Preprocessor macro variables. The variable port, doesn't get set until runtime.

Preprocessor macro variables. I'm going to edit your title to make it clearer what you're asking. #include "variables. > Can I read and write C-MACROs from these scripts? C-MACRO? if you mean, preprocessing C-like expressions with defines and includes - yes, you can run the C compiler and produce preprocessor output. Because preprocessor replacements happen before any C++ syntax check, macro definitions can be a tricky feature. For example, you might decide to create a macro that declares two variables, one with a name based on parameter to the macro and another with a different name, that stores the original value of the variable (perhaps for debugging). Clang command line argument reference ¶ Introduction Actions Compilation options Preprocessor options Include path management Dependency file generation Dumping preprocessor state Diagnostic options Target-independent compilation options Common Offloading options OpenCL options SYCL options CUDA options HIP options HLSL options Target-dependent compilation options AARCH64 AMDGPU ARM Hexagon Learn how to define a C++ macro in CMake in 3 simple steps. Aug 4, 2025 · In C, # and ## operators are preprocessor operators using in macros for token manipulation. Use of the preprocessor is advantageous since it makes Mar 23, 2025 · The preprocessor supports conditional compilation of parts of source file. This is an attempt to define a few variadic macros that can be used to count the number of arguments given. is it possible to write a generic macro that takes a structure May 2, 2025 · The PreProcessor and the Config Parser allow to use macros in configs. The answer to that question (which you didn't actually ask) is no; a macro definition cannot include further preprocessor directives. The preprocessor commands are directives to the C# compiler, specifying the sections of code to compile or identifying how to handle specific errors and warnings within the code. , an executable, a library). - Providers tab Include paths and preprocessor macros for C/C++ indexer and other Language Settings are supplied by Language Settings Providers as part of Scanner Discovery. Nov 2, 2010 · Is there anyway to use a " sizeof " in a pre-processor macro? No. Jan 10, 2016 · Substituting the value of i into the macro is impossible, since macro substitutions happen before your code is compiled. In particular if cond is a value known at compile time, feel free to just use an if. For example, the following fails to compile: Jul 27, 2020 · You are mixing up variable arguments for macros with variable argument access for functions. Syntax of Jan 2, 2024 · A set of C/C++ preprocessor macros that are useful is most any program. To add a project reference to a WiX project: Right-click on the References node of the project in the Solution Explorer and choose Add Reference…. Understanding Macros in C Before May 7, 2020 · I have configured my embedded project to use MSBuild and I wish to pass a value set in User Variables into my code using a pre-processor macro. 1) Macros -: #define Macros are a type of identifiers that are usually written in capital letters to distinguish them from normal variables. Language Settings Providers can be set up and configured for the selected configuration in the "Providers" tab. cpp Apr 18, 2025 · Preprocessor Directives are divided into four parts -: Macros File Inclusion Conditional Compilation Other directives Let’s now understand all these Preprocessor Directives one by one. Aug 2, 2021 · Preprocessor Provides an overview of the traditional and new conforming preprocessors. This CMake command allows you to define preprocessor macros (also known as compile-time definitions) specifically for a particular target (e. It sees: `define `set_value(bit) abc_``bit``_def = 4'b0000 [some verilog] `set_value(i); [some verilog] So 'i' is just i. Unlike normal parameter In the project file my_proj. Dec 19, 2023 · Defining a constant is not one of the things the preprocessor is useful for, unless that constant must be a macro because it's used in preprocessor conditions using #if. The preprocessor modifies the text of the code which is seen by the actual compiler based on its own collection of commands, which are known as preprocessor directives. The most notable omission is #include. MSVC supports the predefined preprocessor macros required by the ANSI/ISO C99, C11, and C17 standards, and the ISO C++14, C++17, and C++20 standards. Dec 6, 2021 · with that, defining CONFIG_SYSTEM_RELEASE_MODE CMake variable (idf. The token pasting (##) operator simply eliminates any white space around it and concatenates (joins together) the non-whitespace characters together. Conditional inclusions (#ifdef, #ifndef, #if, #endif, #else and #elif) These Standard Predefined Macros (The C Preprocessor)3. ) C is a static language and you can not decide symbol names at runtime. I am trying to figure out how to write a macro that will pass both a string literal representation of a variable name along with the variable itself into a function. When the preprocessor encounters the macro name in the code, it replaces it with the corresponding code fragment or value that is defined using the #define preprocessor. In other words, when the lexer encounters something like #include or a macro name, the preprocessor will be used to perform the inclusion or expand the macro name into its replacement list, and return the resulting non-preprocessor token. Specify options for commands. Sep 6, 2024 · Discover advanced preprocessor usage with C macros. In the Preprocessor Definitions dialog box, add, modify, or delete one or more definitions, one per line. These commands can add, remove or change code in a number of ways and together act to simplify the Mar 2, 2025 · gcc / g++ how to dump all preprocessor definitions Mar 2, 2025 • Written by Nadir SOUALEM C++ C gcc g++ Compilation option -E -dM gives the result. Feb 14, 2024 · Preprocessor directives All of the keywords beginning with # are preprocessor directives, much like with C, although #line is different. Macros with arguments take this a step further, enabling you to create versatile and customizable code blocks. When a macro parameter is used with a leading ‘ # ’, the preprocessor replaces it with the literal text of the actual argument, converted to a string constant. What I've discovered recently as I have been writing custom macros is that many, if not all, of the underlying code I invent is already written in the boost. You can use these macros anywhere in a project's Property Pages dialog box where strings are accepted. They act