cpython/Include/internal/pycore_gil.h

#ifndef Py_INTERNAL_GIL_H
#define Py_INTERNAL_GIL_H
#ifdef __cplusplus
extern "C" {
#endif

#ifndef Py_BUILD_CORE
#  error "this header requires Py_BUILD_CORE define"
#endif

#include "pycore_condvar.h"       // PyCOND_T

#ifndef Py_HAVE_CONDVAR
#  error You need either a POSIX-compatible or a Windows system!
#endif

/* Enable if you want to force the switching of threads at least
   every `interval`. */
#undef FORCE_SWITCHING
#define FORCE_SWITCHING

struct _gil_runtime_state { … };

#ifdef __cplusplus
}
#endif
#endif /* !Py_INTERNAL_GIL_H */