=============================================
To compile this file, use CC -DFLAG=??? if.C
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#if FLAG==10

  You will see this line if the symbol "FLAG" is equal to 10

#else

  You will see this line if the symbol "FLAG" is not equal to 10

#endif


You will always see this line, because it is outside any ifdef/ifndef



#if FLAG!=10

  Another line. You will see this line if the symbol "FLAG" is not equal to 10

#else

  Another line. You will see this line if the symbol "FLAG" is equal to 10

#endif