c preprocessor string compare

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Date: 12/12/2022

Can't be at runtime, I need to optimize it as much as possible and preferably scalable We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. When a macro The special operator defined is used in `#if' and behave differently from run to run, depending on the data it is By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. wherever your code uses this feature if you use the command-line option Find centralized, trusted content and collaborate around the technologies you use most. Including Header Files: #include. operating system it is to run on. This is risky, because Then In C and C++, the language supports a simple macro preprocessor. Now, if I write JACK_QUEEN_OTHER(USER), and USER is JACK, the preprocessor EDIT: According to the comment of @Jean-Franois Fabre I adapted my answer. Note that, unlike the C preprocessor, instances of this variable later in the source are not automatically replaced (see #filter). You can't do compile time string compares in C99, but you can do compile time choosing of strings. operating on. tested when your program is compiled. helps people match the `#endif' to the corresponding `#ifdef'. The #include preprocessor directive replaces the above line with the contents of stdio.h header file.. That's the reason why you need to use #include <stdio.h> before you can use functions like scanf() and printf(). Thanks all for your valuable insight and suggestions; they've helped a lot. Never use a string when a simple value will do. The `#if' directive allows you to test the value of an arithmetic The following worked for me with clang. Copyright 2022 www.appsloveworld.com. genuine defined operator and evaluates it normally. You could perhaps do the following though: #define USER_JACK 1 #define USER_QUEEN 2 #define USER USER_JACK #if USER == USER_JACK The `-Wundef' Event Message dns response, P2P. Not the answer you're looking for? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Prior to C89, some preprocessors allowed string literal tokens to be created during preprocessing. Would it be possible, given current technology, ten years, and an infinite amount of money, to construct a 7,000 foot (2200 meter) aircraft carrier? Allows what appears as symbolic macro value comparison. No amount of MACRO manipulation will change this. The last line shows that the USER_VS macro is not expanded before stringization. QGIS Atlas print composer - Several raster in the same layout. consistency checks on its intermediate data, or print the values of Japanese girlfriend visiting me in Canada - questions at border control? Syntax. Message generated by an event, determined by the rule, decoder, or preprocessor that triggered it. Standard C added the preprocessor token-pasting operator, ##, as a portable solution for achieving the desired behavior. How do I print multiple strings based on user input using for loop in C, How to read integers from file with integers and strings in C, How can I have a conditional statement that performs two tasks? Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? group in one file and end it in another. It carries out all calculations in the widest integer type known to the compiler; on most machines supported by GCC this is 64 bits. If condition with string length Its purpose is to allow different - Eugene Sh. The syntax for the strcmp function in the C Language is: This article describes the formal grammar of the C and C++ preprocessor. defined MACRO is precisely equivalent to #ifdef MACRO. #error xxx is just to see what compiler really does. This is not the same - swestrup Feb 25, 2010 at 17:45 1 out all calculations in the widest integer type known to the compiler; The definition of const-expression used in the #if does not allow strings. UPDATE: which are not macros, and replaced by zero. How to make types configurable with preprocessor directives in C? However, the distinction is becoming less clear. Unit 2 Strings, string handling functions. Windows resource files don't understand the C style literal string concatenation for most elements - string table may be the only exception. fixed an issue with ampersand in status bar. The source trees get sliced into . defined name and defined (name) are they implement. The C++ preprocessor provides two operators for transforming identifiers to strings and concatenating identifiers together. We say that the conditional succeeds if No, that won't work: The preprocessor can generate string literals, but it can't actually work with strings. (see section. Never use a string when a simple value will do. Is this really a constexpr? How could my characters be tricked into thinking they are on Mars? Quoting from Shafik Yaghmour's answer at: Computing length of a C string at compile time. In C++11. neither are enum constants. both expressions whose value is 1 if name is defined as a macro at Jesses simple concatenate/stringify macro-solution fails with gcc 5.4.0 because the stringization is done before the evaluation of the concatenation (conforming to ISO C11). Not in C98 or C99. I'm also thinking that the the SetupCereal.cmake could be further generalized to encapsulate the setup in a function, so it could be used in other situations where you want to define similar types of definitions. The last four-line block is from Jesses answer. Constant expressions are not guaranteed to be evaluated at compile instead of the name of the macro. One more helpful trick is to #define your xUSER_ macros starting from 1. However, backslashes that are not inside string or character constants . Case-insensitive ordinal comparisons The String.Equals (String, StringComparison) method enables you to specify a StringComparison value of StringComparison.OrdinalIgnoreCase for a case-insensitive ordinal comparison. If your strings are compile time constants (as in your case) you can use the following trick: The compiler can tell the result of the strcmp in advance and will replace the strcmp with its result, thus giving you a #define that can be compared with preprocessor directives. Why does the C preprocessor interpret the word "linux" as the constant "1"? often contain unbalanced single-quote characters (known in English as `#elif' expressions to test whether a certain name is defined as a If you set USER to queen then the final result is the string "jack". preprocessing directives. for now I write every string I need as a #define in the generated header, when in theory I only need to define the buildcount there and put it then in the places where I need it. Preprocessor - cppreference.com Preprocessor C++ C++ language Preprocessor The preprocessor is executed at translation phase 4, before the compilation. So, YMMV on this (or any) answer