like predefined macros, but you cannot undefine them. Learn object-like, function-like, and conditional macros with practical examples. com To use proper C99 variadic macros, you should be compiling with a C compiler that supports C99 (like gcc), not a C++ compiler, since C++ doesn't have standard variadic macros. "Providers" tab is only shown in properties C Preprocessor Directives, Macros & Macro Functions Tutorial In this C tutorial we learn how to tell the compiler to apply transformations in our code with preprocessor directives and macros. Jan 13, 2023 · 🧩 Using C# Preprocessor Directives For Conditional Compiling Let’s discuss how we can have a dev-friendly environment-dependent compilation process in C#! Let’s say you’re working on a Jan 17, 2017 · I tried to understand the macros in c using the concatenation preprocessor operator ## but I realized that I have problem with tokens. Apr 30, 2012 · Possible Duplicate: Creating C macro with ## and LINE (token concatenation with positioning macro) I am trying to use the __LINE__ macro to generate different variable names. I'll explain how to get it working in Visual Studio 2019. In C++, there is a fourth category, the named operators. Conditional compilation directives allow you to suppress compilation of parts of a source file. add_compile_definitions ¶ Added in version 3. It handles essential tasks like macro expansion, conditional compilation, and text substitution. And passing args to another macro function doesn't help, because string literals can't be pasted together -- "s""1" is not a valid token. Concatenating Tokens The ## preprocessing operator is used to merge two tokens into one while expanding macros, which is called token pasting or token concatenation. I thought it was easy but in practice it is not. Other macro languages you may be familiar with are the C preprocessor, m4, TEX, and macro assemblers. Aug 4, 2025 · In C, the #define preprocessor directive is used to define the macros and symbolic constants. Apr 19, 2024 · On Linux, OS/X you have the native bash and more. Oct 28, 2023 · Macro s in C Programming Macros In C programming, a macro is a way to define a piece of code that can be reused throughout the program. 3. I need to create a demo version of an existing large application consisting of multiple projects. But what about the pr… In this article, I will discuss the Macro Substitution Directives in C. possible duplicate of How to concatenate twice with the C preprocessor and expand a macro as in "arg ## _ ## MACRO"? The same goes for any macro besides __LINE__ (although that is a common use case. Jul 30, 2009 · 3 Use the preprocessor token-pasting operator: ##TOKEN_NAME As previously noted, the preprocessor directives you are using are non-standard, so YMMV. Example: cmake -DDEFINE_MACRO=OFF . json? For instance, a list of preprocessor macros to then be passed to target_compile_definitions? Aug 21, 2014 · How can I pass a macro to the preprocessor? For example, if I want to compile some part of my code because a user wants to compile unit test, I would do this: #ifdef _COMPILE_UNIT_TESTS_ BLA B May 27, 2025 · Define Preprocessor Macros This CMake command allows you to define preprocessor macros (also known as symbols or constants) that will be used during the compilation process. You already have seen a #include directive in all the examples. The for loop is a verilog construct that is evaluated by the compiler. 137 Simple question for which I could not find answer on the net. To make sure the compiler is receiving the definition right, you C/C++ Project properties: Preprocessor Include Paths, Macros, etc. 3 Macro Arguments ¶ Function-like macros can take arguments, just like true functions. NASM contains a powerful macro processor, which supports conditional assembly, multi-level file inclusion, two forms of macro (single-line and multi-line), and a “context stack” mechanism for extra macro power. So the C preprocessor The C preprocessor (CPP) is a text file processor that is used with C, C++ and other programming tools. I'm currently using GCC as my compiler, but If at all possible I'd like the CMake configuration to be compiler agnostic. Preprocessor operators Discusses the four preprocessor-specific operators used in the context of the #define directive. long fooAlloc( Jan 15, 2024 · Is there a way to create list variables in CMakePresets. Jan 20, 2021 · C preprocessor macro multiple arguments Asked 4 years, 7 months ago Modified 4 years, 7 months ago Viewed 5k times All preprocessor directives begin with #, and only white-space characters may appear before a preprocessor directive on a line. Feb 18, 2025 · Preprocessor directives replace character strings, include or exclude parts of code or complete files, and influence the behavior of the compiler. Now, when we include the file for second time the C pre-processor again checks macro definition #ifndef _MY_FILE_. The C preprocessor is a macro processor that is used automatically by the C compiler to transform your program before actual compilation. This is called preprocessing. Note that if you have a variable with the same name in the scope from which the macro is called, using unreferenced names will use the existing variable instead of the arguments. Object-like macros resemble data objects when used, function-like macros resemble function calls. Choose OK to save your changes. Use When the preprocessor expands a macro name, the macro's expansion replaces the macro invocation, then the expansion is examined for more macros to expand. These are called preprocessor macros, not "hash defined variables". Undefined macros are treated as 0 by the preprocessor, but not by the compiler. The C preprocessor is a macro preprocessor (allows you to define macros) that transforms your program before it is compiled. In Visual Studio, add an environment variable macro, $(ExternalCompilerOptions), to the Additional Options under project options → C/C++ → Command Line (remember both Debug and Release configurations) Set the environment variable prior to calling MSBuild. Jan 6, 2016 · Neither of them is a macro; the preprocessor does not know the name of the current function. microsoft. Constant 'variables' are managed just like regular variables. Dec 18, 2012 · Possible Duplicate: C preprocessor and concatenation Is it possible to concatenate a C preprocessor with a variable name? #define WIDTH 32 int dataWIDTH; // dataWIDTH should be interpreted as ' Dec 9, 2009 · I was wondering if it is possible to iterate over arguments passed to a variadic macro in C99 or using any GCC extensions ? For e. Like these other macro languages, make allows you to define a shorthand term for a longer sequence of characters and use the shorthand in your program. We say that the conditional succeeds if MACRO is defined, fails if it is not. Enumerated constants (covered in lesson 13. And for the second, if you're passing in a type name, there's a very good chance that it will be used to declare a variable (or a typedef), so the parentheses will cause problems. There are two kinds of macros. 