Unveiling some Makefile Black Magic

Go back to the shadows.

Reading time: about 3 minutes (652 words).

Whilst my higher education started off in the computer science realm, I quickly became disillusioned and, excluding a decent temporal shift, moved more into the physical sciences. Whilst I never finished my CS degree; what I completed gave me an adequate understanding of development life cycles, program design and sufficient competency in c++ to get shit done. When I started heavily coding again, forces shunted me towards Matlab and higher level quick and dirty rapid prototyping. As we all know; you can only go so far in this world and I've recently found myself back into the depths with c, fortran and even a little assembly.

Ultimately though, my c++ programs never needed to link to external libraries or worry about machine specific configurations the -o switch was the only one I needed when calling gcc pretty much. Now I'm building MPI tools to run on supercomputing clusters that need the highly optimised linear algebra routines; written down by our forefathers in a more civilised age.

Realistically, Makefiles are relatively simple things, but seem to have a stigma associated with them if you're outside the computer science sphere. In fact; here's a quote from my PhD supervisor when I told him about my knowledge gain concerning this post:

Hehe, careful. Those that learn how to write makefiles are usually doomed to vanish … banished to a basement (or IT department of a fortune 500 company) for all eternity.

I guess writing this post and publishing it on the internet is sealing my fate…

The my first Makefile tutorials around the internet are not too bad (take a look at WLUG and Mrbook to get started); but the Black Magic I eluded to in the title of this post is much cooler than just typing make instead of g++ main.cpp interrobang.cpp -o omgwtfbbq.

🔗 Pre-processor macros

The specific problem I needed to overcome was managing one set of code that requires different linking libraries depending on what machine it was running on.

  • Vayu uses intel compilers and requires the MKL libraries
  • Trifid uses gcc compilers and requires the blas and lapack libraries

Because of these conditions, code in certain files differ. For example, calls to linear algebra routines on Vayu require an MKL_int type, whereas the same call on Trifid asks for int. A pre-processor macro defining a generalised int type LP_INT enables me to overcome this problem. This macro uses an if-elif-else formalism to check what machine the code is compiling on and adds additional headers if needed:

    #if defined(VAYU)

    #include <mkl>
    #define LP_INT MKL_INT

    #elif defined(TRIFID)

    #define LP_INT int


    #error "One of VAYU or TRIFID must be defined"


Now, how can we define these VAYU and TRIFID variables? SUMMON THE MAKEFILE:

    HOST_NAME := $(shell hostname)
    ifeq ($(HOST_NAME),trifid)
    LAPACK = /usr/local/lapack/3.4.2/lib/liblapack.so /usr/local/blas/1.0.248/lib/libblas.so -lm
    LIBS = -L/usr/local/lapack/3.4.2/lib
    CXX = g++ #icpc
    INCLUDES += -I$(MKL)/include
    LAPACK = -lmkl_intel_lp64 -lmkl_intel_thread -lmkl_core -liomp5 -lpthread
    LIBS = -L$(MKL)/lib/intel64

Grab the hostname of the machine & check it against known results (in my case I just check for trifid). Any shell call can be used here if hostname isn't appropriate. Then, setup the required libraries, includes and compiler information specific to the identified machine. Most importantly: append CPPFLAGS to incorperate a machine bool set to 1 which the pre-processor macros are looking for.

Et voilà! Call make on either machine and build without a hassle. No more merge conflicts between git branches for me. A shoutout to Ash who put me on the right path with this issue.