为什么sigaction处理程序中的上下文是一个空指针？

Question

在sigaction(2)手册页中：

The siginfo_t argument to a SA_SIGINFO handler
   When the SA_SIGINFO flag is specified in act.sa_flags, the signal
   handler address is passed via the act.sa_sigaction field.  This han‐
   dler takes three arguments, as follows:

       void
       handler(int sig, siginfo_t *info, void *ucontext)
       {
           ...
       }

当手册页声明它是一个void *时，它为什么是一个ucontext_t *？

ucontext
          This is a pointer to a ucontext_t structure, cast to void *.
          The structure pointed to by this field contains signal context
          information that was saved on the user-space stack by the ker‐
          nel; for details, see sigreturn(2).  Further information about
          the ucontext_t structure can be found in getcontext(3).  Com‐
          monly, the handler function doesn't make any use of the third
          argument.

Answer 1

POSIX实际上要求这是一个void *，sigaction的第三个参数是：

void(*) (int, siginfo_t *, void *)

此外，由于void *可以自由地与任何其他类型的数据指针相互转换，所以没有理由不使用一般情况，因为将来您可能希望无缝地添加不同的类型。