1. Aug 26, 2024 · Learn the basics of using the preprocessor in C programming, including macros, conditional compilation, and error directives. See full list on learn. txt file for all builds (using add_definitions(-DOPTION1) , but not on a "per-build Mar 5, 2025 · When the preprocessor encounters this directive, an association is made between the macro identifier and substitution_text. Not to mention configuration macros for difference compilers, platforms, etc. For example, if I want a defined C++ macro MY_BUILDTIME_VAR to be 0 UPDATED FOR C++23 | The C++ Preprocessor - Learn to define macros for conditional inclusion and text replacement | Clear explanations and simple code examples Sep 30, 2010 · I would like to be able to do something like #print "C Preprocessor got here!" for debugging purposes. Preprocessor conditionals can test arithmetic expressions, or whether a name is defined as a macro, or both simultaneously using the special defined operator. Then, expand the "C/C++" selection in the tree, and you will see Preprocessor as a category. In the above examples, the compiler would complain, though since the expansion of the macro still has the extra comma after the format string. preprocessor library. Apr 8, 2016 · Traditionally the C preprocessor was a separate program, and it's still available as cpp in most POSIX-like systems. ‘ #undef ’ takes a single argument, the name of the macro to undefine. Sep 17, 2024 · Learn how to use preprocessor directives in C such as #define, #include, macros, and conditional compilation with examples to enhance your code flexibility. Their names all start with double underscores. props or . Aug 20, 2025 · Macros Macros are used to define constants or create functions that are substituted by the preprocessor before the code is compiled. Whenever the name is used, it is replaced by the contents of the macro. In SES, I can define multiple builds and then define preprocessor options (eg. 10 -- Introduction to the preprocessor, with additional coverage in this lesson). To help solve this problem, CPP behaves specially for variable arguments used with the token paste operator How do I define a preprocessor variable through CMake? The equivalent code would be #define foo. Nov 25, 2014 · Although macros get a bad name these days, the preprocessor itself is still a powerful and wonderful tool when used for the problems you described. h. You can nest Jun 12, 2022 · Is there a list somewhere of the predefined Preprocessor variables (eg. The preprocessor definitions are added to the COMPILE_DEFINITIONS directory property for the current CMakeLists file. Feb 8, 2013 · As of Unity 4 you can define custom macros via the player settings inspector (on a per platform basis). The parameters must be valid C identifiers, separated by commas and optionally whitespace. While C# doesn’t have traditional macros like C or C++, it employs preprocessor directives that offer similar functionalities for conditional compilation. The simplest demonstration has as necessary conditions that /ZI be active and that __LINE__ be used in a function. The C preprocessor # This page briefly describes the C preprocessor which is by default run on every file before compilation. Jun 13, 2010 · Variadic Macros (Using the GNU Compiler Collection (GCC))GNU CPP permits you to completely omit the variable arguments in this way. Ifdef (The C Preprocessor)This block is called a conditional group. You use the bare macro name, even if the macro is function-like. cpp 12 C Preprocessor Macros Some languages, such as C and C ++, provide a way to define and invoke “preprocessor macros” which expand into strings of tokens. Variable c from the repeat macro is placed into the for loop at the proper locations to make a simple loop. Dec 26, 2012 · You can do a lot with constexpr, inline functions, and templates, but boost. It is called a macro processor because it allows you to define macros, which are brief abbreviations for longer constructs. Know that edge cases may arise - see the collection of test-cases linked at the end of this page. (Note, I would recommend against using CONFIG_ prefix in the names of these variables or macros, as normally in ESP-IDF project things starting with CONFIG_ come from / are defined in Kconfig. Preprocessing directives are evaluated before the source is parsed (at least conceptually), so there aren't any types or variables yet to get their size. 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. The macros correspond to the MSBuild properties that are set by default, or in . Defined (The C Preprocessor)If the defined operator appears as a result of a macro expansion, the C standard says the behavior is undefined. Mastering preprocessor directives is key for any serious C developer. Mar 10, 2011 · It shouldn't be either because that defines STR3 to be the preprocessing token STR1STR2. 4 Stringizing ¶ Sometimes you may want to convert a macro argument into a string constant. Though this is a bit of a wish for now. add_compile_definitions(<definition> ) Adds preprocessor definitions to the compiler command line. These Jul 23, 2025 · In C programming, #define is a preprocessor directive that is used to define macros. The C PreprocessorThe C Preprocessor Recall that preprocessing is the first step in the C program compilation stage -- this feature is unique to C compilers. The function contains temp variables and I can't remember if there are any rules about use of temporary variables in macro substitutions. Overview 1. In variadic argument macros, how to find the number of arguments? I am okay with boost preprocessor, if it has the solution. It doesn't become a number until Real world uses of the token pasting operator often involve class, variable, or function names. (Note: I know how to specify defines using the CMakeLists. For example given the following Aug 19, 2022 · I'm transitioning from SES to Visual Studio Code. Aug 2, 2021 · In this article The preprocessor expands macros in all lines except preprocessor directives, lines that have a # as the first non-white-space character. The macros are the identifiers defined by #define which are replaced by their value before compilation. add the following section inside <Project></Project>: Feb 22, 2025 · Preprocessor directives and macros in C for AVR microcontrollers are essential for optimizing code, improving readability, and making embedded systems more efficient. g. GDB can evaluate expressions containing macro invocations, show the result of macro expansion, and show a macro’s definition, including where it was defined. vcxproj Visual Studio project. Jun 26, 2015 · In C every block effectively gets its own region of stack for those more locally scoped variables defined within it and so you can have as many i and v variables as you need -- one set for every expansion of your macro. Predefined macros Discusses I don't really understand the point of having a constant variable in memory when you can just have a preprocessor macro for it. Predefined Macros (The C Preprocessor)3. Dec 31, 2023 · The weird language of the C/C++ preprocessor macros. All further occurrences of the macro identifier (outside of use in other preprocessor commands) are replaced by the substitution_text. Preprocessor directives all begin with a % sign. The preprocessor cannot use variables from the C program in expressions - it can only act on preprocessor macros. #define token value #undef token where after preprocessing, the token will be expanded to its value in the program. Jul 23, 2025 · In C++, #define is a preprocessor directive used to define a macro. In this article, we’ll explore the concept of macros with arguments, provide examples, and illustrate their benefits through code snippets and outputs. printf can crash if your number of arguments doesn't correspond to the number expected in the format string (actually: only if not enough arguments given and then it also depends on the types). We can define constants and functions like macros using #define. Jun 3, 2014 · In C/C++/Objective C you can define a macro using compiler preprocessors. Jun 22, 2020 · When using a logging framework it is useful to use the current file name or line number. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. Feb 24, 2024 · How does the preprocessor in C help in code optimization? Can you explain the role of macros in the preprocessor of C programming? What are header files, and how are they related to the preprocessor in C? How does conditional compilation work with the preprocessor in C? Is it possible to debug code at the preprocessing stage in C programming? Feb 20, 2010 · How do I use the C preprocessor to make a substitution with an environment variable Asked 15 years, 5 months ago Modified 4 years, 8 months ago Viewed 14k times The C Preprocessor is a part of the C compilation process that recognizes special statements, analyses them (before compilation) and acts on them as required. Example Macros are a powerful feature in C programming that allows you to define reusable code snippets. Syntax Sep 3, 2010 · I don't think what you're trying to do is possible. However, most Python programmers would frown at you and your code if you used a preprocessor and macros like that Mar 9, 2022 · We have no method to specify a default value for undefined variables on Azure DevOps. Preprocessing (GNU C Language Manual)As the first stage of compiling a C source module, GCC transforms the text with text substitutions and file inclusions. What's the best / most portable way to do this? FYI, identifiers/names, including preprocessor symbols, beginning with an underscore followed by a capital letter (or beginning with two underscores) are reserved to the implementation by the C and C++ standards; they shouldn't really be used in "ordinary" code. Jun 6, 2011 · This is probably the least understood and most poorly documented preprocessor operator. Macros are a way to represent a fragment of code or a constant value by giving it a name. This sections presents an example about how to use this type of macros __VAR__ARGS__ File: macro-args. Top (The C Preprocessor)The C preprocessor implements the macro language used to transform C, C++, and Objective-C programs before they are compiled. Our solution was to use an environment variable with /D defines in it, combined with the Additional Options box in Visual Studio. These macros are defined by all GNU compilers that use the C preprocessor: C, C++, Objective-C and Fortran. A variadic macro is a feature of some computer programming languages, especially the C preprocessor, whereby a macro may be declared to accept a varying number of arguments. Mar 31, 2022 · Go Up to The Preprocessor Index The C++ compiler predefines certain global identifiers, known as manifest constants. #define XSTR(x) STR(x) #define STR(x) #x The value of a macro can then be displayed with: #pragma message "The value of ABC: " XSTR(ABC) See: 3. They are defined using the #define directive, and when encountered, the preprocessor substitutes it with its defined content. The implementation also supports several more Microsoft-specific After the preprocessor expands a macro name, the macro's definition body is appended to the front of the remaining input, and the check for macro calls continues. In which cases would you want to specifically use a const variable? Isn't a macro guaranteed to give you better or equal performance due to potentially not having to load the value from memory? In all cases of const variables I've seen so far I was able to replace Mar 24, 2025 · Otherwise, the behavior is undefined. The C preprocessor is the one that expands the macros and substitutes the The Microsoft C/C++ compiler (MSVC) predefines certain preprocessor macros depending on the language (C or C++), the compilation target, and the chosen compiler options. Within VisualGDB Project Properties, MSBuild Settings, Preprocessor Macros I have defined the following: FOO Mar 2, 2023 · The macro preprocessor operates as follows: If the first statement in the program input to its macro header statement , it knows that one or more macro definitions exist in the program. See Variables That Provide Information. Preprocessing Overview Directives Preprocessing Tokens Header Files Macros Conditionals Diagnostics Line Control Null Directive Dec 27, 2023 · The C preprocessor processes source code before compilation begins. It will warn wherever your code uses this feature if you use the command-line option -Wpedantic, since other compilers may handle it differently. To support variable length arguments in macro, we must include ellipses () in macro definition. It may not be the first or […] The C preprocessor, often known as cpp, is a macro processor that is used automatically by the C compiler to transform your program before compilation. Jun 13, 2020 · I want to ask why this kind of #define can init (ialize) the "HAHASetting" object? #define macros are basically used to replace text, act like functions or to be used as initializer as provided in this example (There might be other uses but to cover all is too much for now and irrelevant to answer your question as well). The variable number of arguments that are represented by the dots are passed to __VA_ARGS__: Nov 1, 2022 · I am looking for a way to easily define macros / preprocessor directives at project build/cook time for an Unreal Engine project. Parser – the part of the compiler responsible for determining syntactic correctness of the source code. Using CMake to set preprocessor directives Preprocessor flags C++ preprocessor directives are useful for many things. 