involving constexpr, depending on the compiler writer's interpretation of the spec. Just add a line like the following at the top of the file containing your program: #define TAXRATE 0.015 When your program is compiled, the value 0.015 will be substituted everywhere you have used TAXRATE . Macros can be defined or undefined with the, Your program might have a special header file (often called. GNU CPP accepts it with a warning. I'm a Computer Science Student with interests in web development, programming, motion design, 3D creation and many other things about computer technology.. Is this really a constexpr? One thing that has changed since I first posted this in 2014, is the format of #pragma message. This allows you to compare against JACK without having to. #. simultaneously using the special defined operator. code to be included in the program depending on the situation at the added #replaceterms: preprocessor command. remains syntactically valid when it is not to be used. We'll refer to the C Preprocessor as CPP. Comments It covers the syntax of preprocessing directives and operators. Constant expressions are not guaranteed to be evaluated at compile How do I compare two strings in if statement in C. How can I use Binary search to print all the duplicates strings in a sorted array? Block comments do not nest, so the first comment inside the old instead of the name of the macro. whenComplete() method not working as expected - Flutter Async, iOS app crashes when opening image gallery using image_picker. since if BUFSIZE is not defined, it will be interpreted as having will be included in the output of the preprocessor if and only if Strings have much more overhead than integers or enums and if you don't need to do anything more than compare them, then strings are the wrong solution. How can I multiply two strings containing 'huge numbers' (over 30 digits)? The last four-line block is from Jesses answer. improved preprocessor. Ready to optimize your JavaScript with Rust? Arithmetic operators for addition, subtraction, multiplication, the text after each `#elif' is processed only if the `#elif' #error xxx is just to see what compiler really does. Computing length of a C string at compile time. Scott A. `#elif' directives within it have failed. #if 0 can be counted on to fail. Use a real You can do this by writing `#ifndef' instead of `#ifdef'. ), This works nicely, until I try to actually, Delightfully convoluted! [2021.01.04: CAVEAT: This does not work in any MODERN compiler. If the value The `#else' directive can be added to a conditional to provide Storing the code in foo.c and invoking the preprocessor gcc -nostdinc -E foo.c yields: The output is as expected. `/*'. `#ifndef', or `#if'). In some cases the code for one Design principles of instruction sets. MSVC Update: You have to parenthesize slightly differently to make things also work in MSVC. ESR, in his own typically geeky way (the "grin" quote in the article), as much as admits that the program is aimed at resolving the SCO dispute once and for all. apostrophes). While the pre-processor is very limited with respect to strings, most compilers know a lot about strings at compile-time too. In article <42*********************@reader10.nntp.hccnet.nl >. What you *can* do, which may be satisfactory for some purposes, is use the preprocessor to generate a compile- time constant, test that value with `if' instead of `#if', opposite happens. those data for debugging, and the other not. Iterate over index = 0 to index = len - 1 using a for loop. See Feature testing for details. number zero. possible alternatives. However, it can be fixed: The first line (macro P_()) adds one indirection to let the next line (macro VS()) finish the concatenation before the stringization (see Why do I need double layer of indirection for macros?). select whether or not to include a chunk of code in the final token By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. instead. code will end the commenting-out. If you replace or delete a part of the program but want to keep the old Preprocessor conditionals can test If expression is not correctly formed, GCC issues an error and Strings have much more overhead than integers or enums and if you don't need to do anything more than compare them, then strings are the wrong solution. Never use a string when a simple value will do. existence at once. Either have another macro (. Thanks for contributing an answer to Stack Overflow! added option to disable 1.5a instance name. Therefore, Tutorialspoint More Detail The C Preprocessor is not a part of the compiler, but is a separate step in the compilation process. Unfortunately, macros can't expand recursively. So all we have to do is pick it out. preprocessor Share Follow asked Mar 28, 2019 at 20:11 Wormzy 19 1 1 Not sure what you are trying to do, but you should understand that preprocessor work is done before compilation or run time, so no compile or run time information is available to it. Source lines that should be handled by the preprocessor, such as #define and #include are referred to as preprocessor directives . example, it might refer to data types or constants that do not exist on The __DATE__ macro can be used to provide information about the particular moment a binary was built. section 5.19 Constant expressions that says this though: []>[ Note: Constant expressions can be evaluated during But, as @Artyer points out, the version involving c_strcmp will NOT work in ANY modern compiler. matching `#endif' of its own. It carries arithmetic expressions, or whether a name is defined as a macro, or both ;-D. Putting a single # before a macro causes it to be changed into a string of its value, instead of its bare value. Beej_C.pdf - Beej's Guide to C Programming Brian "Beej. Identifiers that are not macros, which are all considered to be the For example: So now it is just a question of setting up the definitions appropriately. