为什么errno线程安全?

# ifndef __ASSEMBLER__  

/* Function to get address of global `errno' variable.  */  

extern int *__errno_location (void) __THROW __attribute__ ((__const__));  

 

#  if !defined _LIBC || defined _LIBC_REENTRANT  

/* When using threads, errno is a per-thread value.  */  

#   define errno (*__errno_location ())  

#  endif  

# endif /* !__ASSEMBLER__ */  

#endif /* _ERRNO_H */

errno