Makefile fails to see main.o one out of two times - c++

Question

Ok, I'm trying to build a makefile in order to compile a program on different environments. On Linux (Mint) it works just fine, but under Mac OS I get this error:
Undefined symbols for architecture x86_64:
"_main", referenced from:
implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [build] Error 1
With the '-v':
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix
"/Library/Developer/CommandLineTools/usr/bin/ld" -demangle -dynamic -arch x86_64 -macosx_version_min 10.9.0 -w -o bin/Darwin/main ./cfprintf.o ./file.o ./inputmanager.o ./neditfile.o ./neditmanager.o ./neditscreen.o ./n_string.o ./sys_booter.o ./sys_display.o ./sys_manager.o ./sys_screen.o -lc++ -lSystem /Library/Developer/CommandLineTools/usr/bin/../lib/clang/5.1/lib/darwin/libclang_rt.osx.a
Undefined symbols for architecture x86_64:
"_main", referenced from:
implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [build] Error 1
Now here's the makefile:
CC = clang++
LIB =
INC = -I src/
OPTIONS = -w
OBJS = $(shell find ./src/obj -name '*.cpp')
IO = $(shell find ./src/io -name '*.cpp')
SYS = $(shell find ./src/sys -name '*.cpp')
APPS = $(shell find ./src/apps -name '*.cpp')
EXEC = bin/$(shell uname)/main
.PHONY: dummy build all objects io sys apps incBC clear run
build: clear incBC objects io sys apps
#echo "-> Compiling main... "
#$(CC) -c src/main.cpp $(INC) $(OPTIONS)
#echo "-> Building environment... "
#$(CC) $(shell find . -name '*.o') $(LIB) $(OPTIONS) -o $(EXEC) -v
#echo "-> Removing all objects file from compilation... "
#rm *.o
objects:
#echo "-> Building objects files and classes... "
#$(CC) -c $(OBJS) $(INC) $(OPTIONS)
io:
#echo "-> Building input/output files and classes..."
#$(CC) -c $(IO) $(INC) $(OPTIONS)
sys:
#echo "-> Building system files and classes..."
#$(CC) -c $(SYS) $(INC) $(OPTIONS)
apps:
#echo "-> Building all apps files and classes..."
#$(CC) -c $(APPS) $(INC) $(OPTIONS)
incBC:
#echo "-> Updated build count..."
#./incBC.sh
clear:
clear
run:
cd bin/$(shell uname)/ && clear && ./main
Now what's (al least to me) really weird, is that when I type make it fails one out of two times... LITERALLY one out of two! And when it does fail, I get the error message above. What I've noticed from that message is that, in the ld command, there is no main.o which is probably why I get the undefined reference. My question is: why? Why specifically one out of two times, and how can I fix this?

Answer 1

This is a bizarre makefile. Why not just write a shell script? There's nothing in this makefile that takes any advantage of make itself: it will always rebuild everything every time. The point of writing a makefile, instead of a shell script, is so you can avoid rebuilding objects that haven't changes.
Anyway, the reason you're seeing failures is that you're using make's $(shell ...) function inside of a recipe. This is not necessary, because a recipe is already running in a shell. The reason this fails for you is that make will expand the entire recipe (all lines) first, before it starts your recipe (it has to do this obviously). That means that because you're using $(shell ...) that find command is run before the recipe runs, and before the recipe runs you haven't compiled your main.o, so the find command doesn't include it in the list of object files found. Then the next time you run it, main.o exists from the previous run so the find command finds it.
Change this line:
#$(CC) $(shell find . -name '*.o') $(LIB) $(OPTIONS) -o $(EXEC) -v
to this:
#$(CC) `find . -name '*.o'` $(LIB) $(OPTIONS) -o $(EXEC) -v
and it will work.
Although it's still not very useful as a makefile.