{"id":14037,"date":"2023-02-23T14:10:48","date_gmt":"2023-02-23T08:40:48","guid":{"rendered":"https:\/\/fastbitlab.com\/?p=14037"},"modified":"2023-09-16T12:53:17","modified_gmt":"2023-09-16T07:23:17","slug":"microcontroller-embedded-c-programming-lecture-182-conditional-compilation-directives","status":"publish","type":"post","link":"https:\/\/fastbitlab.com\/blog\/microcontroller-embedded-c-programming-lecture-182-conditional-compilation-directives\/","title":{"rendered":"Microcontroller Embedded C Programming Lecture 182| Conditional compilation directives"},"content":{"rendered":"
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Conditional compilation directives<\/span><\/strong><\/h1>\n
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 <\/p>\n<\/div>\n<\/div>\n<\/div>\n

Conditional compilation in \u2018C\u2019<\/b><\/span><\/p>\n

There are various conditional compilation directives available in the ‘C’ programming language.  These are some of the conditional compilation directives, as shown below.<\/span><\/p>\n

#if<\/strong><\/span>
\n#ifdef<\/strong><\/span>
\n#endif<\/strong><\/span>
\n#else<\/strong><\/span>
\n#undef<\/strong><\/span>
\n#ifndef<\/strong><\/span><\/p>\n

These directives help you to include or exclude individual code blocks based on various conditions set in the program. <\/span><\/p>\n

\"Conditional
Figure 1. Conditional compilation in \u2018C\u2019<\/span><\/figcaption><\/figure>\n

 <\/p>\n

 <\/p>\n

Now let’s explore how to use these conditional compilation directives.<\/span><\/p>\n