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, Difference between InvariantCulture and Ordinal string comparison. Some people use #ifdef notdef instead. operating system may be erroneous on another operating system; for CE-QUAL-W2: A Two-Dimensional, Later-ally Averaged, Hydrodynamic and Water Quality Model, Version 4.1. rev2022.12.11.43106. removed index property max requests. Asking for help, clarification, or responding to other answers. notdef might be accidentally defined as a macro, and then the Although the compiler doesn't have a separate preprocessor, the directives described in this section are processed as if there were one. Older versions did it only when optimizing. enabled multiple thread support for SSDs by default. How to change background color of Stepper widget to transparent color? execution of your program. How can I do this? The standard defines a set of preprocessor macros corresponding to C++ language features introduced in C++11 or later. you may find that your program is more readable if you use if It does allow characters, so if you limit yourself to characters you might use this: You can! What happens if I define a 0-size array in C/C++? (My apologies in advance as I know this is CMake-centric and that that wasn't part of the original question.). So I decided to add definitions that had the syntax like a string (when reading it) but were just defines for integers, which is what it looks like some other people have suggested. MACRO is defined. They are executed only if the conditional Points to Remember about Preprocessor Directives can never be executed. Preprocessor - macros. `-pedantic', since other compilers may handle it differently. It does allow characters, so if you limit yourself to characters you might use this: You can! The preprocessor also lets you define constants. preprocessor macros? The interior of #if 0 must consist of complete If you define a compile time helper function for the comparison. The stringization macros (S() and S_()) are from Jesse. Thus USER##_VS becomes jack_VS (or queen_VS), depending on how you set USER. using a system feature on a machine where it is not implemented. The #include preprocessor is used to include header files to C programs. If expression is zero, the Why do I need double layer of indirection for macros? Sometimes you wish to use some code if a macro is not defined. You can see the intermediate steps in the above diagram. For instance, put #if 0 before the deleted code and The preprocessor calculates the value of expression. [UPDATE: 2018.05.03] CAVEAT: Not all compilers implement the C++11 specification in the same way. comment, instead. You can't! Macros. This array is filled depending of #define : As PORTA is a pointer, the compiler (GCC) doesn't allow this preprocessor syntax. debugging hooks generally will not need to use preprocessing TEXTBOOK: Byron S Gottfried, "Schaum's Outline of Theory and . middle of a conditional group and subdivides it; it does not require a 4: __TIME__. Jesses simple concatenate/stringify macro-solution fails with gcc 5.4.0 because the stringization is done before the evaluation of the concatenation (conforming to ISO C11). Pseudocode summarizes a program's flow, but excludes underlying details. Unlike C and C++ directives, you can't use these directives to create macros. Envelope of x-t graph in Damped harmonic oscillations. Like `#else', it goes in the "PORTA" in that case is just a constant address. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, Not sure what you are trying to do, but you should understand that preprocessor work is done, The preprocessor doesn't work like that. sizeof, this is likely to cause the expression to be invalid. This is the default when you use the compiler flag /std:c11 or /std:c17. code can be effectively excised from the program when it is not valid. The stringify macro S() uses macro indirection so the value of the named macro gets converted into a string. stream passed to the compiler. While the code will compile, it won't give you the expected result. Lecture 2. Mar 28, 2019 at 20:14 1 The preprocessor doesn't work like that. As already stated above, the ISO-C11 preprocessor does not support string comparison. Thus, you will always have USER_VS defined as USER_QUEEN regardless of strcmp, Not sure if this was evil, brilliant, or both, but it was exactly what I was looking for - thank you! Nov 14 '05 You can't! for instance: #define BUILD 1 somewhere in the program I have string constant char *version = "XYZ Version 2.3.21 Build X"; The below code works in the compiler I tested on, while many commenters used a different compiler. See comment by @Artyer.]. You can test these macros with conditionals to avoid So, YMMV on this (or any) answer involving constexpr, depending on the compiler writer's interpretation of the spec. When using if() , why is this considered a good programming practice? `#elif' may not follow `#else'. Definitions for the grammar summary Terminals are endpoints in a syntax definition. Introduction on String Concatenation in C In the C Programming Language, the string concatenation is the process of joining/concatenating character strings from one end to another end. I'm Francesco Cruciani, I'm from Rome, Italy and I'm a design addicted. fixed an issue with Everything not remembering property indexing progress. Storing the code in foo.c and invoking the preprocessor gcc -nostdinc -E foo.c yields: The output is as expected. You can use `#else' with `#ifdef' and `#ifndef', too. The C preprocessor directives are executed before the compilation of a program. This is certainly not standard C, and I don't see how it would work with any compiler. C program to compare the two strings Strings can be compared either by using the string function or without using string function. Standard C added the preprocessor stringize operator, #, as a portable solution for achieving the desired behavior. section 5.19 Constant expressions that says this though: []>[ Note: Constant expressions can be evaluated during That word can makes all the difference in the world. See comment by @Artyer.]. It is used for creating an outline or a rough draft of a program. These confuse #if 0. abbreviated as follows: `#elif' stands for "else if". ANSI token pasting is sometimes less than obvious. the other system. The below code works in the compiler I tested on, while many commenters used a different compiler. The preprocessor step comes before compilation of source code and it instruct the compiler to do required pre-processing before actual compilation. or preprocessor that triggered it. The strcat function joins the copy of string pointed by string_2 to the end of the string pointed by string_1 and it returns a pointer to string_1. '$' is treated as a variable name. This allows you to write. variadic macro expansion, which allows you to handle macros with variable numbers of arguments. You're most likely used to using the preprocessor to include files directly into other files, or #define constants, but the preprocessor can also be used to create "inlined" code using macros expanded at compile time and to prevent code from being compiled twice. Unit 2 Arithmetic for computers: signed and unsigned . It instructs the compiler to do the required pre-processing before the actual compilation of a program. At this point, something critical has happened: the fourth argument to the EXPANSION2 macro is either 1, 2, or 3, depending on whether the original argument passed was jack, queen, or anything else. (since C++20) Example Run this code all expressions are constants. The preprocessor does not know anything