信号

信号的位置：/usr/include/signal.h

信号的分类

  • 标准信号/不可靠信号：信号标识符1-32,不处于信号队列中，只能接收一次
  • 实时信号/可靠信号：信号标识符32-64,同一信号可以发送多次，被多次接收，存储在sigqueue中

信号的特点

  • 不可预测/随机性

  • 传送数据简单，信号描述符

信号的状态

  • generation

  • delivery

信号的产生

  • 键盘快捷键
  • 系统调用函数
  • 内核产生：硬件异常，被除数为0
    信号可以由线程产生，也可以由内核产生，但内核产生的终止信号通常是致命的

信号的响应

  • 默认:SIG_DEF通常是终止

  • 忽略:SIG_IGN

  • 用户自定义

==SIGKILL和SIGSTOP的默认行为不能修改，不能被忽略==

内核在信号中扮演的角色

当操作系统从内核态切换为用户态时，会检测==pending&~blocked==位图，如果位图置1则执行信号处理程序

多线程程序中的信号处理函数

对于多线程程序，kill函数会把信号传给每一个线程，当只有一个线程会执行信号处理函数

信号描述符

进程

  1. 记录信号是否被接受pending
  2. 记录屏蔽什么信号block
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/*进程描述符*/
struct task_struct {

    /* Signal handlers: */
    struct signal_struct        *signal;
    struct sighand_struct __rcu        *sighand; /*信号处理函数*/
    sigset_t            blocked;    /*64位屏蔽信号字*/
    sigset_t            real_blocked;
    /* Restored if set_restore_sigmask() was used: */
    sigset_t            saved_sigmask;
    struct sigpending        pending;    /*未决信号集合*/
    unsigned long            sas_ss_sp;  /*信号处理函数的栈地址*/
    size_t                sas_ss_size;    /*信号处理函数的栈空间大小*/
    unsigned int            sas_ss_flags;

};

sighand信号处理

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/*信号处理函数*/
struct sigaction {
    __sighandler_t    sa_handler;/*处理函数*/
    unsigned long    sa_flags; /*决定函数如何执行*/
    sigset_t    sa_mask;    /* mask last for extensibility */
};
struct k_sigaction {
    struct sigaction sa;
};

/*信号处理*/
struct sighand_struct {
    spinlock_t        siglock;//旋转锁，保护hander
    refcount_t        count;  //共享此结构体的线程数
    wait_queue_head_t    signalfd_wqh;
    struct k_sigaction    action[64];//每个信号对应的处理函数
};

进程task_struct包含成员sighand_struct,成员sighangd_struct包含一个64位的数组

其中k_sigaction是仅内核可见的部分

image-20230924133956541

未决信号集

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/*sigset_t:64位的位图*/
typedef struct {
    unsigned long sig[2];
} sigset_t;

//未决信号位图
struct sigpending {
    struct list_head list;
    sigset_t signal;
};
struct sigqueue {
    struct list_head list;
    int flags;
    kernel_siginfo_t info;
    struct ucounts *ucounts;
};

信号屏蔽字blocked和未决信号集pending都是sig_set(64位的位图)，使用unsigned int[2]表示

可靠信号多次发送后，会存储在未决信号链sigqueue中，双向链表节点存放着64位的信号位图

共享未决信号集和私有未决信号集

信号结构

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struct signal_struct {

    refcount_t        sigcnt;
    atomic_t        live;
    int            nr_threads;
    struct list_head    thread_head;

    wait_queue_head_t    wait_chldexit;    /* for wait4() */

    /* current thread group signal load-balancing target: */
    struct task_struct    *curr_target;

    /* shared signal handling: */
    struct sigpending    shared_pending;

    /* For collecting multiprocess signals during fork */
    struct hlist_head    multiprocess;
}

signal_struct被同一线程组所共享

信号相关调用

用户级别的系统调用

Kill

kill:向进程发送信号

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#include <sys/types.h>
#include <signal.h>
int kill(pid_t pid, int sig);
pid
pid>0 sent to the process with the ID specified by pid. 给指定pid的线程发送信号
pid=0 sent to every process in the process group of the calling process 给当前进程所在进程组的其他进程发送信号
pid=-1 sent to every process for which the calling process has permission to send signals, except for process 1 给允许接受当前进程发送的信号的进程发送信号，init进程除外
pid<-1 sent to every process in the process group whose ID is -pid. 给进程号为-pid的进程发送信号

