gcc does not resolve extern global variables, with or without -c option

Question

Hello everyone! So i have this issue : i am declaring some extern global variables in my C program. If I don't use the -c option for gcc, i get undefined references errors. But with that -c option, the linking is not done, which means that i don't have an executable generated.

So how do I solve this?

Here is my makefile (written thanks to Alex)

CC = gcc                      # compiler/linker frontend
INCL = -I$(INCL_DIR)          # includes
DEFS = -D_DEBUG_              # defines
CFLAGS = -g $(INCL) $(DEFS)   # compiler flags
LFLAGS = -lpthread -lm -g     # linker flags

OBJ = approx.o producteur.o sequentialApproximation.o main.o
BIN = calculPi.exe
LINKOBJ  = approx.o producteur.o sequentialApproximation.o main.o
RM = rm -fv

all: $(BIN)

clean:
 ${RM} *\~ \#*\# $(OBJ)

clean_all: clean
 ${RM} $(BIN)

cleanall: clean
 ${RM} $(BIN)

$(BIN): $(OBJ)
 $(CC) $(LFLAGS) -o $@ $^

main.o: main.c
approx.o: approx.c approx.h
producteur.o: producteur.c producteur.h
sequentialApproximation.o : sequentialApproximation.c sequentialApproximation.h

.c.o:
 $(CC) $(CFLAGS) -c $<

And here is the output of make : http://pastebin.com/NzsFetrn

I did declare extern variables in approx.h (extern and global) and i try to call them in approx.c, and there it doesn't work.

Answer 1

Solution 1

You can't pass -c in your linking phase, as that will tell the compiler to skip linking.

Your problem is

$(BIN): $(OBJ)
    $(CXX) $(LINKOBJ) -o "calculPi.exe"

with

CXX_FLAGS =-g -c -lpthread -lm

where $(CXX) expands to gcc -g -c -lpthread -lm ...

get rid of the -c there.

Only pass -c when compiling from source to object modules.

Solution 2

Re the second problem, you aren't actually defining the variables anywhere. In your .c file you need the lines:

Coord* production;
int indice_prod;
int indice_cons;

(Assuming you really want global variables). This will fix the unresolved external error. You can't write to them until they've been allocated!

Notes

Also note that you can specify how to build object files from source in one rule:

.c.o:
    $(CC) $(CFLAGS) -c $<

and specify the dependencies without rules:

main.o: main.c
approx.o: approx.c approx.h
producteur.o: producteur.c producteur.h
sequentialApproximation.o : sequentialApproximation.c sequentialApproximation.h

etc.

Other notes.

Also, as general rule of thumb, CXX should only contain the compiler name, no flags. Those should go in CXX_FLAGS. And why are you calling it CXX not CC? Use g++ if you want a C++ compiler, or use CC. Since you have .c files, I'll assume C. So in your case, you should change:

CXX_FLAGS =-g -c -lpthread -lm
CXX = gcc $(CXX_FLAGS) $(INCL) $(DEFS)

to

CC = gcc
CFLAGS = -g -lpthread -lm $(INCL) $(DEFS)  # also removed -c

And you don't need a separate LINK_CXX.

Sample Makefile

So a minimal Makefile should look like:

# Makefile
CC = gcc                      # compiler/linker frontend
INCL = -I$(INCL_DIR)          # includes
DEFS = -D_DEBUG_              # defines
CFLAGS = -g $(INCL) $(DEFS)   # compiler flags
LFLAGS = -lpthread -lm -g     # linker flags

OBJ = approx.o producteur.o sequentialApproximation.o main.o
BIN = calculPi.exe

all: $(BIN)

$(BIN): $(OBJ)
    $(CC) $(LFLAGS) -o$@ $^

main.o: main.c
approx.o: approx.c approx.h
producteur.o: producteur.c producteur.h
sequentialApproximation.o : sequentialApproximation.c sequentialApproximation.h

.c.o:
    $(CC) $(CFLAGS) -c $<

# clean and other stuff.