Что под капотом std::mutex

typedef __gthread_mutex_t           __native_type;
__native_type  _M_mutex;

typedef pthread_mutex_t __gthread_mutex_t;
typedef int __gthread_mutex_t;

/* Check first for thread specific defines.  */
#if defined (_GLIBCXX___tpf_GLIBCXX___)
#include <bits/gthr-tpf.h>
#elif _GLIBCXX__PTHREADS
#include <bits/gthr-posix.h>
#elif _GLIBCXX__PTHREADS95
#include <bits/gthr-posix95.h>
#elif _GLIBCXX__DCE_THREADS
#include <bits/gthr-dce.h>
#elif _GLIBCXX__SOLARIS_THREADS
#include <bits/gthr-solaris.h>
 
/* Include GTHREAD_FILE if one is defined.  */
#elif defined(_GLIBCXX_HAVE_GTHR_DEFAULT)
#if __GXX_WEAK__
/* The pe-coff weak support isn't fully compatible to ELF's weak.
   For static libraries it might would work, but as we need to deal
   with shared versions too, we disable it for mingw-targets.  */
#ifdef _GLIBCXX___MINGW32_GLIBCXX___
#undef _GLIBCXX_GTHREAD_USE_WEAK
#define _GLIBCXX_GTHREAD_USE_WEAK 0
#endif
 
#ifndef _GLIBCXX_GTHREAD_USE_WEAK
#define _GLIBCXX_GTHREAD_USE_WEAK 1
#endif
#endif
#include <bits/gthr-default.h>
 
/* Fallback to single thread definitions.  */
#else
#include <bits/gthr-single.h>
#endif

typedef struct _Mtx_internal_imp_t *_Mtx_imp_t;