返回值：成功返回0，失败返回-1

error

error explain
EINVAL 无效信号值(invalid)
EPERM 无权限发送(permission)
ESRCH 目标进程不存在（search)

signal

signal：设置信号的处理程序

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 #include <signal.h>
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
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void ( *signal(int signum, void (*handler)(int)) ) (int);

signal在不同unix版本中实现并不相同，signal更适合将信号的处理程序设置为忽略或则默认行为，建议使用sigaction替代signal

返回值：返回之前信号执行程序的返回值或者SIG_ERR

sigaction

sigaction:检测或修改信号处理程序

sigaction可携带siginfo，拥有更多的信息

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#include <signal.h>
int sigaction(int signum, const struct sigaction *act,struct sigaction *oldact);
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struct sigaction {
              void     (*sa_handler)(int);
              void     (*sa_sigaction)(int, siginfo_t *, void *);
              sigset_t   sa_mask; /*屏蔽信号作用*/
              int        sa_flags; 
              void     (*sa_restorer)(void);
          };

在一些体系中，sa_handler和sa_sigaction可能是共用体

通常，信号程序函数不使用第三个参数。

sa_flag的取值
SA_NOCLDSTOP If signum is SIGCHLD,do not receive notification when child processes stop 子进程结束后父进程不收到信号
SA_NOCLDWAIT If signum is SIGCHLD, do not transform children into zombies when they terminate. 父进程结束，子进程不变成僵尸态
SA_NODEFER Do not add the signal to the thread’s signal mask while the handler is executing, unless the signal is specified in act.sa_mask.
SA_ONSTACK Call the signal handler on an alternate signal stack provided by sigaltstack(2). 为函数指定栈空间
SA_RESETHAND Restore the signal action to the default upon entry to the signal handler. 恢复默认信号处理函数
SA_SIGINFO The signal handler takes three arguments 让hander强制接受三个参数，即使用sa_action

siginfo_t

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siginfo_t {
              int      si_signo;     /* Signal number */
              int      si_errno;     /* An errno value */
              int      si_code;      /* Signal code */
              int      si_trapno;    /* Trap number that caused
                                        hardware-generated signal
                                        (unused on most architectures) */
              pid_t    si_pid;       /* Sending process ID 目标进程的ID*/
              uid_t    si_uid;       /* Real user ID of sending process */
              int      si_status;    /* Exit value or signal */
              clock_t  si_utime;     /* User time consumed */
              clock_t  si_stime;     /* System time consumed */
              union sigval si_value; /* Signal value */
              int      si_int;       /* POSIX.1b signal */
              void    *si_ptr;       /* POSIX.1b signal */
              int      si_overrun;   /* Timer overrun count;
                                        POSIX.1b timers */
              int      si_timerid;   /* Timer ID; POSIX.1b timers */
              void    *si_addr;      /* Memory location which caused fault 默认执行程序的地址*/
              long     si_band;      /* Band event (was int in
                                        glibc 2.3.2 and earlier) */
              int      si_fd;        /* File descriptor */
              short    si_addr_lsb;  /* Least significant bit of address
                                        (since Linux 2.6.32) */
              void    *si_lower;     /* Lower bound when address violation
                                        occurred (since Linux 3.19) */
              void    *si_upper;     /* Upper bound when address violation
                                        occurred (since Linux 3.19) */
              int      si_pkey;      /* Protection key on PTE that caused
                                        fault (since Linux 4.6) */
              void    *si_call_addr; /* Address of system call instruction
                                        (since Linux 3.5) */
              int      si_syscall;   /* Number of attempted system call
                                        (since Linux 3.5) */
              unsigned int si_arch;  /* Architecture of attempted system call
                                        (since Linux 3.5) */
          }

返回值：成功返回0，失败返回-1

pause

等待一个信号的到来

abort

alarm

定时发送，被打断剩余时间会存起来

sleep

nanosleep

sigprocmask

sigsupen

信号集处理函数

sigaddset

sigfillset

sigaddsetmask

sigdelsetmask

sigemptyset

sigismember