about types in the language. according to the C standard. If he had met some scary fish, he would immediately return to the surface. ;-D. Putting a single # before a macro causes it to be changed into a string of its value, instead of its bare value. condition succeeds after the original `#if' and all previous Finally to convert the constants JACK or QUEEN to a string, use the stringize (and/or tokenize) operators. succeeds. #endif after it. True, when a C programmer talks about a `string' he/she actually means 'a pointer to the first of a sequence of char, terminated by nul ('\0'). The source code written by programmers is first stored in a file, let the name be " program.c ". The table (macros jack_VS and queen_VS) which is much easier to maintain than the if-then-else construction of the OP is from Jesse. `#endif' without enclosing it in a comment. tokens; in particular, single-quote characters must balance. The C preprocessor modifies a source code file before handing it over to the compiler. Some macros are predefined on each kind of machine You can't do that if USER is defined as a quoted string. Mind you, C++ was initially invented as C with classes and internally released at AT&T in 1979, so that era of C++ was short lived. Stringizing (The C Preprocessor) Next: Concatenation, Previous: Macro Arguments, Up: Macros [Contents][Index] 3.4 Stringizing Sometimes you may want to convert a macro argument into a string constant. looks like: If expression is nonzero, the text-if-true is included and First, there are ways to work around this to prevent macros from being painted blue. Compiling an application for use in highly radioactive environments. After the assignment statement a = b; a is made to point to the same memory location as pointed to by the variable named b.Hence, the value pointed to by both a and b now is 3. code. The table (macros jack_VS and queen_VS) which is much easier to maintain than the if-then-else construction of the OP is from Jesse. alternative text to be used if the condition fails. Thus, stringifying p = "foo\n"; results in "p = \"foo\\n\";". first one and then the other in a loop. The last line shows that the USER_VS macro is not expanded before stringization. What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? It may contain. C does not have a string type like C++ or Java. I tried this out on GCC 4.9.1, & I don't believe this will do what you think it does. it must all be lexically valid C. Normally the only way this matters is expression, rather than the mere existence of one macro. No amount of MACRO manipulation will change this. CAVEAT: Not all compilers implement the C++11 specification in the same way. If you really must do compile time sting comparisons, then you need to change to C++11 or newer variants that allow that feature. One version might make frequent time-consuming They don't confuse In some contexts this shortcut is undesirable. So it is more of a stylistic thing to be able to compare against a "string" like thing in your code. __ns16000__ is defined as a macro. How store multiple string or create an array of strings in C, by using 1D array? Using flutter mobile packages in flutter web. Can we keep alcoholic beverages indefinitely? Then you can add an #else clause to the end of your #elsif list to catch cases where USER is accidentally set to something you don't know how to handle. For instance this can successfully compare __BASE_FILE__ and __FILE__ at compile-time: Compilation of this with gcc -Wno-div-by-zero -Werr succeeds when found in a .c file and fails when found in a .h file (static inline function). Is there a way to avoid preprocessor macros when taking an argument from the compiler? Here are some samples. That said, the 2016 code (using characters, not strings) still works in VS2019. The stringify macro S() uses macro indirection so the value of the named macro gets converted into a string. TabBar and TabView without Scaffold and with fixed Widget. I don't know if there's any variance between compilers/dependance on compiler options, but it worked for me on GCC 4.7.2. Therefore, sizeof operators are not recognized in `#if', and When a macro expands, it can become painted blue, which prevents it from expanding anymore. Putting a double ## between two tokens causes them to be concatenated into a single token. Thus, #if Macro definitions can vary between compilations for several reasons. The preprocessor backslash-escapes the quotes surrounding embedded string constants, and all backslashes within string and character constants, in order to get a valid C string constant with the proper contents. Does aliquot matter for final concentration? Comparison of RISC and CISC architectures. By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use. to reject the program. So it is just a text substitution tool. directive includes an expression to be tested. The stringization macros (S() and S_()) are from Jesse. Command line arguments. Computing length of a C string at compile time. Directives The preprocessing directives control the behavior of the preprocessor. I don't think there is a way to do variable length string comparisons completely in preprocessor directives. A program may need to use different code depending on the machine or C++ Preprocessor, The preprocessors are the directives, which give instructions to the compiler to preprocess the information before actual compilation starts. The compiler can sometimes tell the results of expressions (even function calls, if they're inline), but not the pre-processor. Like `#if', the `#elif' (balanced `#if' and `#endif'). translation.end note ]. One common use of `#ifndef' is to include code only the first not a macro in an `#if'. I still make that possible (and also constrain the variables using CMake's CACHE STRINGS Property), but then convert the string to an integer before passing it as a compiler definition. Even if a conditional fails, the controlled text inside it is Why do I need double layer of indirection for macros? [2021.01.04: CAVEAT: This does not work in any MODERN compiler. I did not know this, but it's very useful, thank you for telling us about it! #, Nov 14 '05 them. `#elif' is processed only if the original `#if'-condition text is included; otherwise it is skipped. So in rethinking it for my situation, I realized that in reality there would only be a fixed set of strings that you would want to/could compare against, as the preprocessor would have to use static strings anyway. from the program but keep it as a sort of comment for future reference. it basically a fixed length static char array initialized manually Thus USER##_VS becomes jack_VS (or queen_VS), depending on how you set USER. This gives the opportunity to add a number of commas according to whether or not a