ld "undefined reference to pthread_create" (missing -pthread) in CI

pthread_create and friends live in the threads runtime. Without -pthread on both compile and link, the symbols are undefined at link time.

What this error means

Code that uses std::thread or raw pthreads fails to link with "undefined reference to pthread_create", though it compiles cleanly.

/usr/bin/ld: main.o: in function 'main':
main.cpp:(.text+0x2c): undefined reference to 'pthread_create'
collect2: error: ld returned 1 exit status

Common causes

How to fix it

Add -pthread

1. Pass -pthread to both compilation and linking.
2. In CMake, link Threads::Threads via find_package(Threads REQUIRED).

g++ -pthread main.cpp -o app
# CMake
find_package(Threads REQUIRED)
target_link_libraries(app PRIVATE Threads::Threads)

How to prevent it

• Use -pthread (or CMake Threads::Threads) for any code that touches std::thread or pthreads so the runtime is always linked.

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