转自：

上一篇博文，基本算是给glibc的signal函数翻了个身。现在glibc的signal基本修正了传统的UNIX的一些弊端，我们说signal并没有我们想象的那么不堪。但是signal也有不尽人意的地方。比如信号处理期间，我们期望屏蔽某些信号，而不仅仅是屏蔽自身，这时候signal就不行了。信号既然是进程间通信IPC的一种机制，我们期望获取更多的信息，而不仅仅是signo，这时候signal/kill这个机制就基本不行了。

    上面所说的都是signal的一些毛病，但是这些都不是致命的，致命的问题在于老的signal机制的不可靠。信号分成可靠性信号和非可靠性信号，并不是说用sigaction安装，用sigqueue发送的信号就是可靠性性信号，用signal安装，kill/tkill发送的信号就是非可靠性信号。这种理解是错误的。这在Linux环境进程间通信（二）：信号（上）一文中讲的非常清楚了。

    信号值位于[SIGRTMIN,SIGRTMAX] 之间的信号，就是可靠信号，位于[SIGHUP,SIGSYS]之间信号，都是非可靠性信号，与安装函数是signal还是sigaction无关，与发送函数是kill还是sigqueue无关。

    1～31之间的所有信号都称为不可靠信号，原因就在于信号不可排队，如果kernel发现同一个信号已经有挂起信号，当前信号就会被丢弃，就好象从来没有被发送过一样，无法引起信号的传递，也无法让进程执行信号处理函数。这种实现的机理，造成了这些信号的不可靠。这正所谓：我本将心向明月，奈何明月照沟渠。

    为了解决这个问题，Linux引入了实时信号,信号值在[32~64]区间内，或者称之为可靠信号。这种信号，kernel不会ignore，哪怕已经有了好多同一个信号，kernel会把新收到信号放入queue之中，等待被传递出去。

    空口说白话，不是我们的风格，我现在用代码证明之。我参考了Linux Programming Interface 一书的例子，写了两个程序，一个是signal_receiver ,一个是signal_sender.

    先看signal_receiver的code：    

 manu@manu-hacks:~/code/c/self/signal$ cat signal_receiver.c
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <signal.h>
 #include <string.h>
 #include <errno.h>


 static int sig_cnt[NSIG];
 static volatile sig_atomic_t get_SIGINT = 0;

 void handler(int signo)
 {
     if(signo == SIGINT)
         get_SIGINT = 1;
     else
         sig_cnt[signo]++;
 }

 int main(int argc,char* argv[])
 {
     int i = 0;
     sigset_t blockall_mask ;
     sigset_t pending_mask ;
     sigset_t empty_mask ;
     printf("%s:PID is %ld\n",argv[0],getpid());


     for(i = 1; i < NSIG; i++)
     {
         if(i == SIGKILL || i == SIGSTOP)
             continue;

         if(signal(i,&handler) == SIG_ERR)
         {
             fprintf(stderr,"signal for signo(%d) failed (%s)\n",i,strerror(errno));
 //            return -1;
         }
     }

     if(argc > 1)
     {
         int sleep_time = atoi(argv[1]);
         sigfillset(&blockall_mask);

         if(sigprocmask(SIG_SETMASK,&blockall_mask,NULL) == -1)
         {
             fprintf(stderr,"setprocmask to block all signal failed(%s)\n",strerror(errno));
             return -2;
         }

         printf("I will sleep %d second\n",sleep_time);

         sleep(sleep_time);
         if(sigpending(&pending_mask) == -1)
         {
             fprintf(stderr,"sigpending failed(%s)\n",strerror(errno));
             return -2;
         }

         for(i = 1 ; i < NSIG ; i++)
         {
             if(sigismember(&pending_mask,i))
             printf("signo(%d) :%s\n",i,strsignal(i));
         }

         sigemptyset(&empty_mask);
         if(sigprocmask(SIG_SETMASK,&empty_mask,NULL) == -1)
         {
             fprintf(stderr,"setprocmask to release all signal failed(%s)\n",strerror(errno));
             return -3;
         }

