[libc++] Do not use futex if LIBCXX_HAS_MUSL_LIBC is ON

[pzheng]

Summary

Futex is not available with musl libc.

[__simt__]

What does the Linux syscall have to do with MUSL?

Is the C library calling dibs on the entire platform?

[pzheng]

The build actually fails because of "#include <linux/futex.h>". Musl does not have this header file and the build fails with a missing header file error.

[bcain]

In D76632#1937702, @pzheng wrote:

The build actually fails because of "#include <linux/futex.h>". Musl does not have this header file and the build fails with a missing header file error.

Maybe that header is supposed to come from the kernel and not the C library.

[pzheng]

In D76632#1937829, @bcain wrote:

In D76632#1937702, @pzheng wrote:

The build actually fails because of "#include <linux/futex.h>". Musl does not have this header file and the build fails with a missing header file error.

Maybe that header is supposed to come from the kernel and not the C library.

You are right! This is actually from the linux kernel and not from glibc or musl. Thanks for pointing this out.

libcxx/src/atomic.cpp

 #ifndef _LIBCPP_HAS_NO_THREADS
 
 #include <climits>
 #include <atomic>
 #include <functional>
 
 #include <iostream>
 
-#ifdef __linux__
+#if defined(__linux__) && !defined(_LIBCPP_HAS_MUSL_LIBC)
 
 #include <unistd.h>
 #include <linux/futex.h>
 #include <sys/syscall.h>
 
 #else // <- Add other operating systems here
 
 // Baseline needs no new headers
 
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#ifdef __linux__
+#if defined(__linux__) && !defined(_LIBCPP_HAS_MUSL_LIBC)
 
 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
                                               __cxx_contention_t __val)
 {
     static constexpr timespec __timeout = { 2, 0 };
     syscall(SYS_futex, __ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0);
 }