Linux信号sigaction / signal

Linux信号sigaction / signal

目的

我们可以通过sigaction函数或者signal指定某种特定信号，在程序执行过程中，通过发送信号，达到改变程序运行状态的目的，比如让程序停止等。

函数原型

获取或者设定与指定信号相关联的处理动作。

/* Get and/or set the action for signal SIG.  */
extern int sigaction (int __sig, const struct sigaction *__restrict __act,
		      struct sigaction *__restrict __oact) __THROW;
		      
extern __sighandler_t signal (int __sig, __sighandler_t __handler)
     __THROW;

sigaction函数执行信号处理程序时，会把当前信号加入到进程的信号屏蔽字中，从而防止在进行信号处理期间信号丢失。

对signal()，Linux默认会自动重启动被中断的系统调用；

而对于 sigaction()，Linux默认并不会自动重启动，所以如果希望执行信号处理后自动重启动先前中断的系统调用，就需要为sa_flags指定 SA_RESTART标志。

struct sigaction

当信号到达时，用于描述采取的动作的结构

/* Structure describing the action to be taken when a signal arrives.  */
struct sigaction
  {
    /* Signal handler.  */
#if defined __USE_POSIX199309 || defined __USE_XOPEN_EXTENDED
    union
      {
	/* Used if SA_SIGINFO is not set.  */
	__sighandler_t sa_handler;
	/* Used if SA_SIGINFO is set.  */
	void (*sa_sigaction) (int, siginfo_t *, void *);
      }
    __sigaction_handler;
# define sa_handler	__sigaction_handler.sa_handler
# define sa_sigaction	__sigaction_handler.sa_sigaction
#else
    __sighandler_t sa_handler;
#endif

    /* Additional set of signals to be blocked.  */
    __sigset_t sa_mask;

    /* Special flags.  */
    int sa_flags;

    /* Restore handler.  */
    void (*sa_restorer) (void);
  };

• sa_handler 和signal()的参数__handler相同，代表新的信号处理函数
• sa_mask 表示处理信号时，需要阻止的信号集
• sa_flags 处理信号时的标志位
  • SA_RESETHAND：在调用处理函数前，将信号的处理函数重置为缺省值（即SIG_IGN）
  • SA_RESTART：执行信号处理后，自动重启先前中断的系统调用
  • SA_NODEFER ：一般情况下， 当信号处理函数运行时，内核将阻塞该给定信号。但是如果设置了 SA_NODEFER标记， 那么在该信号处理函数运行时，内核将不会阻塞该信号
  • SA_ONSTACK：使用由sigaltstack()安装的备选栈
  • SA_SIGINFO：调用信号处理程序时携带了额外参数（提供了关于信号的深入信息）

信号枚举值

ISO C99 signals.

符号	数值	含义
SIGINT	2	Interactive attention signal.
SIGILL	4	Illegal instruction.
SIGABRT	6	Abnormal termination.
SIGFPE	8	Erroneous arithmetic operation.
SIGSEGV	11	Invalid access to storage.
SIGTERM	15	Termination request.

Historical signals specified by POSIX.

符号	数值	含义
SIGHUP	1	Hangup.
SIGQUIT	3	Quit.
SIGTRAP	5	Trace/breakpoint trap.
SIGKILL	9	Killed.
SIGBUS	10	Bus error.
SIGSYS	12	Bad system call.
SIGPIPE	13	Broken pipe.
SIGALRM	14	Alarm clock.

New(er) POSIX signals (1003.1-2008, 1003.1-2013).

符号	数值	含义
SIGURG	16	Urgent data is available at a socket.
SIGSTOP	17	Stop, unblockable.
SIGTSTP	18	Keyboard stop.
SIGCONT	19	Continue.
SIGCHLD	20	Child terminated or stopped.
SIGTTIN	21	Background read from control terminal.
SIGTTOU	22	Background write to control terminal.
SIGPOLL	23	Pollable event occurred (System V).
SIGXCPU	24	CPU time limit exceeded.
SIGXFSZ	25	File size limit exceeded.
SIGVTALRM	26	Virtual timer expired.
SIGPROF	27	Profiling timer expired.
SIGUSR1	30	User-defined signal 1.
SIGUSR2	31	User-defined signal 2.

