如何使用makefile调试项目

Question

I'm a newbie both in makefiles and in projects using multiple files. What I'm working on now has one main.c and two user libraries. Here's the makefile:

CC = gcc
OBJECTS = main.o path.o util.o

9_1 : $(OBJECTS)
    $(CC) -g $(OBJECTS) -o 9_1

main.o : main.c path.h util.h
    $(CC) -g -c main.c path.h util.h
path.o : path.c path.h util.h
    $(CC) -g -c path.c path.h util.h
util.o : util.c util.h
    $(CC) -g -c util.c util.h

.PHONY : clean
clean : 
    rm $(OBJECTS)

I'd like to be able to debug this project in Emacs (using the multiple-windows variable) however when I run gdb -i=mi 9_1 (the default option) and try to add breakpoints in main.c it doesn't let me. Specifically: I run b main.c:25 and I get

No symbol table is loaded. Use the "file" command.
Breakpoint 1 (main.c:25) pending

What should I do?

Answer 1

It happened because

• you already built your objects/executable without the -g flag
• you added the -g flag afterwards

Adding an option in the makefile doesn't automatically trigger a recompilation. Since you have a clean target, just do

make clean
make

and the objects are rebuilt with the -g option: executable is produced with symbols now. Happy debugging.

Aside: don't pass header files to the gcc command. They're useful so make knows the dependencies (in the line above) but not when building the object from the .c file. Doing so creates useless .o files.