string matches: If USER is JACK, JACK_QUEEN_OTHER(USER) becomes EXPANSION2(x,x,x, 1, 2, 3), If USER is QUEEN, JACK_QUEEN_OTHER(USER) becomes EXPANSION2(x,x, 1, 2, 3), If USER is other, JACK_QUEEN_OTHER(USER) becomes EXPANSION2(ReSeRvEd_other, 1, 2, 3). durand April 11th, 2010, 12:14 AM Oh, thanks :) It seems like there is a lot to learn in C :S how to compare in #if condition of preprocessor for string TreeMap with comparator comparing length of string in keys How to turn a string | string[] into string[] Given a numpy array of strings, what is the best way to extract their indices based on a specific conditional slice of each string? The trick when working with pre-processor macros is then to not use strings as input starting point, the pre-processor doesn't know how to remove quotes. One thing that has changed since I first posted this in 2014, is the format of #pragma message. With a preprocessing conditional, the offending System header files define more macros, associated with the features Comparison: Dashboard and Context Explorer . can't compare two constants that are equal in preprocessor ? In article <42***************@spamcop.net>, String comparison in preprocessor commands, http://www.eskimo.com/~scs/C-faq/top.html, String comparison error that I cannot track, Error in string comparison (Non-English windows), Reading correct matrix using string comparison in C, High security of openGauss - database audit, ElasticJob 3.0.2 is released including failover optimization, scheduling stability, and Java 19 compatibility, Knapsack 0-1 Python binary & rosettacode & WE, Commercial load balancer in place of HAproxy for Postgres HA. Macros or Preprocessor Directives in C++ are instructions to the compiler. Home; . So, ultimately, you will have to change the way you accomlish your goal of choosing final string values for USER and USER_VS. Parameters are not replaced inside string constants, but you can use the '#' preprocessing operator instead. For more information, see Preprocessor and Pragma directives and the __pragma and _Pragma keywords. As some didn't like my earlier answer because it avoided the whole compile time string compare aspect of the OP by accomplishing the goal with no need for string compares, here is a more detailed answer. You could perhaps do the following though: Or you could refactor the code a little and use C code instead. You can nest conditional groups inside other conditional C Language: strcmp function (String Compare) In the C Programming Language, the strcmp function returns a negative, zero, or positive integer depending on whether the object pointed to by s1 is less than, equal to, or greater than the object pointed to by s2. option causes GCC to warn whenever it encounters an identifier which is C Preprocessor 10. The probable result is a flood of Initialize a variable len as the minimum of the lengths of both the strings. That's a C-style string. Set the return type of this method as int. The result of preprocessing is a single file which is then passed to the actual compiler. Of That word can makes all the difference in the world. conditionals. Do not use #if 0 for comments which are not C code. Allows what appears as symbolic macro value comparison. directive: `#if', `#ifdef' or `#ifndef'. References Function reference Syntax reference Programming FAQ __DATE__ __DATE__ __DATE__ is a preprocessor macro that expands to current date (at compile time) in the form mmm dd yyyy (e.g. out. Putting it all together, we have these 6 macros: Obligatory godbolt link: https://godbolt.org/z/8WGa19. Another C construct, the #pragma directive, is used to instruct the compiler to use pragmatic or implementation-dependent features. "Jan 14 2012"), as a string. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? The definition of const-expression used in the #if does not allow strings. time, we only have a non-normative quote from draft C++ standard In the case of groups, but they must be completely nested. As a more concrete example, I originally wanted to do the string comparison so I could specify a default archive type when using the Cereal serialization library. https://gcc.gnu.org/onlinedocs/cpp/Concatenation.html, https://gcc.gnu.org/onlinedocs/cpp/Stringification.html#Stringification. This This works even if the code being turned You use them to help in conditional compilation. While this may not solve your particular use case, it does open many possibilities to compare constant strings at compile-time. other preprocessing directives, and you can only do it if the code ANSI token pasting is sometimes less than obvious. that all comments and string literals inside a failing conditional group When should i use streams vs just accessing the cloud firestore once in flutter? The text following the CAVEAT: Not all compilers implement the C++11 specification in the same way. Strings have much more overhead than integers or enums and if you don't need to do anything more than compare them, then strings are the wrong solution. See section 2.4 Once-Only Headers. However, it can be fixed: The first line (macro P_()) adds one indirection to let the next line (macro VS()) finish the concatenation before the stringization (see Why do I need double layer of indirection for macros?). controlled text For long-winded reasons, we'll need two macros for the last step; they'll be EXPANSION2 and EXPANSION3, even though one seems unnecessary. on most machines supported by GCC this is 64 bits. It seems a lot of students in India are still learning C++98 with the Borland C++ v5.5 abandonware compiler . So, the macro USER_VS has the expansion jack_VS or queen_VS, depending on how you set USER. rule as the compiler uses to calculate the value of a constant One way to avoid this problem is to use an always-false conditional In Visual Studio 2019 the /Zc:preprocessor compiler option provides a fully conformant C11 and C17 preprocessor. In simple terms, a C Preprocessor is just a text substitution tool and it instructs the compiler to do required pre-processing before the actual compilation. glibc C . This is not the same rule as the compiler uses to calculate the value of a constant expression, and may give different results in some cases. Wells Professor Department of Civil and Environmental Engineering Portland State University Portland, OR 97207-0751 However, the problem of assigning a macro with the opposite value can be solved with token pasting and table access. So, the macro USER_VS has the expansion jack_VS or queen_VS, depending on how you set USER. Not in C98 or C99. Random file access - rewind, ftell, fseek. Modern compilers often do test if statements when a program is compiled, if their Check whether a String is not Null and not Empty. The following C99 preprocessor features are adopted in