     }

     while(!get_SIGINT)
         continue ; //why not use pause ? I will explain later

     for(i = 1; i < NSIG ; i++)
     {
         if(sig_cnt[i] != 0 )
         {
             printf("%s:signal %d caught %d time%s\n",
                     argv[0],i,sig_cnt[i],(sig_cnt[i] >1)?"s":"");
         }
     }

     return 0;

 }

     因为我们知道，SIGKILL和SIGSTOP这两个信号是不能够定制自己的信号处理函数的，当然也不能block，原因很简单，OS或者说root才是final boss，必须有稳定终结进程的办法。假如所有的信号，进程都能ignore，OS如何终结进程？

    这个signal_receiver会等待所有的信号，接收到某信号后，该信号的捕捉到的次数++，SIGINT会终结进程，进程退出前，会打印信号的捕捉统计。

    如果进程有参数，表示sleep时间，signal_receiver会先屏蔽所有信号（当然，SIGKILL和SIGSTOP并不能被真正屏蔽）。然后sleep 一段时间后，取消信号屏蔽。我们可以想象，在信号屏蔽期间，我们收到的信号，都会在kernel记录下来，但是并不能delivery，这种信号称之挂起信号。如果在sleep期间或者说信号屏蔽期间，我收到SIGUSR1 这个信号1次和10000次，对内核来说，都是没差别的，因为后面的9999次都会被ignore掉。SIGUSR1属于不可靠信号，位图表示有没有挂起信号，有的话，直接ignore，没有的话，则记录在kernel。

    然后我们看下，signal_sender： 

 manu@manu-hacks:~/code/c/self/signal$ cat signal_sender.c
 #include <stdio.h>
 #include <stdlib.h>
 #include <getopt.h>
 #include <signal.h>
 #include <string.h>
 #include <errno.h>

 void usage()
 {
     fprintf(stderr,"USAGE:\n");
     fprintf(stderr,"--------------------------------\n");
     fprintf(stderr,"signal_sender pid signo times\n");
 }

 int main(int argc,char* argv[])
 {
     pid_t pid = -1 ;
     int signo = -1;
     int times = -1;
     int i ;


     if(argc < 4 )
     {
         usage();
         return -1;
     }

     pid = atol(argv[1]);
     signo = atoi(argv[2]);
     times = atoi(argv[3]);

     if(pid <= 0 || times < 0 || signo <1 ||signo >=64 ||signo == 32 || signo ==33)
     {
         usage();
         return -1;
     }

     printf("pid = %ld,signo = %d,times = %d\n",pid,signo,times);

     for( i = 0 ; i < times ; i++)
     {
         if(kill(pid,signo) == -1)
         {
             fprintf(stderr, "send signo(%d) to pid(%ld) failed,reason(%s)\n",signo,pid,strerror(errno));
             return -2;
         }
     }
     fprintf(stdout,"done\n");
     return 0;

 }

     signal_sender需要三个参数，pid signo times，就是向拿个进程发送什么信号多少次的意思。如 signal_sender 1234 10 10000,含义是向pid=1234的 进程发送10号信号（SIGUSR1），连续发送10000次。

    有这两个进程，我们就可以实验了  。 

 manu@manu-hacks:~/code/c/self/signal$ ./signal_receiver &
 [1] 23416
 manu@manu-hacks:~/code/c/self/signal$ ./signal_receiver:PID is 23416
 signal for signo(32) failed (Invalid argument)
 signal for signo(33) failed (Invalid argument)

 manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 23416 10 10000
 pid = 23416,signo = 10,times = 10000
 done
 manu@manu-hacks:~/code/c/self/signal$ sleep 20 ; ./signal_sender 23416 2 1
 pid = 23416,signo = 2,times = 1
 done
 ./signal_receiver:signal 10 caught 2507 times
 [1]+ Done ./signal_receiver

    signal_receiver等待signal的来临，singal_sender向其发送SIGUSR1 10000次，然后sleep 20秒，确保sig_receiver处理完成。但是我们发现，其实一共才caught信号SIGUSR1  2507次，7000多次的发送都丢失了，所以我们称SIGUSR1 是非可靠信号，存在丢信号的问题。