Nonstandard signals found in all modern POSIX systems(including both BSD and Linux).

符号	数值	含义
SIGWINCH	28	Window size change (4.3 BSD, Sun).

测试

1 处理信号后重启系统调用

#include 
#include "stdio.h"
#include 
#include 

void signalHandle(int signum) {
	if(signum  = SIGINT) {
        std::cout << "SIGINT recived" << std::endl;
	}
}

int main(void){
	struct sigaction act;

	sigemptyset(&act.sa_mask);
	//这里使用SA_RESTART执行信号处理后自动重启到先前中断的系统调用，可以多次捕捉信号
	act.sa_flags = (SA_SIGINFO|SA_RESTART);
	act.sa_handler = signalHandle;
	sigaction(SIGINT, &act, NULL);

	while(1){

    }
}

当按下 CTRL + c时，会一直打印。

$ ./main 
^CSIGINT recived
^CSIGINT recived
^CSIGINT recived

2 处理信号后不重启系统调用

#include 
#include 
#include 
#include 
#include 

void show_handler(int sig)
{
    printf("I got signal %dn\n", sig);
    int i;
    for(i = 0; i < 5; i++) 
   {
        printf("i = %dn\n", i);
        sleep(1);
    }
}

int main(void)
{
    // section 1
    int i = 0;
    struct sigaction act, oldact;
    act.sa_handler = show_handler;
    sigaddset(&act.sa_mask, SIGQUIT);         
    act.sa_flags = SA_RESETHAND | SA_NODEFER; 
    //act.sa_flags = 0;                       
 
    sigaction(SIGINT, &act, &oldact);
    while(1) 
   {
        sleep(1);
        printf("sleeping %dn\n", i);
        i++;
    }
    return 0;
}

程序起来后，控制台输入CTRL + c，能够接收到信号。再次输入时，程序退出。

(base) qiancj@qiancj-Dell-G15-5510:~/codes/test/build$ ./main 
sleeping 0
sleeping 1
^CI got signal 2
i = 0
i = 1
i = 2
i = 3
i = 4
sleeping 2
^C
(base) qiancj@qiancj-Dell-G15-5510:~/codes/test/build$ 

3 使用 sigaction 函数为 SIGUSR1 和 SIGUSR2 信号注册处理函数

#include 
#include 
#include 
#include 
#include 

static void sig_usr(int signum)
{
    if(signum == SIGUSR1)
    {
        printf("SIGUSR1 received\n");
    }
    else if(signum == SIGUSR2)
    {
        printf("SIGUSR2 received\n");
    }
    else
    {
        printf("signal %d received\n", signum);
    }
}


int main(void)
{
// section 2
   char buf[512];
    int  n;
    struct sigaction sa_usr;
    sa_usr.sa_flags = 0;
    sa_usr.sa_handler = sig_usr;   //信号处理函数
    
    sigaction(SIGUSR1, &sa_usr, NULL);
    sigaction(SIGUSR2, &sa_usr, NULL);
    
    printf("My PID is %dn\n", getpid());
    
    while(1)
    {
        if((n = read(STDIN_FILENO, buf, 511)) == -1)
        {
            if(errno == EINTR)
            {
                printf("read is interrupted by signal\n");
            }
        }
        else
        {
            buf[n] = '0';
            printf("%d bytes read: %s\n", n, buf);
        }
    }
    return 0;
}

当一个终端启动程序，获取当前PID号(13770)，另一个终端输入kill -USR1 32230或者kill -USR2 32230，第一个终端就会收到中断信号

(base) qiancj@qiancj-Dell-G15-5510:~/codes/test/build$ ./main
My PID is 32230n

SIGUSR1 received
read is interrupted by signal
SIGUSR2 received
read is interrupted by signal

实际代码框架

当程序需要一直运行时，需要人为中断停止时，可以使用信号函数，具体框架可以设置为以下2种：

1. signal

#include 
#include 
#include 
#include 
#include 

sig_atomic_t stop_flag = 0;

void INTSigHandler(int32_t num) {
  stop_flag = 1;
  std::cout << "  Signal Interactive attention " << num << " received."
            << std::endl;
}

int main(int argc, char **argv) {
  signal(SIGINT, INTSigHandler);

  // work always....
  ...
   
