Problem:
When compiling your C++ application, you see one or more of the following or similar error messages (complete error message log example shown below)
threads_pthread.c:(.text+0x4a): undefined reference to `pthread_rwlock_init' threads_pthread.c:(.text+0x89): undefined reference to `pthread_rwlock_rdlock' threads_pthread.c:(.text+0xa9): undefined reference to `pthread_rwlock_wrlock' threads_pthread.c:(.text+0xc9): undefined reference to `pthread_rwlock_unlock' threads_pthread.c:(.text+0xee): undefined reference to `pthread_rwlock_destroy' threads_pthread.c:(.text+0x129): undefined reference to `pthread_once' threads_pthread.c:(.text+0x149): undefined reference to `pthread_key_create' threads_pthread.c:(.text+0x17b): undefined reference to `pthread_setspecific' threads_pthread.c:(.text+0x19b): undefined reference to `pthread_key_delete' threads_pthread.c:(.text+0x207): undefined reference to `pthread_once' threads_pthread.c:(.text+0x167): undefined reference to `pthread_getspecific'
Solution:
You need to link the pthread library to your executable. When running gcc manually, use the -lpthread option, for example:
gcc -o myexe main.cpp -lpthread
When using CMake, use
add_executable(myexe main.cpp) target_link_libraries(myexe pthread)
Full error log example:
/usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `CRYPTO_THREAD_lock_new': threads_pthread.c:(.text+0x4a): undefined reference to `pthread_rwlock_init' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `CRYPTO_THREAD_read_lock': threads_pthread.c:(.text+0x89): undefined reference to `pthread_rwlock_rdlock' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `CRYPTO_THREAD_write_lock': threads_pthread.c:(.text+0xa9): undefined reference to `pthread_rwlock_wrlock' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `CRYPTO_THREAD_unlock': threads_pthread.c:(.text+0xc9): undefined reference to `pthread_rwlock_unlock' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `CRYPTO_THREAD_lock_free': threads_pthread.c:(.text+0xee): undefined reference to `pthread_rwlock_destroy' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `CRYPTO_THREAD_run_once': threads_pthread.c:(.text+0x129): undefined reference to `pthread_once' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `CRYPTO_THREAD_init_local': threads_pthread.c:(.text+0x149): undefined reference to `pthread_key_create' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `CRYPTO_THREAD_set_local': threads_pthread.c:(.text+0x17b): undefined reference to `pthread_setspecific' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `CRYPTO_THREAD_cleanup_local': threads_pthread.c:(.text+0x19b): undefined reference to `pthread_key_delete' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `openssl_init_fork_handlers': threads_pthread.c:(.text+0x207): undefined reference to `pthread_once' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(threads_pthread.o): in function `CRYPTO_THREAD_get_local': threads_pthread.c:(.text+0x167): undefined reference to `pthread_getspecific' /usr/bin/ld: /home/uli/.conan/data/openssl/1.1.1k/_/_/package/6af9cc7cb931c5ad942174fd7838eb655717c709/lib/libcrypto.a(dso_dlfcn.o): in function `dlfcn_globallookup':