    俗话说不怕不识货，就怕货比货 ，我们让可靠信号参战，看下效果：

 manu@manu-hacks:~/code/c/self/signal$ ./signal_receiver &
 [1] 26067
 ./signal_receiver:PID is 26067
 signal for signo(32) failed (Invalid argument)
 signal for signo(33) failed (Invalid argument)
 manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 26067 10 10000
 pid = 26067,signo = 10,times = 10000
 done
 manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 26067 36 10000
 pid = 26067,signo = 36,times = 10000
 done
 manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 26067 2 1
 pid = 26067,signo = 2,times = 1
 done
 ./signal_receiver:signal 10 caught 2879 times
 ./signal_receiver:signal 36 caught 10000 times
 [1]+ Done ./signal_receiver

    可靠性信号36，发送10000次，signal_receiver全部收到，不可靠性信号10，共收到2879次。这个数字是不可预期的，取决于内核进程的调度。

    这个如果还不够直观，我们在比较一次，让signal_receiver先屏蔽所有信号一段时间，如30s，然后解除屏蔽。

 manu@manu-hacks:~/code/c/self/signal$ ./signal_receiver 30 &
 [1] 27639
 manu@manu-hacks:~/code/c/self/signal$ ./signal_receiver:PID is 27639
 signal for signo(32) failed (Invalid argument)
 signal for signo(33) failed (Invalid argument)
 I will sleep 30 second

 manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 27639 10 10000
 pid = 27639,signo = 10,times = 10000
 done
 manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 27639 36 10000
 pid = 27639,signo = 36,times = 10000
 done
 manu@manu-hacks:~/code/c/self/signal$
 manu@manu-hacks:~/code/c/self/signal$ signo(10) :User defined signal 1
 signo(36) :Real-time signal 2

 manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 27639 2 1
 pid = 27639,signo = 2,times = 1
 done
 ./signal_receiver:signal 10 caught 1 time
 ./signal_receiver:signal 36 caught 10000 times
 [1]+ Done ./signal_receiver 30

      这个比较反差比较大，不可靠signal10 共收到1次，可靠性信号36 共caught到10000次。原因就在于sigprocmask将所有的信号都屏蔽了，造成所有的信号都不能delivery。对1～31的信号，内核发现已经有相应的挂起信号，则ignore到新来的信号。但是可靠性信号则不同，会添加队列中去，尽管已经有了相同的信号。需要注意的是，signal pending有上限，并不能无限制的发：

 manu@manu-hacks:~/code/c/self/signal$ ulimit -a
 core file size (blocks, -c) 0
 data seg size (kbytes, -d) unlimited
 scheduling priority (-e) 0
 file size (blocks, -f) unlimited
 pending signals (-i) 15408
 max locked memory (kbytes, -l) 64
 max memory size (kbytes, -m) unlimited
 open files (-n) 1024
 pipe size (512 bytes, -p) 8
 POSIX message queues (bytes, -q) 819200
 real-time priority (-r) 0
 stack size (kbytes, -s) 8192
 cpu time (seconds, -t) unlimited
 max user processes (-u) 15408
 virtual memory (kbytes, -v) unlimited
 file locks (-x) unlimited

