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