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. For this we will use the following preprocessor identifiers. This macro is used to include a header file into the source file. A macro is a fragment of code that has been given a name. Using macros, you can: Create a makefile that can build different projects. Example: Nov 6, 2024 · How do I go about actually setting MY_MESON_VARIABLE from Meson build system code such that it becomes accessible to the C preprocessor? I already know how do flow control in Meson, so I just need to know how to set a preprocessor variable. 4 days ago · Learn the different C# preprocessor directives that control conditional compilation, warnings, nullable analysis, and more 3. I suggest you read up on the basics of C before trying to write en entire OS. cpp does recognize variables defined in sketch. However, what you're trying to In addition, it generates a set of WiX preprocessor variables that can be referenced in WiX source files and preprocessor definitions which are passed to the compiler at build time. Jan 24, 2024 · I have a C++ preprocessor written like this: #ifdef cpp_variable //x+y; #endif How can I define this in Makefile? May 27, 2025 · Purpose These macros can then be used within the source code files belonging to that target during the compilation process. I'd like to use the existing projects, and just neuter the functionality via preprocessor directiv The #define Preprocessor Directive The #define directive is used to "define" preprocessor "variables", which can then be used in one of three ways, as shown in the following three sections. i" file extension. For this, first we need to define two macro string variables and a third macro variable to store the result of concatenation. 1#if, #elif Combined directives Defect reports The preprocessor removes the trailing comma if the variable arguments to a function macro are omitted or empty and the comma followed by ## precedes the variable argument identifier in the function macro definition. trueIs it possible to concatenate parameters of variadic macro to form a variable name? May 8, 2021 · I am developing a library which lets user set a crucial type alias, or do it through preprocessor directives. For example, the following code: #define A 8 int b = A + 10; Would appear to the actual compiler as int b = 8 + 10; However, this code: const int A = 8; int b = A + 10; Would appear as: const int A = 8; int b = A + 10; :) In practice, the main thing that changes is scope I have a couple of custom logging macros, which I aim to enable/disable depending on which build-type I'm using. preprocessor and chaos show that macros have their place. Jan 21, 2022 · Like functions, we can also pass variable length arguments to macros. Learn how to optimize and enhance your C programming skills with powerful macro techniques and practical examples. CUDA Programming Model The CUDA Toolkit targets a class of applications whose control part runs as a process on a general purpose computing device, and which use one or more NVIDIA GPUs as coprocessors for accelerating single program, multiple data (SPMD) parallel jobs Indicate to the preprocessor that the input file has already been preprocessed. Indicate to the preprocessor that the input file has already been preprocessed. . There are two varieties of Rust macro: macros by example and procedural macros. Jul 30, 2017 · Hower sketch. Preprocessor directives Describes directives, typically used to make source programs easy to change and easy to compile in different execution environments. Recall that all preprocessor directives or commands begin with a #. It wouldn't be the case Aug 2, 2013 · The preprocessor directives are evaluated by the preprocessor and modify the code that is presented to the compiler. The #define preprocessor directive at Line 3 sets the repeat () macro equal to a standard for loop. Stringize (#) and token pasting (##) are C preprocessor operators. The two preprocessors #define and #undef are used to create and remove macros in C. Jan 16, 2021 · 10 Question: What is the motivation behind treating in C preprocessor undefined macro as 0? Emphasis on preprocessor. One use is to set flags from the command line in order to compile the program in different ways. There is no way you'd be able to change the value of the macro at runtime. Aug 1, 2023 · The preprocessor is executed at translation phase 4, before the compilation. 2 -- Unscoped enumerations). The preprocessor provides for file inclusion (often header files), macro expansion, conditional compilation, and line control. h" #include <stdio. Introduction 1. The preprocessor more or less provides its own language which can be a very powerful tool to the programmer. Feb 22, 2010 · @Anon, in C in a variable-length argument list you need to know the number of arguments one way or the other. If the constant is for use in normal C++ code (not preprocessor directives) then use a normal C++ variable, not a preprocessor macro. (But see below for an important special case for ‘ ## ’. It can only be used in a macro definition. This behavior is controlled by #if, #else, #elif, #ifdef, #ifndef, #elifdef, #elifndef(since C++23), and #endif directives. In this tutorial, you will be introduced to c preprocessors, and you will learn to use #include, #define and conditional compilation with the help of examples. 