1 Overview
A signal is a small message that notifies a process that an event of some type has occurred in the system.
- Kernel abstraction for exceptions and interrupts.每种信号类型都对应某个类型的系统事件
- Sent from the kernel (sometimes at the request of another process) to a process.
- Different signals are identified by small integer ID’s (1-30)
- The only information in a signal is its ID and the fact that it arrived.
2 Signal Concepts
2.1 Sending a signal
2.1.1 What
Kernel sends (delivers) a signal to a destination process by updating some state in the context of the destination process.
2.1.2 Why
Kernel sends a signal for one of the following reasons:
- Kernel has detected a system event such as divide-by-zero (SIGFPE) or the termination of a child process (SIGCHLD)
- Another process has invoked the kill system call to explicitly request the kernel to send a signal to the destination process.
2.1.3 How
2.1.3.1 Sending Signals with kill Program
kill program sends arbitrary signal to a process or process group
2.1.3.2 Sending Signals from the Keyboard
Typing ctrl-c (ctrl-z) sends a SIGINT (SIGTSTP) to every job in the foreground process group.
- SIGINT – default action is to terminate each process
- SIGTSTP – default action is to stop (suspend) each process
2.1.3.3 Sending Signals with kill Function
int kill(pid_t pid, int sig);
2.1.3.4 Sending Signals to itself with alarm Function
unsigned int alarm(unsigned int secs);
2.2 Receiving a signal
2.2.1 What
A destination process receives a signal when it is forced by the kernel to react in some way to the delivery of the signal.
Three possible ways to react:
- Ignore the signal (do nothing)
- Terminate the process (with optional core dump).
- Catch the signal by executing a user-level function called a signal handler. Akin to a hardware exception handler being called in response to an asynchronous interrupt.
2.2.2 How
Suppose kernel is returning from an exception handler and is ready to pass control to process p.
Kernel computes pnb = pending & ~blocked
- The set of pending nonblocked signals for process p
If (pnb == 0)
- Pass control to next instruction in the logical flow for p.
Else
- Choose least nonzero bit k in pnb and force process p to receive signal k.
- The receipt of the signal triggers some action by p
- Repeat for all nonzero k in pnb.
- Pass control to next instruction in logical flow for p.
2.2.2.1 default action
Each signal type has a predefined default action, which is one of:
- The process terminates
- The process terminates and dumps core.
- The process stops until restarted by a SIGCONT signal.
- The process ignores the signal.
For SIGKILL and SIGSTOP the action is "always", not just "default".
2.2.2.2 Installing Signal Handlers
The signal function modifies the default action associated with the receipt of signal signum:
handler_t *signal(int signum, handler_t *handler)
Different values for handler:
- SIG_IGN: ignore signals of type signum
- SIG_DFL: revert to the default action on receipt of signals of type signum.
Otherwise, handler is the address of a signal handler
- Called when process receives signal of type signum
- Referred to as “installing” the handler.
- Executing handler is called “catching” or “handling” the signal. When the handler executes its return statement, control passes back to instruction in the control flow of the process that was interrupted by receipt of the signal.
2.3 pending signal
A signal is pending if it has been sent but not yet received.
There can be at most one pending signal of any particular type.
Important: Signals are not queued
- If a process has a pending signal of type k, then subsequent signals of type k that are sent to that process are discarded.
A process can block the receipt of certain signals.
- Blocked signals can be delivered, but will not be received until the signal is unblocked.
A pending signal is received at most once.
How
Kernel maintains pending and blocked bit vectors in the context of each process.
pending – represents the set of pending signals
- Kernel sets bit k in pending whenever a signal of type k is delivered.
- Kernel clears bit k in pending whenever a signal of type k is received
blocked – represents the set of blocked signals
- Can be set and cleared by the application using the sigprocmask function.
Signal handling problems.
Pending signals are not queued
For each signal type, just have single bit indicating whether or not signal is pending
Even if multiple processes have sent this signal
Living With Nonqueuing Signals
Signal arrival during long system calls (say a read)
Signal handler interrupts read() call
Linux: upon return from signal handler, the read() call is restarted automatically
Some other flavors of Unix can cause the read() call to fail with an EINTER error number (errno) in this case, the application program can restart the slow system call
Subtle differences like these complicate the writing of portable code that uses signals.
UNIX signal 集合
信号名字 BSD Linux Mac Solaris 默认动作 说明
SIGABRT x x x x 终止+core 异常终止(abort)
SIGALRM x x x x 终止 闹钟超时(alarm)
SIGBUS x x x x 终止+core 总线错误
SIGCANCEL x 忽略 线程库内部使用
SIGCHLD x x x x 忽略 子进程暂停或终止
SIGCONT x x x x 忽略 通知已暂停进程继续
SIGEMT x x x x 终止+core 来自PDP-11之(emulator trap)指令
SIGFPE x x x x 终止+core 算术异常
SIGFREEZE x 忽略 检查点冻结
SIGHUP x x x x 终止 拨号连接被挂断，通常由会话首进程接收
SIGILL x x x x 终止+core 非法硬件指令
SIGINFO x x 忽略 C-T产生
SIGINT x x x x 终止 DELETE或C-C产生
SIGIO x x x x 终止/忽略 异步I/O
SIGIOT x x x x 终止+core PDP-11之指令，四个系统将其值定义为SIGABRT
SIGKILL x x x x 终止 **终止，无法忽略和捕捉**
SIGLWP x 忽略 线程库内部使用
SIGPIPE x x x x 终止 写管道或SOCK_STREAM时接收数据进程已经终止
SIGPOLL x x 终止 轮询事件，俩系统将其等同于SIGIO
SIGPROF x x x x 终止 setitimer设置的梗概统计间隔计时器到期
SIGPWR x x 终止/忽略 如果存在电池，则当电量低时警告
SIGQUIT x x x x 终止+core C-\产生
SIGSEGV x x x x 终止+core 内存段访问异常
SIGSTKFLT x 终止 协处理器故障，早期Linux定义信号
SIGSTOP x x x x 暂停 **暂停，无法忽略和捕捉**
SIGSYS x x x x 终止+core 无效系统调用
SIGTERM x x x x 终止 kill发送之默认终止信号
SIGTHAW x 忽略 检查点解冻
SIGTRAP x x x x 终止+core PDP-11的TRAP指令
SIGTSTP x x x x 暂停 C-Z产生
SIGTTIN x x x x 暂停 后台读控制tty
SIGTTOU x x x x 暂停 后台写控制tty
SIGURG x x x x 忽略 紧急情况（套接字）
SIGUSR1 x x x x 终止 用户定义信号
SIGUSR2 x x x x 终止 用户定义信号
SIGVALRM x x x x 终止 虚拟时间闹钟（setitimer）
SIGWAITING x 忽略 线程库内部使用
SIGWINCH x x x x 忽略 终端窗口大小改变
SIGXCPU x x x x 终止+core 超过CPU资源限制（setrlimit）
SIGXFSZ x x x x 终止+core 超过文件长度限制（setrlimit）
SIGXRES x 忽略 超过资源限制
