Question

How do I change variable value dynamically in makefile?
I want to call specific target depending on value of macro.

For eg.

STATIC_LIB = TRUE

all: makelib $(var)

makelib:
ifeq (${STATIC_LIB}, TRUE)
    var=staticlib
else
    var=sharedlib
endif

Here i want to call either staticlib target or sharedlib target depending on the value of target.

Code:

.SUFFIXES: .cpp .hpp

# Programs
SHELL   = bash
CC      = g++
LD  = ld
RM  = rm
ECHO    = /bin/echo
CAT = cat
PRINTF  = printf
SED = sed
DOXYGEN = doxygen
STATIC_LIB = TRUE
######################################
# Project Name (generate executable with this name)
TARGET = cs296_exe_28

# Project Paths
PROJECT_ROOT=$(HOME)/Desktop/cs296/cs296_base_code
EXTERNAL_ROOT=$(PROJECT_ROOT)/external
SRCDIR = $(PROJECT_ROOT)/src
OBJDIR = $(PROJECT_ROOT)/myobjs
BINDIR = $(PROJECT_ROOT)/mybins
LIBDIR = $(PROJECT_ROOT)/mylibs
DOCDIR = $(PROJECT_ROOT)/doc

# Library Paths
BOX2D_ROOT=$(EXTERNAL_ROOT)
GLUI_ROOT=/usr
GL_ROOT=/usr

#Libraries
LIBS = -lBox2D -lglui -lglut -lGLU -lGL

# Compiler and Linker flags
CPPFLAGS =-g -O3 -Wall 
CPPFLAGS+=-I $(BOX2D_ROOT)/include -I $(GLUI_ROOT)/include
LDFLAGS+=-L $(BOX2D_ROOT)/lib -L $(GLUI_ROOT)/lib

######################################

NO_COLOR=\e[0m
OK_COLOR=\e[1;32m
ERR_COLOR=\e[1;31m
WARN_COLOR=\e[1;33m
MESG_COLOR=\e[1;34m
FILE_COLOR=\e[1;37m

OK_STRING="[OK]"
ERR_STRING="[ERRORS]"
WARN_STRING="[WARNINGS]"
OK_FMT="${OK_COLOR}%30s\n${NO_COLOR}"
ERR_FMT="${ERR_COLOR}%30s\n${NO_COLOR}"
WARN_FMT="${WARN_COLOR}%30s\n${NO_COLOR}"
######################################

SRCS := $(wildcard $(SRCDIR)/*.cpp)
INCS := $(wildcard $(SRCDIR)/*.hpp)
OBJS := $(SRCS:$(SRCDIR)/%.cpp=$(OBJDIR)/%.o)
OBJSNMAIN := $(filter-out $(OBJDIR)/main.o,$(OBJS))

.PHONY: all setup doc clean distclean

ifndef STATIC_LIB
makelib : sharedlib # line 5
else ifeq ( ${STATIC_LIB}, TRUE )
makelib : staticlib # line 7
else
makelib : sharedlib # line 9
endif


all: setup makelib exelib

setup:
    @$(ECHO) "Setting up compilation..."
    @mkdir -p myobjs
    @mkdir -p mybins
    @mkdir -p mylibs

$(BINDIR)/$(TARGET): $(OBJS)
    @$(PRINTF) "$(MESG_COLOR)Building executable:$(NO_COLOR) $(FILE_COLOR) %16s$(NO_COLOR)" "$(notdir $@)"
    @$(CC) -o $@ $(LDFLAGS) $(OBJS) $(LIBS) 2> temp.log || touch temp.err
    @if test -e temp.err; \
    then $(PRINTF) $(ERR_FMT) $(ERR_STRING) && $(CAT) temp.log; \
    elif test -s temp.log; \
    then $(PRINTF) $(WARN_FMT) $(WARN_STRING) && $(CAT) temp.log; \
    else $(PRINTF) $(OK_FMT) $(OK_STRING); \
    fi;
    @$(RM) -f temp.log temp.err

-include $(OBJS:.o=.d)

$(OBJS): $(OBJDIR)/%.o : $(SRCDIR)/%.cpp
    @$(PRINTF) "$(MESG_COLOR)Compiling: $(NO_COLOR) $(FILE_COLOR) %25s$(NO_COLOR)" "$(notdir $<)"
    @$(CC) $(CPPFLAGS) -c $< -o $@ -MD 2> temp.log || touch temp.err
    @if test -e temp.err; \
    then $(PRINTF) $(ERR_FMT) $(ERR_STRING) && $(CAT) temp.log; \
    elif test -s temp.log; \
    then $(PRINTF) $(WARN_FMT) $(WARN_STRING) && $(CAT) temp.log; \
    else printf "${OK_COLOR}%30s\n${NO_COLOR}" "[OK]"; \
    fi;
    @$(RM) -f temp.log temp.err

exe: $(BINDIR)/$(TARGET)

makelib:
ifeq (${STATIC_LIB}, TRUE)
LIB_TYPE = staticlib
else
LIB_TYPE = sharedlib
endif

staticlib:
    @$(ECHO) "static"
    @ar rcs $(LIBDIR)/libCS296.a $(OBJSNMAIN)

sharedlib:
    @$(ECHO) "shared"
    @$(CC) -shared -o $(LIBDIR)/libCS296.so $(OBJSNMAIN)

exelib:
    $(CC) -L $(LIBDIR) $(LDFLAGS) -o cs296_exelib_28 $(OBJDIR)/main.o -lCS296 $(LIBS)

doc:
    @$(ECHO) -n "Generating Doxygen Documentation ...  "
    @$(RM) -rf doc/html
    @$(DOXYGEN) $(DOCDIR)/Doxyfile 2 > /dev/null
    @$(ECHO) "Done"

clean:
    @$(ECHO) -n "Cleaning up..."
    @$(RM) -rf $(OBJDIR) *~ $(DEPS) $(SRCDIR)/*~ $(LIBDIR)
    @$(ECHO) "Done"

distclean: clean
    @$(RM) -rf $(BINDIR) $(DOCDIR)/html

Answer 1

The solution that you came up with would not work since $(var) would be evaluated before makelib is executed.

What you could do instead is to define makelib conditionally:

STATIC_LIB = TRUE
all : makelib

ifndef STATIC_LIB
makelib : sharedlib # line 5
else ifeq (${STATIC_LIB}, TRUE)
makelib : staticlib # line 7
else
makelib : sharedlib # line 9
endif

An ifndef conditional directive saves from an error that would occur in case STATIC_LIB is not defined.