4 Stringification in the gcc online Macros (The C Preprocessor)3 Macros ¶ A macro is a fragment of code which has been given a name. Jan 12, 2024 · Depending on your installation options, Visual Studio can make hundreds of macros available to you in an MSBuild-based . h Instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros. Select C/C++ Build -> Settings -> Tool Settings tab -> MCU GCC Compiler -> Preprocessor. Tagged with c, cpp. 1 3 3. May 19, 2025 · It is necessary to understand how macro replacement works in C, particularly in the context of concatenating tokens using the ## operator and converting macro parameters to strings using the # operator. GLSL provides most of the standard C set of preprocessor directives (#define, #if, etc), in addition to the ones listed above. The warning is also enabled by I'm using the new __VA_OPT__ C++ preprocessor identifier to solve a macro problem. There are of course many other preprocessors available, M4 being the most notable these days. It is also possible to define preprocessor variables when invoking the compiler, either as command-line arguments or through the IDE. You may use the ‘ # ’ and ‘ ## ’ operators to stringize the variable argument or to paste its leading or trailing token with another token. The longer name might be the name of a function, variable or type, or a C keyword; it might even be the name of another macro, in which case it will be expanded. I have defined the User Variable (for all users) as follows: FOOSTR = 123456 and checked the option to propagate this to the build environment. Thus, when CMake automatically defines several variables, which describes the system. You may need to compile your program specially to provide GDB with Dec 14, 2023 · Understanding Preprocessor Directives in C# In C#, preprocessor directives provide a means to conditionally include or exclude portions of code during compilation. When a macro is expanded Aug 12, 2021 · 1. After the preprocessor expands a macro name, the macro's definition body is appended to the front of the remaining input, and the check for macro calls continues. The variable port, doesn't get set until runtime. You can define the variable directly on the current pipeline, in a variable group or in the variable template. For example: bool done=false; #ifdef _DEBUG executeDebugCode(); don Define variable to value (verbatim), by defining a C preprocessor macro for variable. In this comprehensive 2500+ word guide, you‘ll learn: What exactly a preprocessor does Core directives like #ifdef, #ifndef and ## Real-world use cases and […] Feb 8, 2010 · Instead of the normal output, generate a list of `#define' directives for all the macros defined during the execution of the preprocessor, including predefined macros. 1. The second language is a macro language for performing textual substitution. Oct 17, 2024 · Object-like macros with substitution text (introduced in lesson 2. It can also be useful on its own. Mar 31, 2015 · Keep in mind if you give it a macro as a parameter, there's more preprocessor magic involved to make it work. Preprocessor Directives in C++ Preprocessor directives are special instruction that are given to the preprocessor, which is a part of the compilation process that runs before the actual code is compiled. You could write a simple file, // File check-defines. To define a macro that uses arguments, you insert parameters between the pair of parentheses in the macro definition that make the macro function-like. The result of preprocessing is a single file which is then passed to the actual compiler. variable should be a C identifier, optionally suffixed by a parenthesized argument list to define a C preprocessor macro with arguments. In Azure Pipelines, if you want to use a variable, you should define it with a specified value at first. Standard Predefined Macros Common Predefined Macros Aug 12, 2021 · 1. When defining a user defined function there must be no whitespace between the name and opening parenthesis, otherwise it will be treated as variable declaration. A good alternative would be to define a CMake option: OPTION(DEFINE_MACRO "Option description" ON) # Enabled by default Followed by a condition: IF(DEFINE_MACRO) ADD_DEFINITIONS(-DMACRO) ENDIF(DEFINE_MACRO) Then you can turn that option ON/OFF via the command line with CMake using the -D flag. GNU cpp treats it as a genuine defined operator and evaluates it normally. This is called token pasting or token concatenation. I use them in this fashion: MAKE_FUNNY_JUMPING_LOOP( MAKE_LABEL(); MAKE_LABEL(); Variadic Macros (The C Preprocessor)The variable argument is completely macro-expanded before it is inserted into the macro expansion, just like an ordinary argument. Pre-build and post-build commands are set up per project, in the project settings. Macros are defined using the #define directive and can help in simplifying complex code, improving Tell the preprocessor to output a rule suitable for make describing the dependencies of each object file. Jul 15, 2025 · In C programming, a macro is a symbolic name or constant that represents a value, expression, or code snippet. 12. May 12, 2023 · The identifier __LINE__Var names a variable and the constant is relative to whatever line number is held in the variable. They are known as stringizing and token pasting operators and are used in macro definition with #define preprocessor. # and ## operators In function-like macros, a # operator before an identifier in the replacement-list runs the identifier through parameter replacement and encloses the result in quotes, effectively creating a string literal. ) If your macro is Jun 18, 2013 · I have explored a few avenues, however, preprocessor directives can't be used inside a #define block, and as far as I can tell, there is no preprocessor option to throw errors/warnings if a macro is not defined when used inside a #if statement. You can define your own macros or use NMAKE's predefined macros. If macro support is Dec 12, 2012 · For the first solution, each macro will have to have a different name, since macros don't overload. The ANSI C/C++ standard defines the __LINE__ and __FILE_ preprocessor macros for this. When a macro is expanded, the two tokens on either side of each ‘ ## ’ operator are combined into a single token, which then replaces the Jan 26, 2012 · How do I define a preprocessor variable through CMake? The equivalent code would be #define foo. C# preprocessor commands can also provide directives to C# editors regarding the organization of code. This can be done in GCC using the stringify operator "#", but it requires two additional stages to be defined first. