Conditional compilation of C language

Conditional compilation is achieved through precompiled directives, the main methods are:

1, #if, #elif, #else, #endif

#if conditions 1
Code Snippet 1
#elif Conditions 2
Code Snippet 2
...
#elif condition N
Code Snippet N
#else
Code Snippet N+1
#endif

#if的含义是, if the constant expression after # if is true, compile the code that it controls, such as when condition 1 is set up in code Snippet 1, Condition 1 does not set up again to see if the condition 2 is set up, if the condition 2 is set up the code segment 2, otherwise, and so on to judge other conditions, The final code snippet N+1 is compiled if the condition 1-n is not force-coded.

2, #ifdef, #else, #endif or #ifndef, #else, #endif

#ifdef: Indicates "If this macro is defined"

#ifndef: Indicates "If this macro is not defined"

#ifdef Windows//If there is a macro defined for Windows, execute Code Snippet 1, otherwise execute code snippet 2
Code Snippet 1
#else
Code Snippet 2
#endif

#ifndef Windows//If you do not have a macro defined for Windows, execute Code Snippet 1, otherwise execute code snippet 2
Code Snippet 1
#else
Code Snippet 2
#endif

3, through the macro function defined (macro_name)

The parameter is a macro name (no need to add ""), if the macro_name is defined and returns True, otherwise false, the function can write more complex conditional compilation instructions such as

#if defined (Macro1) | | (!defined (MACRO2) && defined (MACRO3))
...
#else
...
#endif

    Directive         Purpose    #           NULL instruction, no effect    #include    contains a source code file     #define     define a macro     #undef      cancel a defined macro    #if         If the given condition is true, compile the following code     #ifdef      If the macro is already defined, compile the following code    #ifndef     If the macro is not defined, compile the    following code #elif       If the previous # if given condition is not true and the current condition is true, compile the code below, which is actually else If the shorthand    #endif      End an # if ... #else条件编译块    #error      Stop compiling and display an error message

Conditional compilation of C language