Undefined reference to header only library [duplicate]

Question

Goal

Use concise makefile with organized directory structure. In particular, I don't want to have to edit the makefile to manage dependencies with every new source addition. The assumed directory structure is:

./src      # Source files written by me (.cpp, .h)
./build    # Build artifacts (*.o, *.d)
./include  # Source files not written by me

For now the ./include directory contains the header-only implementation of concurrentqueue.

I am using the following makefile.

# Compiler flags
CXXFLAGS += -std=c++11 -Wall -Wextra -lpthread
PREFLAGS := -MMD -MP

# Executable
TARGET := main

# Source
SRC := src
INC := include

# Build
BUILD := build

# Include directories
INC_DIRS := $(INC) $(shell find $(SRC) -type d)
INC_FLAGS := $(addprefix -I,$(INC_DIRS))

# Dependency/output filenames
SRCS := $(shell find $(SRC) -name *.cpp)
OBJS := $(subst $(SRC)/,$(BUILD)/,$(addsuffix .o,$(basename $(SRCS))))
DEPS := $(OBJS:.o=.d)

$(TARGET): $(OBJS)
        $(CXX) $(OBJS) -o $@

$(BUILD)/%.o: $(SRC)/%.cpp
        mkdir -p $(dir $@)
        $(CXX) $(CXXFLAGS) $(PREFLAGS) $(INC_FLAGS) -c -o $@ $<

-include $(DEPS)

Issue

Everything was working fine until I tried to use the types from the concurrentqueue library. I created a simple header file with some typedefs.

// types.h
#pragma once

#include "message.h"
#include "blockingconcurrentqueue.h"

typedef moodycamel::BlockingConcurrentQueue<Message> message_queue_t;
typedef moodycamel::BlockingConcurrentQueue<std::pair<uint32_t, Message>> tagged_message_queue_t;

and then tried to instantiate the queues in my main.cpp,

message_queue_t send_queue;
tagged_message_queue_t receive_queue;

which resulted in the following linker errors on the final compilation step:

mkdir -p build/
g++ -std=c++11 -Wall -Wextra -lpthread -MMD -MP -Iinclude -Isrc -c -o build/main.o src/main.cpp
g++ build/main.o build/protocol.o build/args.o build/config.o build/message.o build/util.o -o main
build/main.o: In function `moodycamel::details::Semaphore::Semaphore(int)':
main.cpp:(.text._ZN10moodycamel7details9SemaphoreC2Ei[_ZN10moodycamel7details9SemaphoreC5Ei]+0x3e): undefined reference to `sem_init'
build/main.o: In function `moodycamel::details::Semaphore::~Semaphore()':
main.cpp:(.text._ZN10moodycamel7details9SemaphoreD2Ev[_ZN10moodycamel7details9SemaphoreD5Ev]+0x14): undefined reference to `sem_destroy'
collect2: error: ld returned 1 exit status
make: *** [main] Error 1

What I've Tried

I have completed another project that used this same concurrentqueue library and had no issues. However, on that project, I wrote specific makefile targets for each *.cpp file, which required me to manage the dependencies myself. From my understanding, I should not have to do that. The only other differences between the previous project and this one I can think of is that

There was no ./build directory. All build artifacts were placed in ./src
There was no ./include directory. The concurrentqueue src files were placed in ./src.

I'm at a loss for what the important difference is between my projects. For a sanity check, I tried getting rid of the ./include directory and placing the concurrentqueue header files back in ./src, but that led to the same linker errors.

Looks like your header-only library has dependencies on sem_init etc. Try linking with the pthread library (-lpthread). — G.M.
– G.M., Commented Sep 22 at 13:50
This may help: What is an undefined reference/unresolved external symbol error and how do I fix it?. — Jesper Juhl
– Jesper Juhl, Commented Sep 22 at 14:20

Kungfunk · Accepted Answer · 2025-09-22 13:55:12Z

3

My final compilation step was not linking to -lpthread, which was why it was failing. Adding $(CXXFLAGS) to the main target resolved the issue.

$(TARGET): $(OBJS)
        $(CXX) $(CXXFLAGS) $(OBJS) -o $@

answered Sep 22 at 13:55

Kungfunk

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3 Comments

John Bollinger Sep 22 at 14:07

With g++ (or gcc) you should turn on pthreads support with the -pthread option, at both compile time and link time, instead of explicitly linking libpthread (-lpthread). Especially so now that glibc is moving away from providing libpthread as a separate library.

Kungfunk Sep 22 at 14:51

Originally, the g++ i was using did not understand the -pthread option. However, it looks like my school updated g++ to a newer version, so I will make the change immediately.

Jesper Juhl Sep 22 at 14:56

Gcc/g++ has accepted -pthread for decades. If your version did not then you are stuck in the last century and you really need to upgrade.

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