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A control line of the form
    # define identifier token-sequence
causes the preprocessor to replace subsequent instances of the identifier with the given sequence of tokens; leading and trailing white space around the token sequence is discarded. A second #define for the same identifier is erroneous unless the second token sequence is identical to the first, where all white space separations are taken to be equivalent. (more…)

A control line of the form
  # include <filename>
causes the replacement of that line by the entire contents of the file filename. The characters in the name filename must not include > or newline, and the effect is undefined if it contains any of “, ‘, \, or /*. The named file is searched for in a sequence of implementation-defined places. (more…)

Parts of a program may be compiled conditionally, according to the following schematic syntax.
    preprocessor-conditional:
      if-line text elif-parts else-partopt #endif (more…)

For the benefit of other preprocessors that generate C programs, a line in one of the forms
    # line constant “filename”
    # line constant
causes the compiler to believe, for purposes of error diagnostics, that the line number of the next source line is given by the decimal integer constant and the current input file is named by the identifier. If the quoted filename is absent, the remembered name does not change. Macros in the line are expanded before it is interpreted.