 <\/p>\n

#if and #endif directive<\/span><\/strong><\/span><\/p>\n

\"Figure
Figure 2. #if and #endif directive<\/span><\/figcaption><\/figure>\n

 <\/p>\n

Syntax:<\/b><\/span> <\/b><\/p>\n

#if<constant expression><\/span>\r\n\r\n#endif<\/span><\/pre>\n
You have to write <\/span>#if<\/b> followed by a constant expression. So, this must be a number or an integer. Like 0 or non-zero value.<\/span><\/span><\/p>\n
 <\/p>\n
Example<\/b><\/span><\/p>\n
Here, #if 0, then you can write your code block, and the scope of the <\/span>if<\/b> is marked or decided by the following <\/span>endif<\/b><\/span>. That’s why the endif always should follow an if.<\/span><\/span><\/p>\n
This directive checks whether the constant expression is a zero or non-zero value.<\/span><\/p>\n
If the constant is 0, then the code block will not be included for the code compilation. <\/span><\/p>\n
If the constant is non zero, the code block will be included for the code compilation. <\/span><\/p>\n
endif<\/b><\/span> directive marks the end of scope of #if, #ifdef, #ifndef, #else, #elif directives.<\/span> <\/span><\/p>\n
 <\/p>\n
Please don’t confuse this ‘<\/span><\/span>if<\/span>‘<\/b> with your decision-making if statement, both are entirely different. This is a conditional compilation directive, that \u2018if\u2019 is a ‘C’ statement. So, that \u2018if\u2019 what we use in decision-making is an if statement. So, that is checked during the runtime of the program. But this #if and #endif is checked during the pre-processor stage of the compilation, whether to include a code block or not in the final executable of the program.<\/span><\/span><\/p>\n
In this case, this code block will not be part of the final executable of the program. <\/span><\/p>\n
How to use  #if and #endif in a program?<\/span><\/p>\n
#if<\/span> 0\r\n   float<\/span> radius = 0;\r\n   printf<\/span>(\"This is circle area calculation program\\n\");\r\n   fflush<\/span>(stdout);\r\n   printf<\/span>(\"Enter the radius :\");\r\n   fflush<\/span>(stdout);\r\n   scanf<\/span>(\"%f\", &radius);\r\n   printf<\/span>(\"Area of circle = %f\\n\",(3.1415 * radius * radius));\r\n   fflush<\/span>(stdout);\r\n#endif<\/span>\r\n\r\n   float<\/span> base, height;\r\n   printf<\/span>(\"This is Triangle area calculation program\\n\");\r\n   fflush<\/span>(stdout);\r\n   printf<\/span>(\"Enter base and height: \");\r\n   fflush<\/span>(stdout);\r\n   scanf<\/span>(\"%f%f\",&base, &height);\r\n   printf<\/span>(\"Area of triangle = %f\\n\", (0.5 * base * height));\r\n   return<\/span> 0;<\/pre>\n
Now here in this program, there are 2 code blocks. The first code block actually computes the area of a circle and the second code block computes the area of a triangle.<\/span><\/p>\n
\"Conditional
Figure 3. Example<\/span><\/figcaption><\/figure>\n
In programs, you use comments to exclude the sentences. But comments are generally used to give some description or to provide some information. So, using that to remove a code block is not good.<\/span><\/p>\n
Instead of that you can use <\/span>pre-processor directive #if<\/span><\/b>. <\/span><\/span><\/p>\n
Please note that you should remember 2 points:<\/span><\/p>\n
    \n
  1.  It must be a constant expression here.<\/span><\/li>\n
  2.  And this <\/span>#if<\/b><\/span> must be terminated by its associated <\/span>#endif<\/b><\/span>, otherwise you will face problems. <\/span><\/span><\/li>\n<\/ol>\n
     <\/p>\n
    Look at Figure 3, I can use <\/span>#if<\/b> 0<\/b><\/span>. A <\/span>#if<\/b><\/span> must be terminated by its associated <\/span>#endif<\/b><\/span>. #endif actually decides the scope of this. So, now this code block is excluded from the build itself.<\/span><\/span><\/p>\n
    In the output, you can see that it directly goes to the second code block. It is not executing the first code block(Figure 3). <\/span><\/p>\n
     <\/p>\n
     <\/p>\n
    #if \u2026#else \u2026\u2026#endif<\/b><\/span><\/p>\n
    The #if, #else, and #endif are conditional compilation directives that are used in programming languages like C, C++, and C# to include or exclude parts of the code based on certain conditions.<\/span><\/p>\n
    \"Figure
    Figure 4. #if \u2026#else \u2026#endif<\/span><\/figcaption><\/figure>\n
     <\/p>\n
    Syntax:<\/b><\/span><\/p>\n
    You can also use <\/span>#else<\/b><\/span> in between <\/span>#if<\/b><\/span> and <\/span>#endif<\/b><\/span>, as shown in Figure 4. <\/span><\/span><\/p>\n
    The #if directive tests whether a particular condition is true or false. And if it is true, the code between the #if and the corresponding #else or #endif directive is included in the compiled output. If the condition is false, the code between the #if and #else directives is skipped, and the code between #else and #endif directives (if present) is included instead.<\/span><\/p>\n