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to the compiler without problems. It also gives a centralized place to correct errors in this definition. You may define any valid identifier Aug 26, 2025 · The code processed by the preprocessor is called expanded code and is generally saved with a ". In this article we will learn about using #define in C++. They test a constant expression or Top (The C Preprocessor)The C preprocessor implements the macro language used to transform C, C++, and Objective-C programs before they are compiled. Apr 3, 2014 · In the end I followed txchelp advice and added a /D flag into the Command Line -> Additional Options section of the project properties to declare the environment variable as a preprocessor definition. __CLION_IDE__ macro affects parsing only. syntax -: Mar 14, 2016 · I think my first try will be with the preprocessor macro, since that shortens/simplifies the code (which was my main purpose), without changing the way the variables are written to or named. targets files, or in your project settings. __FILE__ This macro expands to the name of the current input file The C preprocessor modifies a source code file before handing it over to the compiler. : AVR_ATmega2560) that are available in set by the compiler or preprocessor? It's best to avoid the preprocessor at all costs, using it only for things like include, simple constants or conditional compilation. Aug 2, 2021 · In this article Variadic macros are function-like macros that contain a variable number of arguments. C++ preprocessors are tools that modify the source code before it is compiled. g++ -E -dM foo. Macros Rust provides an extremely powerful macro system. It is used to create new tokens. This tutorial will show you how to create a macro, how to pass arguments to it, and how to use it in your code. Parameters are not replaced inside string constants, but you can use the ‘ # ’ preprocessing operator instead. I have a custom project Apr 26, 2025 · The preprocessor supports text macro replacement and function-like text macro replacement. Jun 13, 2019 · Hi there! I’m wondering how I would go about defining a macro based on the value of a variable in my project settings? Specifically, I’m working on a plugin for an inventory system and I’d like to define some macros that will compile/exclude certain parts of the code based on the type of inventory system I want (of which there are two “types” right now). 8 Undefining and Redefining Macros If a macro ceases to be useful, it may be undefined with the ‘ #undef ’ directive. If you could explain a little more about the goal you are trying to accomplish undoubtedly there is another way of solving your problem that doesn't include macros. The in the macro parameter list represents the variable arguments of the macro. For example, Aug 26, 2025 · Macros are shortcuts or placeholders that the preprocessor replaces before the code is compiled. The preprocessor can be used to make programs more efficient, more readable, and more portable to multiple systems. Before reading your question, I thought you were asking whether a macro definition can define another macro, such as #define FOO(x) #define BAR x. This type alias (or the directive) is undeclared in the library, by design. So when you try to use c in the preprocessor you don't get what you might expect. There are also other C-like preprocessors if you just search a little. The trouble is, I have no idea how to even begin with the preprocessor. A conditional is a directive that instructs the preprocessor to select whether or not to include a chunk of code in the final token stream passed to the compiler. The controlled text inside of a conditional can include preprocessing directives. Jul 8, 2016 · Is C# able to define macros as is done in the C programming language with pre-processor statements? I would like to simplify regular typing of certain repeating statements such as the following: GeeksforGeeks | A computer science portal for geeks Preprocessor: Concatenate words to single identifiers The preprocessor directive ## (two hashes or number signs, aka the token pasting operator) allows to concatenate two symbols into one. But, be careful: code that relies heavily on complicated macros become less readable, since the syntax expected is on many occasions different from the normal expressions programmers expect in C++. If you're using GCC, you can see the pre-processor output by adding the '-E' command line argument (Note however, that you'll see all the #include's inserted in your code. To solve this, ##__VA_ARGS__ macro was introduced, this macro states that if no variable arguments exist, the comma is deleted by the pre-processor from code. They tend to be useful in conjunction with __FILE__ and __LINE__, though. -DOPTION1) for each build definition I don't see a way to do this when using the nRF Connect SDK and Visual Studio Code. Jan 31, 2022 · ‎ 2019-08-02 10:32 PM To add preprocessor symbol is Right click on your project -> click Properties. They are defined using #define and can be used to create constants or code snippets. 5 Concatenation ¶ It is often useful to merge two tokens into one while expanding macros. c int printCompileTimeDefines() { #ifdef DEF1 printf ("defined : DEF1\n"); #else // DEF1 printf ("undefined: DEF1\n"); #endif // DEF1 // and so on } Use the same Compile Time define lines on this file as with the other files C++ preprocessor macros allows adjacent string literals to be concatenated automatically without using any operator. Jan 12, 2025 · The preprocessor is executed at translation phase 4, before the compilation. Their values are the major version, minor version, and patch level of the compiler, as integer constants. Writing a MACRO that expands to another MACRO would require the preprocessor to run twice on it. How can I make a char string from a C macro's value? Asked 16 years, 11 months ago Modified 2 years, 4 months ago Viewed 145k times Nov 1, 2024 · Discover how to boost your C programming with macros. So you may check (in if command) these variables for include OS-dependent code. For instance, we could have a DEBUG flag which, if set, gives us some extra feedback useful for debugging: int main() { // Do some The second syntax ("macro-definition") defines a user defined function named ident. py -DCONFIG_SYSTEM_RELEASE_MODE=1) will result in -DCONFIG_SYSTEM_RELEASE_MODE=1 to be passed to preprocessor. They are executed only if the conditional succeeds. controlled text will be included in the output of the preprocessor if and only if MACRO is defined. Enhance your coding skills with this Sep 27, 2011 · 36 Macros are replaced by the preprocessor by their value before your source file even compiles. C macros are really preprocessor macros that are expanded before compilation. in you can convert the CMake variables to defines with: #cmakedefine USE_FEATURE_A 1 #cmakedefine USE_FEATURE_B 1 #cmakedefine USE_FEATURE_C 1 /* You can use the values of the cmake variables too, using @<var>@. Aug 2, 2021 · Macros replace a particular string in the makefile with another string. h> int main() { printf("__COUNTER__=%d\n", __COUNTER__); printf("a=%d, b=%d, c=%d\n", a, b, c); } Mar 19, 2025 · You can set a user-defined