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-WH: Send additional arguments to the C preprocessor

When the -H option (see -H_) is used, the C preprocessor is invoked to scan include header files for typedef's and class declarations. That is called with a standard set of options. (Presently, gcc is actually called to invoke the preprocessor; it is sent the options -E, -P, and -I.) Occasionally it may be necessary to send additional options. Those can be specified as the (string) argument to -WH. Thus, to define two macros to the preprocessor, one could say either of

-WH-Dtest1=1 -WH-Dtest2=2
-WH"-Dtest1=1 -Dtest2=2"

The first form shows that -WH accretes to earlier uses. The second form shows how to handle embedded blanks (in a UNIX shell). Then, if one were programming in C, use of -H would issue the system command

gcc -E -P -Dtest1=1 -Dtest2=2