    The #else directive is optional and is used to specify an alternate code block that should be included in the compiled output when the condition tested by the preceding #if directive is false.<\/span><\/p>\n
    The #endif directive is used to mark the end of a conditional block and is required after every #if or #else directive.<\/span><\/p>\n
     <\/p>\n
    Example<\/b><\/span><\/p>\n
    Here, a  #if constant expression is 1. So, code block-1 will be included for the build, and code block-2 will not be considered for the build.<\/span><\/p>\n
    Suppose, #if constant expression is 0, then code block-2 will be considered for the build.  <\/span><\/p>\n
     <\/p>\n
    Let\u2019s see how you use this in a program, as shown in below.<\/span><\/p>\n
    In the program, I use #if constant expression 1, after the code block-1 I will use #else, and then after the code block-2 I use #endif.  <\/span><\/p>\n
    In this case, the #if block will be included in the compiled output, and it prints an area calculation program.<\/span><\/p>\n
    So I use if 0, in that case, the second code block will be the output. <\/span><\/p>\n
    That’s why use  #else when you want to execute either one of the code blocks.<\/span><\/p>\n
    #if<\/span> 1\r\n   float<\/span> radius = 0;\r\n   printf<\/span>(\"This is circle area calculation program\\n\");\r\n   fflush<\/span>(stdout);\r\n   printf<\/span>(\"Enter the radius :\");\r\n   fflush<\/span>(stdout);\r\n   scanf<\/span>(\"%f\", &radius);\r\n   printf<\/span>(\"Area of circle = %f\\n\",(3.1415 * radius * radius));\r\n   fflush<\/span>(stdout);\r\n\r\n\r\n#else<\/span>\r\n  float<\/span> base, height;\r\n  printf<\/span>(\"This is Triangle area calculation program\\n\");\r\n  fflush<\/span>(stdout);\r\n  printf<\/span>(\"Enter base and height: \");\r\n  fflush<\/span>(stdout);\r\n  scanf<\/span>(\"%f%f\",&base, &height);\r\n  printf<\/span>(\"Area of triangle = %f\\n\", (0.5 * base * height));\r\n#endif<\/span>\r\n\r\nreturn<\/span> 0;<\/pre>\n
     <\/p>\n
     <\/p>\n
    #ifdef<\/b><\/span><\/p>\n
    Here as its name indicates so the evaluation of this pre-processor directive depends upon the definition of a macro. That’s why it is called <\/span>#ifdef<\/b> (that is if defined).<\/span><\/span><\/p>\n
    That’s the reason we use an identifier for the evaluation, so not a constant expression.<\/span> <\/span><\/span><\/p>\n
    \"Figure
    Figure 5. #ifdef<\/span><\/figcaption><\/figure>\n
     <\/p>\n
    \n
    #ifdef directive checks whether the identifier is defined in the program or not. If the identifier is defined, the code block will be included for compilation. <\/b><\/span><\/p>\n<\/blockquote>\n
     <\/p>\n
    Syntax:<\/b><\/span> <\/b><\/span>#ifdef, there is a space and you should use an identifier. And as usual, you have to use endif to terminate the scope. <\/span><\/p>\n
    Example<\/b><\/span><\/p>\n
    If the NEW_FEATURE macro is defined in the program, then only include the code block, otherwise exclude the code block. That’s the meaning of those statements. <\/span><\/p>\n
     <\/p>\n
     <\/p>\n
    #ifndef<\/b><\/span><\/p>\n
    The meaning of this code block is, if a new feature is not defined then it executes the code block.  <\/span><\/p>\n
    \"Figure
    Figure 6. #ifndef<\/span><\/figcaption><\/figure>\n
     <\/p>\n
     <\/p>\n
    #ifdef and #else<\/b><\/span><\/p>\n
    Now you can also use #else along with  #ifdef.<\/span><\/p>\n
    \"Figure
    Figure 7. #ifdef and #else<\/span><\/figcaption><\/figure>\n
    If the NEW_FEATURE macro is really defined, then the code block of the new feature will be included. If the NEW_FEATURE macro is not defined, then the code block for the old feature will be included.<\/span><\/p>\n
    In the following article, let’s learn about the #if and ‘defined’<\/a> operators.<\/span><\/p>\n
     <\/p>\n
    Get the Microcontroller Embedded C Programming Full Course<\/strong> Here<\/a>.<\/span><\/span><\/p>\n
    FastBit Embedded Brain Academy Courses<\/b><\/span><\/p>\n
    Click here:<\/span> <\/span><\/span><\/span>https:\/\/fastbitlab.com\/course1<\/span><\/a><\/span><\/p>\n
     <\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
      Conditional compilation directives   Conditional compilation in \u2018C\u2019 There are various conditional compilation directives available in the ‘C’ programming language.  These are some of the conditional compilation directives, as shown below. #if #ifdef #endif #else #undef #ifndef These directives help you to include or exclude individual code blocks based on various conditions set in 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