Advanced Signal Management高级信号管理

#include <signal.h>
int sigaction(int signo, const struct sigaction *act, struct sigaction *oldact);

如果act不是NULL，系统调用将更改act指定的信号的当前行为。如果oldact不是null，则调用将存储以前(或当前，如果为空)行为。

struct sigaction {
     void (*sa_handler)(int); /* signal handler or action */
     void (*sa_sigaction)(int, siginfo_t *, void *);
     sigset_t sa_mask; /* signals to block */
     int sa_flags; /* flags */
     void (*sa_restorer)(void); /* obsolete and non-POSIX */
};

sa_handler字段指示接收信号时要采取的操作。
sa_mask字段提供一组信号，该信号在信号处理器的执行期间该系统应该阻塞。
sa_flags字段是一个零、一个或多个标志的位掩码，这些标志改变了signo给出的信号的处理。

• SA_NOCLDSTOP
  如果signo是SIGCHLD，则此标志指示系统在子进程停止或恢复时不提供通知。
• SA_NOCLDWAIT
  如果signo是SIGCHLD，则此标志启用自动子进程捕获：子进程在终止时不会变成僵尸，并且父节点不需要(也不能)对它们调用wait()
• SA_NOMASK
  同SA_NODEFER
• SA_ONESHOT
  通SA_RESETHEAD
• SA_ONSTACK
• SA_RESTART
• SA_RESETHEAD
  此标志启用“一击”模式。一旦信号处理程序返回，给定信号的行为将重置为默认值。
  Sigaction()在成功时返回0。如果失败，调用将返回−1并将errno设置。

typedef struct siginfo_t {
  int si_signo; /* signal number */
  int si_errno; /* errno value */
  int si_code; /* signal code */
  pid_t si_pid; /* sending process's PID */
  uid_t si_uid; /* sending process's real UID */
  int si_status; /* exit value or signal */
  clock_t si_utime; /* user time consumed */
  clock_t si_stime; /* system time consumed */
  sigval_t si_value; /* signal payload value */
  int si_int; /* POSIX.1b signal */
  void *si_ptr; /* POSIX.1b signal */
  void *si_addr; /* memory location that caused fault */
  int si_band; /* band event */
  int si_fd; /* file descriptor */
};

The Wonderful World of si_code

si_code字段指示信号的原因。对于用户发送的信号，字段指示信号是如何发送的.对于内核发送的信号，字段指示发送信号的原因。

• SI_ASYNCIO
  The signal was sent due to the completion of asynchronous I/O (see Chapter 5).
• SI_KERNEL
  The signal was raised by the kernel.
• SI_MESGQ
  The signal was sent due to a state change of a POSIX message queue (not covered
  in this book).
• SI_QUEUE
  The signal was sent by sigqueue() (see the next section).
• SI_TIMER
  The signal was sent due to the expiration of a POSIX timer (see Chapter 11).
• SI_TKILL
  The signal was sent by tkill() or tgkill(). These system calls are used by threading libraries and are not covered in this book.
• SI_SIGIO
  The signal was sent due to the queuing of SIGIO.
• SI_USER
  The signal was sent by kill() or raise().
  以下si_code值仅对SIGBUS有效。它们指示发生的硬件错误的类型：
• BUS_ADRALN进程发生了对齐错误。
• BUS_ADRERR进程访问无效的物理地址。
• BUS_OBJERR进程导致了其他形式的硬件错误。
  对于SIGCHLD，以下值标识子程序如何生成发送给其父节点的信号：
• CLD_CONTINUED
  子进程被停止了，但是已经恢复了。
• CLD_DUMPED
  不正常地终止了子进程。
• CLD_EXITED
  子程序通过EXIT()终止。
• CLD_KILLED
  子进程被杀了。
• CLD_STOPPED
  子进程停了下来。
  -CLD_TRAPPED
  子进程被困在陷阱里。
  以下值仅对SIGFPE有效。它们解释了所发生的算术错误的类型：
• FPE_FLTDIV
  The process performed a floating-point operation that resulted in division by zero.
• FPE_FLTOVF
  The process performed a floating-point operation that resulted in an overflow.
• FPE_FLTINV
  The process performed an invalid floating-point operation.
• FPE_FLTRES
  The process performed a floating-point operation that yielded an inexact or invalid result.
• FPE_FLTSUB
  The process performed a floating-point operation that resulted in an out-of-range subscript.
• FPE_FLTUND
  The process performed a floating-point operation that resulted in an underflow.
• FPE_INTDIV
  The process performed an integer operation that resulted in division by zero.
• FPE_INTOVF
  The process performed an integer operation that resulted in an overflow.
  以下si_code值仅对SIGILL有效。他们解释了非法执行指令的性质：
• ILL_ILLADR
  The process attempted to enter an illegal addressing mode.
• ILL_ILLOPC
  The process attempted to execute an illegal opcode.
• ILL_ILLOPN
  The process attempted to execute on an illegal operand.
• ILL_PRVOPC
  The process attempted to execute a privileged opcode.
• ILL_PRVREG
  The process attempted to execute on a privileged register.
• ILL_ILLTRP
  The process attempted to enter an illegal trap.
  对于SIGPOLL，下列值标识生成信号的I/O事件：
• POLL_ERR
  An I/O error occurred.
• POLL_HUP
  The device hung up or the socket disconnected.
• POLL_IN
  The file has data available to read.
• POLL_MSG
  A message is available.
• POLL_OUT
  The file is capable of being written to.
• POLL_PRI
  The file has high-priority data available to read.

以下代码对SIGSEGV有效，描述了两种类型的无效内存访问：

• SEGV_ACCERR
  The process accessed a valid region of memory in an invalid way—that is, the process violated memory-access permissions.
• SEGV_MAPERR
  The process accessed an invalid region of memory.
  对于SIGTRAP，这两个si_code值标识陷阱命中的类型：
• TRAP_BRKPT
  The process hit a break point.
• TRAP_TRACE
  The process hit a trace trap.

Sending a Signal with a Payload

正如我们在上一节中所看到的，在SA_SIGINFO标志中注册的信号处理程序被传递一个siginfo_t参数。此结构包含一个名为si_value的字段，它是可选的有效载荷。 从信号发生器到信号接收器。

#include <signal.h>
int sigqueue (pid_t pid, int signo, const union sigval value);

union sigval {
    int sival_int;
    void *sival_ptr;
};

sigval value;
int ret;
value.sival_int = 404;
ret = sigqueue (1722, SIGUSR2, value);
if (ret)
     perror ("sigqueue");