Using The -D And -U Command-line Options
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Identifiers can be defined and undefined using the command-line compiler options -D and -U. The following syntax can be used:
-Dname -Dname=string
and
-Uname
The -Dname option defines the identifier name to the null string. -Dname=string defines name to string. In this assignment, string cannot contain spaces or tabs.
The -Uname command-line option undefines the previous definition of the identifier name.
You can also define multiple identifiers with a single #define option. Use a semicolon (;) to separate definitions of identifiers.
For example, the command line
BCC32.EXE -Ddebug=1; paradox=0; X -Umysym myprog.c
is equivalent to placing
#define debug 1 #define paradox 0 #define X #undef mysym
in the myprog.c file.