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-- part contents for background part 4
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source code between the #ifdef and the #else (if present) is compiled.  Otherwise the source code between #else and #endif is compiled.  The programmer places source code appropriate to only one circumstance following the #ifdef and code for the other circumstance following the #else.  Thus, with only a single change — incorporation of the appropriate #define directive — source code appropriate to the chosen circumstance may be compiled.

For example, it may be desired that a program compile under two slightly different C compilers, having different names for the header file for standard string functions:

    # define    CCOMP1       /* remove this line for alternate compiler */
              .
              .
    # ifdef       CCOMP1
    # include  <string.h>
    # else
    # include  <strings.h>
    # endif

-- part contents for background part 7
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