage. (The list is given in section 7.31: "Future library directions".) There are also reserved names for type definitions and macros, but I won't list them here. +---------------------------------------------------+ | Pattern | Header | |------------------------------+--------------------| | cerfc?[fl]?, cexp2[fl]?, | | | cexpm1[fl]?, clog1[0p][fl]?, | complex.h | | clog2[fl]?, c[lt]gamma[fl]? | | |------------------------------+--------------------| | is[a-z].*, to[a-z].* | ctype.h, wctype.h | |------------------------------+--------------------| | atomic_[a-z].* | stdatomic.h | |------------------------------+--------------------| | str[a-z].* | stdlib.h, string.h | |------------------------------+--------------------| | mem[a-z].* | string.h | |------------------------------+--------------------| | wcs[a-z].* | string.h, wchar.h | |------------------------------+--------------------| | cnd_[a-z].*, mtx_[a-z].*, | thread.h | | thrd_[a-z].*, tss_[a-z].* | | +---------------------------------------------------+ It may come as a surprise that the C language reserves identifiers like strong, island, and together, but it does. Bonus chatter: Windows header files have historically not been conscientious about avoiding these reserved names. We're trying to do better for new headers, but not everyone has gotten the memo. Update: Added special "internal double-underscore" rule. [png] Raymond Chen Follow Tagged Code Read next What does it mean when I get a mismatch from MSVC for _COROUTINE_ABI? The two different kinds of coroutine interfaces shouldn't be mixed and matched. [png]Raymond Chen January 11, 2023 0 comment 13 comments Leave a commentCancel reply Log in to join the discussion. * [png] Andreas Schulz January 9, 2023 9:06 am 0 collapse this comment copy link to this comment not everyone has gotten the memo. Not even the Windows Terminal team, eh? https://github.com/microsoft/terminal/issues/3512 Log in to Vote or Reply + [png] Dustin Howett January 9, 2023 4:58 pm 0 collapse this comment copy link to this comment We're definitely behind on fixing that one Log in to Vote or Reply * [png] Adam Rosenfield January 9, 2023 10:22 am 0 collapse this comment copy link to this comment Even if they'd be within their rights to do so, I'd wager that the ISO C committee would try to avoid introducing certain new functions like `strafe` that could potentially collide with identifiers being used by real code that didn't follow the rules. Log in to Vote or Reply + [png] Hong Lou Tou January 9, 2023 7:13 pm 0 collapse this comment copy link to this comment Good C code nowadays always use "poor man's namespaces" by prefixing the library name to global names, and really the C language standard should start doing the same, even if old names cannot be changed. Log in to Vote or Reply o [png] aaaaaa 123456789 January 11, 2023 10:21 am 0 collapse this comment copy link to this comment And that's exactly what happened: C23 uses stdc_ for a bunch of functions it introduces. Log in to Vote or Reply * [png] Henke37 January 9, 2023 1:33 pm 0 collapse this comment copy link to this comment I'd argue that the windows api counts as part of the implementation, and as such is allowed to use the reserved names. Log in to Vote or Reply + [png] MGetz January 10, 2023 5:27 am 0 collapse this comment copy link to this comment And yet the windows headers include macros for min and max which clobber the standard std::min and std::max creating what is technically undefined behavior unless the dev defines NOMINMAX. Which is also why that's a standard define in all my projects, because while the macro versions are technically faster they are also technically undefined behavior. Log in to Vote or Reply * [png] James Touton January 9, 2023 4:12 pm 1 collapse this comment copy link to this comment Note the wording for the double-underscore prohibition: > Each identifier that contains a double underscore __ [...] The prohibition covers all names that contain a double underscore anywhere in the identifier, not just at the start. Log in to Vote or Reply + [png] Lukas Mai January 10, 2023 9:47 am 0 collapse this comment copy link to this comment Yeah, that's why "the rules for C happen to match the C++ rules" is not quite true. C++ reserves identifiers that contain double underscores anywhere, but C does not. In other words, int a__b; is perfectly valid C, but uses a reserved identifier in C++. Log in to Vote or Reply * [png] Michael Spam January 10, 2023 8:57 am 0 collapse this comment copy link to this comment How many C++ programs have a variable name called total . Millions? Had no idea it was reserved! Log in to Vote or Reply * [png] Letao Wang January 10, 2023 6:22 pm 0 collapse this comment copy link to this comment These rules are kinda yikes. They barely ever get mentioned, let alone taught systematically. Compilers don't seem to care to enforce them either. They are like traps that will happily let people walk in, and everything will feel fine until a random day when it explodes. Either that or they are "rules in name only", the kind that everyone violates and there are no consequences. Log in to Vote or Reply * [png] Daniel Roskams January 11, 2023 3:32 am 0 collapse this comment copy link to this comment The C standard is kind of a joke that no one actually follows. I don't have hard numbers, but I bet I could count on one hand the number of non-trivial programs on my computer that 100% comply to the C standard. Even people that postulate about writing "standards compliant" or "portable" etc. code have probably named a variable "total" or "strict" or "structureSize" or "isvalid" at some point. Log in to Vote or Reply + [png] George Tokmaji January 11, 2023 9:25 am 0 collapse this comment copy link to this comment Normal variables don't have external linkage, and the C11 rules in the table only applies to symbols with external linkage. 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