mongoc_init未定义参考

Question

I am trying to use the mongo C driver in a Trivia server program I am making to use it to track login information scores etc.

It took me a decent amount of time to figure out how to compile the stuff in a makefile but I was able to include mongoc.h so things like mongoc_client work fine. As soon as I try to actually use a function though, such as mongoc_init I get

> undefined reference to `mongoc_init'  

I feel like things should be linked though since the mongoc.h include all other header files and it looks like it is linking properly. Here is my makefile - I'm not sure what to do or even what information to provide to solve this issue.

client:
    make -f makefile.client

server:
    make -f makefile.server

clean:
    rm -f *~ *.o tserve core tclient core *.tar *.zip *.gzip *.bzip *.gz  

^ Makefile

C = gcc
CFLAGS = -g -Wall
LDFLAGS = -lpthread
INFO = -L/usr/local/lib -I/usr/local/include/libmongoc-1.0 -I/usr/local/include/libbson-1.0 -lmongoc-1.0 -lbson-1.0

all: tserve

csapp.o: csapp.c csapp.h
        $(CC) $(CFLAGS) -c csapp.c

tserve.o: tserve.c readq.h csapp.h
        $(CC) $(CFLAGS) -c tserve.c $(INFO)

readq.o: readq.c readq.h csapp.h
        $(CC) $(CFLAGS) -c readq.c

tserve: tserve.o readq.o csapp.o

^ makefile.server

C = gcc
CFLAGS = -g -Wall
LDFLAGS = -lpthread

all: tclient

csapp.o: csapp.c csapp.h
        $(CC) $(CFLAGS) -c csapp.c

tclient.o: tclient.c csapp.h
        $(CC) $(CFLAGS) -c tclient.c

tclient: tclient.o csapp.o

^ makefile.client

If it is worth noting - the system is question had the driver installed using the following code

system("wget https://github.com/mongodb/mongo-c-driver/releases/download/1.0.2/mongo-c-driver-1.0.2.tar.gz");
system("tar -xzf mongo-c-driver-1.0.2.tar.gz");
system("rm mongo-c-driver-1.0.2.tar.gz");
if (chdir("mongo-c-driver-1.0.2"))
    perror("error:");
system("./configure --prefix=/usr --libdir=/usr/lib64");
system("make");
system("sudo make install");

the code is also at https://www.github.com/decix/TriviaServer

Answer 1

You only use the INFO variable, which contains -lmongoc-1.0 -lbson-1.0 (which is what you need), to build tserve.o . In this build step, the linker is not involved, so the -l options are not evaluated and don't do anything. Instead, you need to specify them when you build tserve .

You could solve this by supplying a recipe for tserve that does this, such as

tserve: tserve.o readq.o csapp.o
    $(CC) $(LDFLAGS) $(INFO) -o $@ $+

...but it would be better, in my opinion, to split INFO and put the parts into the variables that the predefined rules use for the relevant purposes. Then your whole Makefile could be replaced with

# C compiler
CC       = gcc
# C compiler flags
CFLAGS   = -g -Wall
# C preprocessor flags
CPPFLAGS = -I/usr/local/include/libmongoc-1.0 -I/usr/local/include/libbson-1.0 -pthread
# Linker flags
LDFLAGS  = -L/usr/local/lib -pthread
# Libraries
LDLIBS   = -lmongoc-1.0 -lbson-1.0

all: tserve

csapp.o: csapp.c csapp.h
tserve.o: tserve.c readq.h csapp.h
readq.o: readq.c readq.h csapp.h

tserve: tserve.o readq.o csapp.o

The .o files will then be built through implicit rules that generate them from corresponding .c files and use CPPFLAGS and CFLAGS , saving you the trouble of specifying the recipes explicitly -- the rules only exist to track dependencies -- and the linking step uses an implicit rule that uses LDFLAGS and LDLIBS . Note that I supplied -pthread in both CPPFLAGS and LDFLAGS because it is relevant for both the preprocessor and the linker.

For further information, see the bit about implicit rules in the GNU make manual, and speaking of dependency tracking, you may find this article about automating it enlightening.

Answer 2

the following file is expected to be named Makefile.mak


# note: with very minor changes this should work for most C projects
#       irregardless of the OS being used

SHELL := /bin/sh

.PHONY: all clean

#
# macro for all *.c files
# (NOTE:
# (the following 'wildcard' will pick up ALL .c files
# (like FileHeader.c and FunctionHeader.c
# (which should not be part of the build
# (so be sure no unwanted .c files in directory
# (or change the extension
#
SRC := $(wildcard *.c)
OBJ := $(SRC:.c=.o)
DEP := $(SRC:.c=.d)
INC := $(SRC:.c=.h)


CC       := /usr/bin/gcc
MAKE     := /usr/bin/make


name := tserve

CFLAGS := -c -g -std=c11 -Wall -Wextra -pedantic-errors
#CPPFLAGS += =MD

# when #(DEBUG) is used in the compile/link steps,
# them max info for the gdb debugger is generated
DEBUG   := -ggdb3
LDFLAGS := -L/lib -L/usr/lib -L/usr/local/lib

LIBS :=  -lpthread -lmongoc-1.0 -lbson-1.0

INC := -I/usr/local/include/libmongoc-1.0 -I/usr/local/include/libbson-1.0
name := tserve

all: $(name)


#
# link the .o files into the executable
# using the linker flags
# -- explicit rule
#
$(name): $(OBJ)
    #
    # ======= $(name) Link Start =========
    $(CC) $(LDFLAGS) -o $@ $(OBJ) $(LIBS)
    # ======= $(name) Link Done ==========
    #

#
#create dependancy files
# -- inference rule
#
%.d: %.c Makefile.mak
    #
    # ========= START $< TO $@ =========
    $(CC) -M $(CPPFLAGS) $< > $@.$$$$;                      \
    sed 's,\($*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@;     \
    rm -f $@.$$$$
    # ========= END $< TO $@ =========

#
# compile the .c file into .o files using the compiler flags
# -- inference rule
#
%.o: %.c %.d
    #
    # ========= START $< TO $@ =========
    $(CC) $(CCFLAGS) -c $< -o $@ -I. $(INC)
    # ========= END $< TO $@ =========
    #


clean:
    # ========== CLEANING UP ==========
    rm -f *.o
    rm -f $(name).map
    rm -f $(name)
    rm -f *.d
    # ========== DONE ==========


ifneq "$(MAKECMDGOALS)" "clean"
-include $(DEP)
endif