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Variations (cont)

How do we set a cpp definition such as BSD or linux?
  • via an explicit #define in the source code   (e.g. config.h) 

    #define BSD 4
#define linux

  • via the -D flag to the C compiler 

    gcc -DBSD=4 blah.c

  • some definitions are built-in to the pre-processor 

    #define i386

A cpp trick:   #if 0   causes code to be ignored   ("commented out").