    我发送100万，最终会收到15408个可靠信号：

 manu@manu-hacks:~/code/c/self/signal$ ./signal_receiver 30 &
 [1] 16488
 manu@manu-hacks:~/code/c/self/signal$ ./signal_receiver:PID is 16488
 signal for signo(32) failed (Invalid argument)
 signal for signo(33) failed (Invalid argument)
 I will sleep 30 second

 manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 16488 36 1000000
 pid = 16488,signo = 36,times = 1000000
 done
 manu@manu-hacks:~/code/c/self/signal$ signo(36) :Real-time signal 2

 manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 16488 2 1
 pid = 16488,signo = 2,times = 1
 done
 ./signal_receiver:signal 36 caught 15408 times
 [1]+ Done ./signal_receiver 30

      内核是怎么做到的？

    

    上图是内核中signal相关的数据结构。其中task_struct中有sigpending类型的成员变量pending

    

 struct task_struct {
     volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
     void *stack;
     atomic_t usage;
     unsigned int flags; /* per process flags, defined below */
     unsigned int ptrace;
         ...
         ...
 /* signal handlers */
     struct signal_struct *signal;
     struct sighand_struct *sighand;

     sigset_t blocked, real_blocked;
     sigset_t saved_sigmask; /* restored if set_restore_sigmask() was used */
     struct sigpending pending;

        ...
 }

 struct signal_struct {
       atomic_t     sigcnt;
       atomic_t     live;
       int     nr_threads;
       ...
       ...
       /* shared signal handling: */
       struct sigpending    shared_pending;
       ...
 }

 struct sigpending {
     struct list_head list;
     sigset_t signal;
 };

 #define _NSIG        64

 #ifdef __i386__
 # define _NSIG_BPW    32
 #else
 # define _NSIG_BPW    64
 #endif

 #define _NSIG_WORDS    (_NSIG / _NSIG_BPW)

 typedef unsigned long old_sigset_t;        /* at least 32 bits */

 typedef struct {
     unsigned long sig[_NSIG_WORDS];
 } sigset_t;

      task_struct中的pending，和signal->shared_pending都是记录挂起信号的数据结构，读到此处，你可能会迷惑，为何有两个这样的结构。这牵扯到thread与信号的一些问题，我们此处简化，就认为是一个就好，后面讲述线程与信号关系的时候，再展开。

      

    我们看到了，kill也好，tkill也罢，最终都走到了_send_signal.当然了kill系统调用根据pid的情况会分成多个分支如pid >0 pid = 0 pid=-1;pid < 0&pid !=-1,总之了，我的图只绘制了pid >0 的分支。tkill也有类似情况。

    那么kernel是怎么做到的非可靠信号和可靠信号的的这些差别的呢？

 static int __send_signal(int sig, struct siginfo *info, struct task_struct *t,
             int group, int from_ancestor_ns)
 {
     struct sigpending *pending;
     struct sigqueue *q;
     int override_rlimit;
     int ret = 0, result;

     assert_spin_locked(&t->sighand->siglock);

     result = TRACE_SIGNAL_IGNORED;
     if (!prepare_signal(sig, t,
             from_ancestor_ns || (info == SEND_SIG_FORCED)))
         goto ret;

     pending = group ? &t->signal->shared_pending : &t->pending;
     /*
      * Short-circuit ignored signals and support queuing
      * exactly one non-rt signal, so that we can get more
      * detailed information about the cause of the signal.
      */
     result = TRACE_SIGNAL_ALREADY_PENDING;
     if (legacy_queue(pending, sig)) //如果是低于32的信号，并且已经在pending中出现了的信号，就直接返回了，ignore
         goto ret;

     result = TRACE_SIGNAL_DELIVERED;
     /*
      * fast-pathed signals for kernel-internal things like SIGSTOP
      * or SIGKILL.
      */
     if (info == SEND_SIG_FORCED)
         goto out_set;

