<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/87778>87778</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            libc++ does not link to pthread with GNU ld and older glibc
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc++
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          andreyv
      </td>
    </tr>
</table>

<pre>
    Commit 36bb134ac79c91129d6ea551953ce67ed776123d removed the explicit linking of `pthread` and other libraries in libc++ on GNU/Linux. This is fine with glibc version 2.34 or later, as libpthread functionality [has been moved to libc](https://sourceware.org/git/?p=glibc.git;a=blob;f=NEWS;h=da4b2223e946368018dfb5ad73532ebf7dd84181;hb=HEAD#l1048). However, with older glibc versions libc++ now fails to link:
```
[ 98%] Linking CXX shared library llvm-18.1.3.build/lib/x86_64-unknown-linux-gnu/libc++.so
CMakeFiles/cxx_shared.dir/condition_variable.cpp.o: In function `std::__1::notify_all_at_thread_exit(std::__1::condition_variable&, std::__1::unique_lock<std::__1::mutex>)':
condition_variable.cpp:(.text._ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE+0x19): undefined reference to `pthread_getspecific'
condition_variable.cpp:(.text._ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE+0x3e): undefined reference to `pthread_setspecific'
condition_variable.cpp:(.text._ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE+0x4a): undefined reference to `pthread_getspecific'
```

libc++ sources have code such as
```cpp
#if defined(__ELF__) && defined(_LIBCPP_LINK_PTHREAD_LIB)
#pragma comment(lib, "pthread")
#endif
```
But this looks to be an LLVM extension and is not supported by the GNU linker, which is used by default when building libc++ on GNU/Linux.

Other libraries removed in the commit may have similar issues.

A workaround is to use `-DLIBCXX_ADDITIONAL_LIBRARIES=pthread`.
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJzMVt-Pm7wS_Wucl9EisIHAQx6SJWmjm6bVbnvv6r4gg4fgG8fOxSY__vtPhmw37bbSJ30vlaIkmMEzZ47nHLi1cqcRZyRZkKSY8N61pptxLTq8niaVEdfZozkcpAOWVlXEYl5P8zqPIpqLFHmSRHnCakynKKbTNKJMQIcHc0IBrkXAy1HJWjpQUu-l3oFpgKTh0bUdckHSELgWYFyLHShZdbyTaEFqf1ETuiB0AUbDh-03QlcbqftLAF9baUFaaKRGOEvXws5Hwwk7K40GGrAYTAeKO-wIfQRu_Xa3nND0unbSaK6kuwJJFi23UCFquJVtxuRJQWjWOne0hM0JXRG6sqbvajzzDgPT7Qhd7aTzd9jqSFgxVBH4JbbghBWVMhVhi4awYrv8zzNhi5awQvC4opQyzOOUpVkYZaKpEi6mLGEUq2YqRBZHWeTDK8KKj8t5QShTURhnhOYBfDRnPI3ABvBGCex-bIG9b582Z2i4VHZEpvceTliQcE7S8PYZL5MF5BmhCUkK2Nz4enx5AdvyDsWNnysodTo8RFkQBSyoeqkEoSslK0JXlywt0_ih13ttzvpBeb4edrofA24FBdaM-R4_8T2upEJL6Kq-XMoxTyBk5xeMFtLzVJ54J3mlMKiPx8AQNoe1_s6iP03WCY-JzcsyGv9o42RzLblSJXflSHyJF89W9j76fSpCU9_f96G9lv_vsVSm3hP2-P7-oXd4IWxJaE7o9Hunfw1mOFdZ4PDigvK_22fHyjKiyW-LXz5tn8soe7_Z0q9Hd7Wtt89lMtSyXC4JXYSXKPclsTn0WqCfHD-nDXaoa_QH420oyx06e8RaNrL2GP4QAAz_NgD7RwKI-T9i4OdhHb7vxnzUJgstPyHURiDYvm6B258e96jHFcpkA7dSCM3KcrlZlSWhOQzHP72_t1kvHr98KTfr7b_KL18_Pi3nhV_ziF43O3Z8d-BQm8MBtR-0QRMegVD6KveU3j-AWsjml-AWvQPnVV4Zsx90q0LgGjabf38CvDjUg9B765AWtHFg--PRdA4FVNfBeD5svw1idxPKVtatj-3tGCKw4b1ycG5RwyBiXux-bzr3Tf_8k1u9-p3UQ-Z6tMsDv45cWHmQincgre3R_rDTHM6m2_PO9CMSZ3yB_iQ8FL7jLy_lvCjWX9eft_ON7_fT_Gm9fCaseDPQYCJmTOQs5xOcRdOIMkopjSbtDLM0abIa6yTGkDYsFw1LRFJHgqU0y-uJnNGQxmEcJhELWRwFVVbXeRwlfEqjhkUpiUM8cKkCL_ne8iYDiFk2nU6zieIVKju8OVD61jrPclJMutngE1W_syQOlbTOvm3jpFM4u2u3MDgS6TnzfXh168HiBjLF-Krw5naTvlOzHw16J13bV0FtDt5y1On15-HYmf9h7c16pIHQ1QDirwAAAP__VDbl5w">