How to pass macro definition from "make" command line arguments (-D) to C source code?

Question

Call make command this way:

make CFLAGS=-Dvar=42

And be sure to use $(CFLAGS) in your compile command in the Makefile. As @jørgensen mentioned , putting the variable assignment after the make command will override the CFLAGS value already defined the Makefile.

Alternatively you could set -Dvar=42 in another variable than CFLAGS and then reuse this variable in CFLAGS to avoid completely overriding CFLAGS.

Just use a specific variable for that.

$ cat Makefile 
all:
    echo foo | gcc $(USER_DEFINES) -E -xc - 

$ make USER_DEFINES="-Dfoo=one"
echo foo | gcc -Dfoo=one -E -xc - 
...
one

$ make USER_DEFINES="-Dfoo=bar"
echo foo | gcc -Dfoo=bar -E -xc - 
...
bar

$ make 
echo foo | gcc  -E -xc - 
...
foo

Because of low reputation, I cannot comment the accepted answer.

I would like to mention the predefined variable CPPFLAGS. It might represent a better fit than CFLAGS or CXXFLAGS, since it is described by the GNU Make manual as:

Extra flags to give to the C preprocessor and programs that use it (the C and Fortran compilers).

Examples of built-in implicit rules that use CPPFLAGS

• n.o is made automatically from n.c with a recipe of the form:
  • $(CC) $(CPPFLAGS) $(CFLAGS) -c
• n.o is made automatically from n.cc, n.cpp, or n.C with a recipe of the form:
  • $(CXX) $(CPPFLAGS) $(CXXFLAGS) -c

One would use the command make CPPFLAGS=-Dvar=123 to define the desired macro.

More info

• Variables Used by Implicit Rules
• Catalogue of Built-In Rules

Call make this way

make CFLAGS=-Dvar=42

because you do want to override your Makefile's CFLAGS, and not just the environment (which has a lower priority with regard to Makefile variables).

$ cat x.mak
all:
    echo $(OPTION)
$ make -f x.mak 'OPTION=-DPASSTOC=42'
echo -DPASSTOC=42
-DPASSTOC=42