     /*
      * Real-time signals must be queued if sent by sigqueue, or
      * some other real-time mechanism. It is implementation
      * defined whether kill() does so. We attempt to do so, on
      * the principle of least surprise, but since kill is not
      * allowed to fail with EAGAIN when low on memory we just
      * make sure at least one signal gets delivered and don't
      * pass on the info struct.
      */
     if (sig < SIGRTMIN)
         override_rlimit = (is_si_special(info) || info->si_code >= 0);
     else
         override_rlimit = 0;

      //分配sigqueue结构，并且链入到相应的pending。

     q = __sigqueue_alloc(sig, t, GFP_ATOMIC | __GFP_NOTRACK_FALSE_POSITIVE,
         override_rlimit);
     if (q) {
         list_add_tail(&q->list, &pending->list);
         switch ((unsigned long) info) {
         case (unsigned long) SEND_SIG_NOINFO:
             q->info.si_signo = sig;
             q->info.si_errno = 0;
             q->info.si_code = SI_USER;
             q->info.si_pid = task_tgid_nr_ns(current,
                             task_active_pid_ns(t));
             q->info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
             break;
         case (unsigned long) SEND_SIG_PRIV:
             q->info.si_signo = sig;
             q->info.si_errno = 0;
             q->info.si_code = SI_KERNEL;
             q->info.si_pid = 0;
             q->info.si_uid = 0;
             break;
         default:
             copy_siginfo(&q->info, info);
             if (from_ancestor_ns)
                 q->info.si_pid = 0;
             break;
         }

         userns_fixup_signal_uid(&q->info, t);

     } else if (!is_si_special(info)) {
         if (sig >= SIGRTMIN && info->si_code != SI_USER) {
             /*
              * Queue overflow, abort. We may abort if the
              * signal was rt and sent by user using something
              * other than kill().
              */
             result = TRACE_SIGNAL_OVERFLOW_FAIL;
             ret = -EAGAIN;
             goto ret;
         } else {
             /*
              * This is a silent loss of information. We still
              * send the signal, but the *info bits are lost.
              */
             result = TRACE_SIGNAL_LOSE_INFO;
         }
     }

 out_set:
     signalfd_notify(t, sig);
     sigaddset(&pending->signal, sig);  //加入位图
     complete_signal(sig, t, group);
 ret:
     trace_signal_generate(sig, info, t, group, result);
     return ret;
 }

 static inline int legacy_queue(struct sigpending *signals, int sig)
 {
     return (sig < SIGRTMIN) && sigismember(&signals->signal, sig); //是不可靠信号，并且该信号已经存在挂起信号，

    那么15408的限制是在哪里呢？在__sigqueue_alloc 里面。

 static struct sigqueue *
 __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimit)
 {
     struct sigqueue *q = NULL;
     struct user_struct *user;

     /*
      * Protect access to @t credentials. This can go away when all
      * callers hold rcu read lock.
      */
     rcu_read_lock();
     user = get_uid(__task_cred(t)->user);
     atomic_inc(&user->sigpending);    //计数器+1
     rcu_read_unlock();

     if (override_rlimit ||
      atomic_read(&user->sigpending) <=
             task_rlimit(t, RLIMIT_SIGPENDING)) {
         q = kmem_cache_alloc(sigqueue_cachep, flags);
     } else {
         print_dropped_signal(sig);
     }

     if (unlikely(q == NULL)) {
         atomic_dec(&user->sigpending);
         free_uid(user);
     } else {
         INIT_LIST_HEAD(&q->list);
         q->flags = 0;
         q->user = user;
     }

     return q;
 }

     我们看到，legacy_queue就是用来判断是否是非可靠信号（signo低于32），并且相同signo值已经存在在挂起信号之中，如果是，直接返回。

     而对于可靠信号，会分配一个sigqueue的结构，然后讲sigqueue链入到sigpending结构的中链表中。从而就不会丢失信号。当然对pending信号的总数作了限制，限制最多不可超过15408.当然了这个值是可以修改的：

    

参考文献：

1 Linux programming interface

2 深入理解linux内核

3 linux kernel 3.8.0 内核源码