Passing parameters to make to change #define variable [duplicate]

Question

I would like to compile a C code multiple times through a Makefile by passing different parameters to the #define variable. Where should I put my noobie hands on?

In my C code I have a size definition for a matrix' dimensions through

#define N 500

I would like to compile it multiple times in a loop, but I can't get my head around on how to pass the

-DN=(different sizes) 

inside the Makefile.

The job would be structured as following: 1. make clean 2. make with -DN=certain size 3. run performance test and acquire results 4. repeat with different size.

My Makefile now looks like this:

exe_name = exe
CC = gcc
CFLAGS =  -g -O2 -ggdb
LDFLAGS = -lm -L. -Wall -Wextra
OMPFLAGS = -fopenmp

OBJS = ompmatmul.o

default: $(exe_name)

$(exe_name): $(OBJS) 
    $(CC) -o $@ $^ $(LDFLAGS) $(OMPFLAGS)

%.o: %.c
    $(CC) -c $< -o $@ $(CFLAGS) $(OMPFLAGS) 

clean: 
    rm -f $(OBJS) $(exe_name) *~

.PHONY: clean #Don't look for a file named 'clean' 

While the loop I would insert the make into is the following:

for size in $(seq 500 500 3000); do

    make clean
    make #define_new_size
        #do_performance_and_acquire_results
done

Answer 1

To pass a parameter to the make file, add the parameter like this:

make N=500

Then your makefile would contain:

ifndef N
    # provide a default
    N=500
endif

...

$(exe_name): $(OBJS) 
    $(CC) -DN=$(N) -o $@ $^ $(LDFLAGS) $(OMPFLAGS)    

Answer 2

To automate the tests, based on the excellent solution proposed by dbush, you could add the following to your Makefile:

SEQS := $(shell seq 500 500 3000)
TSTS := $(addprefix test-,$(SEQS))

.PHONY: tests $(TSTS)

tests: $(TSTS)

test-%:
    $(MAKE) clean
    $(MAKE) $(exe_name) N=$*
    $(MAKE) performance-test