  while ((!stop_flag)) {
    LOG_INFO << "I am working...";
    std::this_thread::sleep_for(std::chrono::seconds(1));
  }

  // Stop working! Go to rest!

  return 0;
}

2 将程序放入线程中

#include 
#include 
#include 
#include 
#include 

namespace {

std::atomic_bool continueExecution{true};

void SigTermHandler(int signal) {
  if (signal == SIGTERM || signal == SIGINT) {
    // set atomic exit flag
    continueExecution = false;
  }
}

bool RegisterSigTermHandler() {
  struct sigaction sa;
  sa.sa_handler = SigTermHandler;
  sa.sa_flags = 0;
  sigemptyset(&sa.sa_mask);
  // register signal handler
  if (sigaction(SIGTERM, &sa, NULL) == -1) {
    // Could not register a SIGTERM signal handler
    return false;
  }

  if (sigaction(SIGINT, &sa, NULL) == -1) {
    // Could not register a SIGTERM signal handler
    return false;
  }
  return true;
}

}  // namespace

void ThreadAct1();

void ThreadAct1() {
  while (continueExecution) {
    // always work...
    ...
    // sleep
    std::this_thread::sleep_for(std::chrono::milliseconds(10000));
  }
}

int main(int argc, char *argv[]) {

  if (!RegisterSigTermHandler()) {
    std::cout << "Unable to register signal handler" << std::endl;
  }

  // start thread
  std::thread act1(ThreadAct1);
  act1.join();

  return 0;
}

终端间通过kill发送信号

(base) qiancj@qiancj-Dell-G15-5510:~$ kill --help
kill: kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigspec]
    Send a signal to a job.
    
    Send the processes identified by PID or JOBSPEC the signal named by
    SIGSPEC or SIGNUM.  If neither SIGSPEC nor SIGNUM is present, then
    SIGTERM is assumed.
    
    Options:
      -s sig	SIG is a signal name
      -n sig	SIG is a signal number
      -l	list the signal names; if arguments follow `-l' they are
    		assumed to be signal numbers for which names should be listed
      -L	synonym for -l
    
    Kill is a shell builtin for two reasons: it allows job IDs to be used
    instead of process IDs, and allows processes to be killed if the limit
    on processes that you can create is reached.
    
    Exit Status:
    Returns success unless an invalid option is given or an error occurs.

信号种类

(base) qiancj@qiancj-Dell-G15-5510:~$ kill -l
 1) SIGHUP	 2) SIGINT	 3) SIGQUIT	 4) SIGILL	 5) SIGTRAP
 6) SIGABRT	 7) SIGBUS	 8) SIGFPE	 9) SIGKILL	10) SIGUSR1
11) SIGSEGV	12) SIGUSR2	13) SIGPIPE	14) SIGALRM	15) SIGTERM
16) SIGSTKFLT	17) SIGCHLD	18) SIGCONT	19) SIGSTOP	20) SIGTSTP
21) SIGTTIN	22) SIGTTOU	23) SIGURG	24) SIGXCPU	25) SIGXFSZ
26) SIGVTALRM	27) SIGPROF	28) SIGWINCH	29) SIGIO	30) SIGPWR
31) SIGSYS	34) SIGRTMIN	35) SIGRTMIN+1	36) SIGRTMIN+2	37) SIGRTMIN+3
38) SIGRTMIN+4	39) SIGRTMIN+5	40) SIGRTMIN+6	41) SIGRTMIN+7	42) SIGRTMIN+8
43) SIGRTMIN+9	44) SIGRTMIN+10	45) SIGRTMIN+11	46) SIGRTMIN+12	47) SIGRTMIN+13
48) SIGRTMIN+14	49) SIGRTMIN+15	50) SIGRTMAX-14	51) SIGRTMAX-13	52) SIGRTMAX-12
53) SIGRTMAX-11	54) SIGRTMAX-10	55) SIGRTMAX-9	56) SIGRTMAX-8	57) SIGRTMAX-7
58) SIGRTMAX-6	59) SIGRTMAX-5	60) SIGRTMAX-4	61) SIGRTMAX-3	62) SIGRTMAX-2
63) SIGRTMAX-1	64) SIGRTMAX	

Reference

• linux中sigaction函数详解
• sigaction详细解析