Is it possible to define a C macro in a makefile?

Question

Is it possible to put the equivalent of #define VAR (in a C program) into a makefile, so that one can control which part of the program should be compiled?

score 22 · Accepted Answer · answered Mar 14 '11 at 17:59

22

Accordingly to cc manpage on linux

-D name=definition
           The contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive.  In
           particular, the definition will be truncated by embedded newline characters.

answered Mar 14 '11 at 17:59

Zimbabao

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Note that this is not Linux-specific. POSIX specifies `-D` for the `c99` command which is the standard name for the C compiler: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html – R.. GitHub STOP HELPING ICE Mar 15 '11 at 03:48

score 14 · Answer 2 · answered Mar 14 '11 at 18:54

Edit your Makefile to show

CFLAGS=-D VAR1 -D VAR2=*something*

If you are using default rules in the Makefile, this should work automatically. If you do not, and are invoking the C compiler explicitely, just make sure you are writing something along the lines of

$(CC) $(CFLAGS) -c -o $@ $<

Even more cute if the fact the CFLAGS=...above can be used on the command line rather than written in the Makefile (read man(1) manual page); this allows for easy reconfiguration of your compilation parameters at last moment, but the parameters won't be kept.

Best practices include using CPPFLAGS instead of CFLAGS, and using += instead of =; however support for these features are not as universal as the above, and depend on your make system.

score 7 · Answer 3 · answered Mar 14 '11 at 17:56

7

Yes.

Most compilers support command line options for specifying #define's. For Visual C++, this is the /D option.

answered Mar 14 '11 at 17:56

Michael

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In fact, `-D` was used by some of the earliest C compilers, and remains nearly universally accepted. The only others I can think of that are nearly as close to universal are `-c` and maybe `-O`. – Jerry Coffin Mar 14 '11 at 18:00
Most of Microsoft's command line tools (`cl` included) accept `/` as well as `-` as argument prefix. Which means you can use `-D` and be compatible to `cl` as well as `gcc`. – Max Truxa Sep 15 '14 at 17:56

Is it possible to define a C macro in a makefile?

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