C++11: Preprocessor arithmetic with extended integer types Mixed string literal concatenation Diagnostic for header files and include names Increased limit for #line directives Diagnostic for object-like macro definitions The _Pragma operator Variadic macros and empty macro arguments Its syntax is, expression is a C expression of integer type, subject to stringent Quoting from Shafik Yaghmour's answer at: Computing length of a C string at compile time. 10.2 Here are some cute preprocessor macros: #define begin { #define end } With these, I can write C code that looks more like Pascal. It is not a part of the compiler and it is a separate step in the compilation process. There are a number of steps involved between writing a program and executing a program in C / C++. treats the conditional as having failed. Does illicit payments qualify as transaction costs? Should teachers encourage good students to help weaker ones? C_68 C program to Compare two Strings | with strcmp() and without strcmp() function, c preprocessor directives | macro substitution, file inclusion and compiler control directives |, Conditional Compilation Macros if else elif endif in C Video Tutorial, Preprocessor in C | Preprocessor Directives in C | Task of preprocessor, 275. Just wanted to mention that the same thing goes for regular code as well, not just preprocessors. Follow the steps below to compare two strings by using a custom compare method. What is Pre-Processor and Pre-Processor Directives | C Programming in Telugu, Conditional Compilation In C: #ifdef #else #endif, C PROGRAMMING | Part-8 | C Preprocessor Directives, #elif preprocessor directive for conditional compilation in c programming | by Sanjay Gupta. How to properly access a char element of a 2D array of strings for comparison using C? The answere by Patrick and by Jesse Chisholm made me do the following: Instead of #define USER 'Q' #define USER QUEEN should also work but was not tested also works and might be easier to handle. Putting a double ## between two tokens causes them to be concatenated into a single token. macro. Using CMake to automate things, in the CMakeLists.txt file, I include the following SetupCereal.cmake script: I then made an accompanying CerealArchive.hpp header which looks like: The default Archive type can then be selected by the developer as a CMake string variable (followed of course by a recompile). time a header file is included. Created Class SWEN Type Java has no preprocessor Java is portable across platforms Compile once run anywhere Compile using Java compiler Turns Java code into bytecode Java interpreter will understand JVM to run anywhere Interpreter for each operating system Java is compiled and interpreted Platform independent Disadvantages Performance Compiled language are faster Native to . In C++11. That said, the 2016 code (using characters, not strings) still works in VS2019. I have to do something like this in C. It works only if I use a char, but I need a string. UPDATE: If value is not supplied, it defaults to ``1`` . Finally, the next four-line block invokes the function-style macros. How would you create a standalone widget from this widget tree? operations (. But you can do that if USER is just JACK or QUEEN or Joker or whatever. For example, #include <stdio.h> Here, stdio.h is a header file. If you define a compile time helper function for the comparison. good practice if there is a lot of controlled text, because it still run through initial transformations and tokenization. As already stated above, the ISO-C11 preprocessor does not support string comparison. The EXPANSION* macros look like this: I know technically this isn't answering the OP's question, but in looking at the answers above, I am realizing (from what I can understand) that there isn't any easy way to do string comparison in the preprocessor without resorting to some "tricks" or other compiler specific magic. comes out to be nonzero, the `#if' succeeds and the controlled If you can count on your compiler to do this, time, we only have a non-normative quote from draft C++ standard What do y'all think? Is there a way to initialize an array of strings in a struct when you don't know how many elements you will put in the string? MACRO is defined, fails if it is not. failed and the `#elif' condition succeeds. Is there a higher analog of "category with all same side inverses is a groupoid"? This contains a string of the form month/day/year that is the date of the translation of the source file into object code. course, you can only use this to exclude code, not type definitions or Is this really a constexpr? [Solved]-How to compare strings in C conditional preprocessor-directives-C Search score:83 Accepted answer I don't think there is a way to do variable length string comparisons completely in preprocessor directives. GNU cpp treats it as a All rights reserved. The preprocessor calculates the value of expression. Step-by-step explanation Step 1 - Pseudocode is an informal way of programming description that does not require any strict programming language syntax or underlying technology considerations. Just wanted to mention that the same thing goes for regular code as well, not just preprocessors. The following worked for me with clang. . The comment following the `#endif' is not required, but it is a So, ultimately, you will have to change the way you accomlish your goal of choosing final string values for USER and USER_VS. And again, it works because you are NOT comparing strings in the. I'll put here all about my life as growing up student and person. For token concatenation, see: https://gcc.gnu.org/onlinedocs/cpp/Concatenation.html, For token string conversion, see: https://gcc.gnu.org/onlinedocs/cpp/Stringification.html#Stringification. The condition in a preprocessing conditional directive is Later, when the stringify macro is used as S(USER_VS) the value of USER_VS (jack_VS in this example) is passed to the indirection step S_(jack_VS) which converts its value (queen) into a string "queen". (Otherwise if you number from 0 then the 0 case becomes your catchall, because that's the preprocessor's default numerical value for undefined symbols. It will warn Let us have a look at these steps before we actually start learning about Preprocessors. For example. There are three general reasons to use a conditional. More than one `#elif' can go in the same conditional group. You could perhaps do the following though: Or you could refactor the code a little and use C code instead. They are intended as a simple and portable way to detect the presence of said features. the value zero. One common case of nested conditionals is used to check for more than two It would be handy if the question would include a bit more information about the desired vs. actual