Undefined reference to pthread CLion

Question

I'm trying to run this simple thread c++ program in CLion

#include <iostream>
#include <thread>

using namespace std;


//Start of the thread t1
 void hello() {
     cout << "Hello,concurrent world!" << endl; }



 int main() {
     thread t1(hello);     // spawn new thread that calls hello()

     cout << "Concurrency has started!" << endl;
     t1.join();            

     cout << "Concurrency completed!";

     return 0;
   }

My problem is that there's an error of undefined reference to pthread, and I don't undestand what I'm doing wrong... please notice that I'm doing this on CLion.

Wreckfish

You need to link with -lpthread. – Baby

@Sam ... or even better with -pthread. – Hamlett

It's much better to use cross-platform style, listed here: https://mcmap.net/q/108997/-cmake-and-libpthread – Petry

Answer 1

In CLion, to compile with flag -pthread you should add the following line to CMakeLists.txt (I've tested and it works):

SET(CMAKE_CXX_FLAGS -pthread)

Tinctorial

It worked for me but I also had to add find_package(Threads). – Cirsoid

@PaulLaffitte It worked for me without having to add find_package(Threads) – Beep

I downvoted not because this is a bad answer, it was ok at the time of writing. However, best practice is now to use find_package(…) and link against Threads::Threads as in https://mcmap.net/q/1121533/-undefined-reference-to-pthread-clion – Uteutensil

Answer 2

In CMake first find the package:

find_package(Threads REQUIRED)

Then link against it:

target_link_libraries(${PROJECT_NAME} Threads::Threads)

Now the build will succeed the linking step.

Dispatch

Best "modern" cmake answer, thx! – Pheon

Answer 3

You have to compile with flag -lpthread or -pthread (its usually advised to usepthread). If you're using CLion then you will need to edit CMakeLists.txt to ensure that your code is compiled with this flag by setting compiler flags with a command like:

SET( CMAKE_CXX_FLAGS  "<other compiler flags> -pthread")

You can learn more about these options on this post.

Pancreatin

with -pthread flag? First I hear about that. Where you found that information? – Guddle

I learned about it here #23251363 – Pancreatin

As the answer in the linked question states, you should use -pthread, not -lpthread. – Supranational

Answer 4

Make sure you are linking to pthread in the end of your CMakeLists.txt

target_link_libraries(${PROJECT_NAME} pthread)

Bowling

That is not enough. You also have to set the compiler flag -pthread – Supranational

Answer 5

As per CLion 2021.3.3, no need to specify any additional settings:

Simply be sure to start from C and not C++ and select C 99.

--

cmake_minimum_required(VERSION 3.21)
project(threads01 C)

set(CMAKE_C_STANDARD 99)

add_executable(threads01 main.c)

Cathycathyleen