Question

I’m looking for help getting my Makefile to do what I want it to do.

I have figured out how to add preprocessing code to my C source which will compile only if I’m compiling for debug:

#if DEBUG
  printf("main()\n");

  {
    /* Pauses execution so gdb can attach. */
    int i=9;
    pid_t PID;
    char hostname[256];
    gethostname(hostname, sizeof(hostname));
    printf("PID %d on %s ready for attach.\n", PID=getpid(), hostname);
    fflush(stdout);
    while (i>0) {
      sleep(5);
      i--;
    }
  }
#endif

And I’ve figured out that if I add -DDEBUG=1 to my compile statement, that the above code will be compiled (otherwise it’s not compiled).

Next, I want to pass a flag to my Makefile which will either include, or not include the -D option. Currently, I have two separate compile lines which I comment and uncomment as appropriate. Here is my Makefile (which I inherited from someone and am having a difficult time understanding). See the lines that say CFLAGS?:

SHELL = /bin/sh

prefix       = /home/schwarz/sundials/instdir
exec_prefix  = ${prefix}
includedir   = ${prefix}/include
libdir       = ${exec_prefix}/lib

CPP         = cc -E
CPPFLAGS    =
CC          = cc
# CFLAGS      = -Wall -g
CFLAGS      = -Wall -g -DDEBUG=1
# CFLAGS      = -g -O2
LDFLAGS     =
LIBS        = -lm
MPICC       = /usr/local/mpi/bin/mpicc
MPI_INC_DIR = /usr/local/mpi/bin/../include
MPI_LIB_DIR = /usr/local/mpi/bin/../lib
MPI_LIBS    =
MPI_FLAGS   =

INCLUDES = -I${includedir} -I${MPI_INC_DIR}
LIBRARIES = -lsundials_cvode -lsundials_nvecparallel ${LIBS}
LIBRARIES_BL =

EXAMPLES = FPU          # cvAdvDiff_non_p cvDiurnal_kry_bbd_p cvDiurnal_kry_p


OBJECTS = ${EXAMPLES:=.o}

# -----------------------------------------------------------------------------------------

.SUFFIXES : .o .c

.c.o :
        ${MPICC} ${CPPFLAGS} ${CFLAGS} ${MPI_FLAGS} ${INCLUDES} -c $<

# -----------------------------------------------------------------------------------------

all: ${OBJECTS}
        @for i in ${EXAMPLES} ; do \
          echo "${MPICC} -o $${i} $${i}.o ${MPI_FLAGS} ${CFLAGS} ${LDFLAGS} -L${libdir} ${LIBRARIES} -L${MPI_LIB_DIR} ${MPI_LIBS} ${LIBRARIES_BL}" ; \
          ${MPICC} -o $${i} $${i}.o ${MPI_FLAGS} ${CFLAGS} ${LDFLAGS} -L${libdir} ${LIBRARIES} -L${MPI_LIB_DIR} ${MPI_LIBS} ${LIBRARIES_BL}; \
        done

clean:
        rm -f ${OBJECTS}
        rm -f ${EXAMPLES}

I have searched the web – I swear! – but cannot figure out how to pass a parameter into the makefile. How can I set up my Makefile so that it does different types of compiles of the same code for different compile needs? Even better would be if I could pass a specific value to the Makefile, which would pass a specific value to the compile, which I could use to initialize i in the code. (IOW, I compile as make 8 and the C is compiled as int i=8;). Is any of that even possible in Make?

As an aside, I’ve read about Makefiles and how they work by listing files with labels (colons), followed by their dependencies. But I can’t understand how this Makefile works or why it issues two compile statements. So a brief explanation would also be helpful.

Answer 1

To choose between debug and release versions:

ifeq($(debug),1)
  CFLAGS      = -Wall -g -DDEBUG=1
else
  CFLAGS      = -Wall -g
endif

and then invoke make:

make debug=1

to compile with DEBUG, or:

make

to compile without debug.

For initializing i:

#include <stdio.h>

#ifndef INIT_I
# define INIT_I 9
#endif
int main() {
  int i = INIT_I;
  return 0;
}

and makefile:

ifneq ($(init_i),)
CFLAGS = "-DINIT_I=$(init_i)"
endif

all:
  gcc d.c $(CFLAGS) -E

to set initial i:

make init_i=10

or use default value:

make