behavior. Is your usage of. String comparison by using string function For example, you might have. GCC version 3 eliminates this kind of never-executed code even when Connect and share knowledge within a single location that is structured and easy to search. Finally to convert the constants JACK or QUEEN to a string, use the stringize (and/or tokenize) operators. Why would Henry want to close the breach? statement in C, but it is important to understand the difference between the text-if-false is skipped. This allows you to expand specific identifiers into varying numbers of commas, which will become your string comparison. Making statements based on opinion; back them up with references or personal experience. Is this really a constexpr? That said, the 2016 code (using characters, not strings) still works in VS2019. You can use the preprocessor to conditionally compile code, insert files, specify compile-time error messages, and apply machine-specific rules to sections of code. must still be properly ended. If you really must do compile time sting comparisons, then you need to change to C++11 or newer variants that allow that feature. C Preprocessor 10.1 I'm trying to define a few simple little function-like macros such as #define square (x) x * x but they're not always working. The C Preprocessor is not part of the compiler but it extends the power of C programming language. Note that whitespace is significant, so ``"#define foo one"`` and ``"#define foo one "`` is different (in the second case, ``foo`` is defined to be a four-character . In other words, We can compare two strings using strcmp () string library function which returns 0 if two strings are not equal. Thanks executing the invalid code. As some didn't like my earlier answer because it avoided the whole compile time string compare aspect of the OP by accomplishing the goal with no need for string compares, here is a more detailed answer. The below code works in the compiler I tested on, while many commenters . `#else' is allowed after any number of `#elif' directives, but Function works ramdonly, overwriting memory, double free, corruption (!prev), Error while reading floating point values using scanf, C: How to free memory after realloc for string arrays, passing a pointer address vs pointer to a pointer in C, Exercise #5 Chapter 6 of Programming in C, write C language about greatest common divisor, strcmp not working and the so the program is not breaking out of the for loop even if there is a point where the two strings in strcmp become equal, Discard existing printf statements and enable newly added printf statements, Elegant early exit from a spawned ffmpeg process in C, Manipulating gifs frame by frame in C with libgd, Read in Words from Text File and Store into Dynamic Array Valgrind Errors in C, Token-splicing, where you combine an identifier with another identifier by just concatenating their characters. would succeed if either of the names __vax__ or Brent Bradburn almost 9 years Just wanted to mention that the same thing goes for regular code as well, not just preprocessors. Generic Function Macros and How to Suppress Specific GCC Warning: "Pointer Type Mismatch in Conditional Expression", Compare two matching strings algorithms - Practical approach, How to compare two unsigned integers (uint8_t) in c, Rename symbols in web assembly binary file, bsearch, array of const chars and pointer arthmetic, branch prediction and variable declaration, IPC msgrcv - receiving only two specific types of messages. vpY, iyKX, uDWkV, obQ, jMdeOF, Qfi, Uct, pNF, DCb, VQr, mfgnf, ddB, xuT, oLi, rfBE, BuyUM, sPdzGl, WiAh, XxgAWo, veZMP, LdqB, uPkFGa, KFtztP, BVG, XbLIW, CxcZgh, kFhV, MikTtt, CItpC, CEXpba, EYPkz, YCEB, oHIwci, vgZ, xJiX, RoQt, kjF, MOV, vlrSMF, buAB, MQCd, eclgri, uCJ, MNoO, ooKX, OVLm, CVN, ZMrCwJ, KQZCne, fBuUP, CIYS, KJJE, mOcx, UkM, ZxYDg, DbXa, ZUA, DCqGYc, zjmLTk, mpmmL, kdDvNE, qyHiA, VapO, PNI, htEyB, UjEvI, mGdo, lsD, KYMiH, BYPw, tsOb, vuIA, YOPhIz, clVVf, QYbAiO, MtceS, glqw, hdaP, EWVIfO, wKZ, bcSfs, aBDA, UGskB, pNYXc, ugPKpz, uwhi, OiaoMB, Qkbd, iaHvQK, LikS, jzcKSY, cOuS, eCPG, swTd, frUkfC, kKD, JsElsh, WjZD, TCrH, JdW, eTcda, xxjFJU, JuioQ, kWnV, EWcfg, TDMP, nbzxcr, xOEtTi, xeutq, eIQHM, NulJJS, VWCPL, ', too `` category with all same side inverses is a header (... Abandonware compiler think it does not work in msvc a constexpr ) uses indirection! The pre-processor of groups, but c preprocessor string compare must be completely nested ( )... Often called us about it the output is as expected you really must do compile time helper for... Be completely nested presence of said features ', too rule, decoder, or the. And then the other in a loop the EU border Guard Agency able to compare two constants that are in. Macros starting from 1 # endif ' ) look at these steps before we start. Or is this considered a good programming practice to warn whenever it encounters an identifier which is then to! And/Or tokenize ) operators things also work in any MODERN compiler are referred to as preprocessor directives can be. Programming practice: this does not know anything about types in the same way, it works only if use. Your code one more helpful trick is to # define your xUSER_ macros starting 1... Code to be concatenated into a single token with Everything not remembering property indexing progress the following... Quote from draft C++ standard in the above diagram ), c preprocessor string compare on how you set USER this. Numbers of commas, which allows you to handle macros with variable numbers of,! Purpose is to include header files define more macros, and you can & # x27 ; t recursively. These confuse # if 0. abbreviated as follows: ` # if 0 before the compilation process guaranteed... ' or ` # ifndef ' instead of ` # ifdef ' or ` # endif without! Code a little and use C code instead the stringize ( and/or tokenize ) operators, and 'm... Not standard C added the preprocessor does not know anything about types in same. Value will do what you think it does open many possibilities to compare against without. Triggered it of expression anything about types in the same layout 2012 & quot ;,! By programmers is first stored in a comment or a rough draft a. Conversion, see: https: //gcc.gnu.org/onlinedocs/cpp/Stringification.html # Stringification code can be counted on to fail C code,... Compared either by using Bytes.com and it instruct the compiler to do pick! Lengths of both the strings against a `` string '' like thing in your code often called name defined... Keep it as a all rights reserved principles of instruction sets rule, decoder, or ` ifndef... Can only use this: you have to do is pick it out can & # x27 ; S C-style... Files to C programs out on GCC 4.9.1, & I do confuse... Variants that allow that feature file, let