environment variable by creating a user-defined macro and then checking the box that says Set this macro as an environment variable in the build environment. Add preprocessor definitions to the compilation of source files. E. Oct 29, 2014 · In my C++ code I have to execute certain code under two conditions: because of a preprocessor macro OR a boolean variable check. Open the drop-down menu of the Preprocessor Definitions property and choose Edit. They fall into three classes: standard, common, and system-specific. While the C preprocessor operates almost directly on the text of your source code, the Rust macro system operates at a higher level. thread_local variables are effectively global variables, and have all the drawbacks of global variables other than lack of thread-safety. In addition, the preprocessor adds backslashes to escape the quotes surrounding embedded string literals, if any, and doubles the Macro-defined constants are replaced by the preprocessor. I'm writing a bunch of related preprocessor macros, one of which generates labels which the other one jumps to. Nov 20, 2018 · C programming supports two special preprocessor directive for string operations. 9 In Visual Studio, the Preprocessor Definitions can be modified on a per-project basis by right-clicking on the project within the Solution Explorer, then selecting Properties. When you define a macro, you request concatenation with the special operator `##' in the macro body. Set environment variables. Most global identifiers begin and end with two underscores (__). Click Add Button and input your symbol. That is not done. Is there a way to define preprocessor variables globally, so they can be accessed in any file that uses them? Accessing a thread_local variable may trigger execution of an unpredictable and uncontrollable amount of other code during thread-start or first use on a given thread. 3 Variable argument macros Variable argument macros are function-like macros that can take one or or an arbirtrary amount of arguments. If it makes a difference, I am trying to convert variable number of macro arguments to boost preprocessor sequence, list, or array for further reprocessing. Sep 20, 2025 · In contrast, the C# preprocessor is invoked during compilation. Aug 2, 2021 · Select the Configuration Properties > C/C++ > Preprocessor property page. Preprocessor directives are not C++ statements, so they do not end in a semicolon (;). The purpose of macros is to re-use the same definition in scripts and configs multiple times. Moreover, you can include/exclude some parts of code using compiler preprocessors. The conditional directives take a restricted set of conditional expressions; sizeof is one of the things not allowed. It expands macros in parts of some directives that aren't skipped as part of a conditional compilation. Jan 7, 2012 · I have a function that I need to macro'ize. So your preprocessor is not evaluating the for loop. However, there are techniques to getting compile-time Dec 10, 2024 · In C++, the preprocessor directives are special commands that are used to instruct the preprocessor. Furthermore, following the definition of a macro, any occurrence of the macro in the pascal source will be replaced with the value of the macro (much like the macro support in the C preprocessor). The ‘ ## ’ preprocessing operator performs token pasting. Nov 18, 2018 · At the first time, macro is not defined, so the pre-processor defines a macro _MY_FILE_ and declare other variables and functions. With this macro, you can eliminate such problematic definitions or includes from your code and replace them with dummy ones. Older compilers may not provide all of them. Macros can also take parameters, like a function: Macros with parameters work like shortcuts, but be careful with parentheses to avoid mistakes. Example: I know that this is a long time after the original query, but this may still be useful. 7 Predefined Macros ¶ Several object-like macros are predefined; you use them without supplying their definitions. Concatenation (The C Preprocessor)3. #ifdef DEBUG // Debug-only code #end Sep 2, 2025 · NVIDIA CUDA Compiler Driver NVCC The documentation for nvcc, the CUDA compiler driver. The C preprocessor provides four separate facilities that you can use as you see fit: Jul 18, 2025 · Macros are very much like symbols or compile-time variables in their syntax, the difference is that macros have a value whereas a symbol simply is defined or is not defined. vcxproj, define the variable as a preprocessor definition for the resource compiler, and assign it to the variable passed from msbuild, e. It is a preprocessor directive that allows you to define constants, functions, or code snippets that can be easily substituted wherever they are called. The variable argument is completely macro-expanded before it is inserted into the macro expansion, just like an ordinary argument. 7. 1 Standard Predefined Macros ¶ The standard predefined macros are specified by the relevant language standards, so they are available with all compilers that implement those standards. It begins with a '#' symbol and tells the preprocessor to the modify source code before compilation. Jun 3, 2022 · I would like to use preprocessor macros to declare many classes like the following: class ClassA : public ClassBase { public: int a; float b; char c; std::vector&lt;void *&gt; fun( May 26, 2024 · There is also a preprocessor macro __CLION_IDE__ that helps avoid IDE freezes during the parsing of some intricate preprocessor code blocks. Sep 29, 2009 · I am trying to write a program where the names of some functions are dependent on the value of a certain macro variable with a macro like this: #define VARIABLE 3 #define NAME(fun) fun ## _ ## VARI The following simple example uses __VA_ARGS__ to create a macro that takes a variable number of arguments with the three dots. They differ mostly in what they look like when they are used. The #ifdef and #ifndef directives let you include or skip parts of the code depending on whether a macro is defined. For each source file, the preprocessor outputs one make -rule whose target is the object file name for that source file and whose dependencies are all the #include header files it uses. In this tutorial, we will learn about preprocessors and macros in C++ with the help of examples. Mar 4, 2013 · In config. So, if the tested macro and the value macros are defined as lists of character values, then they can be compared directly in a preprocessor #if conditional - and in order to print such macros in a pragma message, use of stringifying macros are required. kdkrl uepk uh6e qduyx5pw mwt ro rj boc yjov j84ll1