Clang

X Macro

1. X 宏：

   #include <stdio.h>
   #include <unistd.h>
   
   #define REGS_FOREACH(_)  _(X) _(Y)
   #define RUN_LOGIC        X1 = !X && Y; \
                           Y1 = !X && !Y;
   #define DEFINE(X)        static int X, X##1;
   #define UPDATE(X)        X = X##1;
   #define PRINT(X)         printf(#X " = %d; ", X);
   
   int main() {
    REGS_FOREACH(DEFINE);
    while (1) { // clock
      RUN_LOGIC;
      REGS_FOREACH(PRINT);
      REGS_FOREACH(UPDATE);
      putchar('\n'); sleep(1);
    }
   }
   

2. gcc -E x_macro.c，最后几行结果：

   int main() {
    static int X, X1; static int Y, Y1;;
    while (1) {
      X1 = !X && Y; Y1 = !X && !Y;;
      printf("X" " = %d; ", X); printf("Y" " = %d; ", Y);;
      X = X1; Y = Y1;;
      putchar('\n'); sleep(1);
    }
   }
   

   printf("1" " 2" " 3\n"); 在编译的时候会把多个双引号的内容合并，相当于printf("1 2 3\n");

3. man gcc，查看-E：

         -E  Stop after the preprocessing stage; do not run the
             compiler proper.  The output is in the form of
             preprocessed source code, which is sent to the standard
             output.
   
             Input files that don't require preprocessing are ignored.
   

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