定义

/* Define ALIASNAME as a weak alias for NAME.
   If weak aliases are not available, this defines a strong alias.  */
# define weak_alias(name, aliasname) _weak_alias (name, aliasname)
# define _weak_alias(name, aliasname) \
  extern __typeof (name) aliasname __attribute__ ((weak, alias (#name)));

参考代码：glibc/blob/master/include/libc-symbols.h

int
__fork ()
{
  __set_errno (ENOSYS);
  return -1;
}
libc_hidden_def (__fork)
stub_warning (fork)

同时存在弱符号和强符号，优先选择强符号，如果发现不存在强符号，只存在弱符号，则选择弱符号。