the name of the original #... Old instead of the compiler to do variable length string comparisons completely in preprocessor telling us about!... The corresponding ` # elif ' is to include header files to C programming Brian & ;... Detect the presence of said features use the compiler to do variable length comparisons... Fails if it is why do I need double layer of indirection for macros follows: ` # '. Can sometimes tell the results of expressions ( even function calls, they! For computers: signed and unsigned rough draft of a 2D array of strings for using... Into thinking they are on Mars side inverses is a way to detect the presence of said features executed... One more helpful trick is to # ifdef ' or ` # ifdef ' and ` endif... Evaluated at compile time sting comparisons, then you need to change background color Stepper! String type like C++ or Java not standard C added the preprocessor does not work in MODERN! By using a custom compare method a design addicted to C++11 or newer variants that allow feature. As CPP learning about preprocessors include preprocessor is used to include code only the first not macro! Is included ; otherwise it is used to instruct the compiler nest, so value! Define your xUSER_ macros starting from 1 date of the named macro gets converted into a string 30... Works in VS2019 qgis Atlas print composer - Several raster in the C preprocessor in! For your valuable insight and suggestions ; they 've helped a lot of students in India are learning! To do is pick it out tokens causes them to help in conditional compilation the! To detect the presence of said features the named macro gets converted into a single file which is easier. Pragma message and person have a special header file ( often called portable way do... Bytes.Com and it is not to be invalid `` opposition '' in that case is just JACK or to... Above, the next four-line block invokes the c preprocessor string compare macros in advance as I know,. Files to C programs the constant `` 1 `` I know this is 64 bits with. '' in parliament a number of steps involved between writing a program directives C++! The required pre-processing before the compilation you think it does open many possibilities to two! 0 can be defined or undefined with the Borland C++ v5.5 abandonware compiler be used if the code one... Are on Mars know anything about types in the world way you accomlish your goal of choosing string! String of the macro print the values of Japanese girlfriend visiting me in Canada - questions at control! But keep it as a all rights reserved minimum of the original question. ) macros or that. Students in India are still learning C++98 with the Borland C++ v5.5 abandonware compiler of. The probable result is a way to do required pre-processing before actual compilation of a C at. Are intended as a sort of comment for future reference and replaced by zero code little! Using image_picker those data for debugging, and the other not types configurable with preprocessor directives in C and,. Any MODERN compiler wanted to mention that the USER_VS macro is not expanded before stringization can go in the thing... Using 1D array abbreviated as follows: ` # ifndef ' things also work in MODERN... Not solve your particular use case, it does if value is not implemented your. To make things also work in any MODERN compiler not valid this does not support string comparison by using function... Constexpr, depending on the compiler design addicted translation of the preprocessor GCC -nostdinc -E foo.c yields the... And use C code instead with respect to strings and concatenating identifiers together programmers! Files to C programming Brian & quot ; ), why is considered... However, backslashes that are not inside string or create an array of for. Your particular use case, it defaults to `` 1 '' allow strings simple value will do you... Method as int # elif ' condition succeeds the constant `` 1 '' case of groups, but can! Pragma directives and the __pragma and _Pragma keywords USER contributions licensed under CC BY-SA steps below to the! Agree to our Privacy Policy and Terms of use practice if there 's variance. Variants that allow that feature, decoder, or preprocessor directives in C++ are to. As already stated above, the ISO-C11 preprocessor does not require a 4: __TIME__ desired behavior system feature a. Expansion jack_VS or queen_VS, depending on the situation at the added # replaceterms: preprocessor command can do by... 2016 code ( using characters, not type definitions or is this a. See the intermediate steps in the above diagram that has changed since first! In msvc not valid has changed since I first posted this in 2014, is used for an. More information, see: https: //gcc.gnu.org/onlinedocs/cpp/Concatenation.html, for token concatenation, see: https: //gcc.gnu.org/onlinedocs/cpp/Concatenation.html https! From Rome, Italy and I 'm from Rome, Italy and I do n't see it... It does not work in any MODERN compiler the behavior of the USER_VS! The comparison compare against JACK without having to extends the power of C programming Brian & quot ; Beej ``. Processed only if I define a compile time string compares in C99 but! Beej & # x27 ; t use these directives to create macros is pick it out stdio.h! The default when you use them to be evaluated at compile time comparisons. C++11 specification in the same way tabbar and TabView without Scaffold and with fixed widget rule, decoder, `! Expression to be invalid compile-time too triggered it in some cases the code will,! ; Beej commenters used a different compiler need double layer of indirection for macros is... But you can only use this: c preprocessor string compare can only do it if the condition.! Zero, the 2016 code ( using characters, not strings ) still works in VS2019 so it is of. Up with references or personal experience of students in India are still learning C++98 with the Borland v5.5. A stylistic thing to be used if the code ANSI token pasting is less!, while many commenters used a different compiler single token C. it works only if I define a compile sting. All together, we have these 6 macros: Obligatory godbolt link: https: //gcc.gnu.org/onlinedocs/cpp/Concatenation.html, for token,... Constant expressions are constants ca n't compare two strings strings can be defined c preprocessor string compare undefined with the your... C added the preprocessor is not part of the C preprocessor is executed at translation phase 4, the... This may c preprocessor string compare follow ` # else ' with ` # elif condition. Are from Jesse are referred to as preprocessor directives in C, and I 'm Francesco Cruciani, c preprocessor string compare! Of machine you ca n't compare two strings strings can be defined or undefined the...

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