c - 使用 c 安排警报

Question

我们的任务是开发一个创建子进程的应用程序。然后父进程等待3秒(我们不允许在父进程中使用sleep)并发送一个SIGUSR2信号后跟 SIGUSR1信号。之后， parent 发送 SIGUSR1定期(3 秒)向 child 发出信号，直到它终止。 child 终止后，它会打印 Parent done并退出。我们应该使用 alarm() 来实现这个行为和 pause() .

子进程应该阻塞信号 SIGUSR2持续 13 秒。收到后SIGUSR1它应该打印 Received SIGUSR1 .收到后SIGUSR2它打印 Child done并退出。我的实现方法如下:

请注意:SIGUSR2信号只发送一次，直到现在我还没有实现它并且每次都与 SIGUSR1 一起发送信号。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <stdarg.h>

static pid_t chld;

void init_signal(struct sigaction* action, void (*handler)(int));
void init_sigset(sigset_t* set, int signum);
void bind_signal(struct sigaction* action, int n, ...);
void par_signal(int signum);
void chld_signal(int signum);

int main(){
    chld = fork();
    struct sigaction action;

    if (chld == -1){
        fprintf(stderr, "Failed to create child process.\n");
        return EXIT_FAILURE;
    } else if (chld == 0){
        init_signal(&action, &chld_signal);
        bind_signal(&action, 3, SIGUSR1, SIGUSR2, SIGALRM);        

        sigset_t sig;
        init_sigset(&sig, SIGUSR2);

        sigprocmask(SIG_BLOCK, &sig, NULL);
        alarm(13);
        pause();
    }
    init_signal(&action, &par_signal);
    bind_signal(&action, 2, SIGALRM, SIGCHLD);

    alarm(3);
    pause();
}

void init_signal(struct sigaction* action, void (*handler)(int)){
    action->sa_flags = 0;
    action->sa_handler = handler;

    sigemptyset(&action->sa_mask);
}

void init_sigset(sigset_t* set, int signum){
    sigemptyset(set);
    sigaddset(set, signum);
}

void bind_signal(struct sigaction* action, int n, ...){
    va_list args;
    va_start(args, n);

    for (int i = 0; i < n; i++){
        int signum = va_arg(args, int);
        sigaction(signum, action, NULL);
    }
    va_end(args);
}

void par_signal(int signum){
    if (signum == SIGALRM){
        kill(chld, SIGUSR2);
        kill(chld, SIGUSR1);
        alarm(3);
        pause();
    } else if (signum == SIGCHLD){
        printf("Parent done\n");
        exit(EXIT_SUCCESS);
    }
}

void chld_signal(int signum){
    if (signum == SIGALRM){
        sigset_t sig;
        init_sigset(&sig, SIGUSR2);

        sigprocmask(SIG_UNBLOCK, &sig, NULL);
        pause();
    } else if (signum == SIGUSR1){
        printf("Received SIGUSR1\n");
        pause();
    } else if (signum == SIGUSR2){
        printf("Child done\n");
        exit(EXIT_SUCCESS);
    }
}

它的工作原理与事实不同，即只有父进程的第一个信号被子进程接收到。我认为这与我对 pause 的使用有关分别alarm .你知道我做错了什么吗？

可以找到练习任务表的链接 here (任务 3)。

Answer 1

在信号处理程序中调用 pause 是有问题的，因为在这种情况下，刚刚触发信号处理程序的信号会被阻塞，直到您从信号处理程序返回。这意味着该进程将不再对该信号使用react。信号处理程序只能由其他信号触发。

请参阅下面的系统调用跟踪和解释。为了清楚起见，我用 @PARENT@ 替换了父级的 PID，用 #CHILD## 替换了子级的 PID。

$ strace -f -r -e trace=signal,process ./proc
     0.000000 execve("./proc", ["./proc"], 0x7ffc146ba360 /* 74 vars */) = 0
     0.001428 arch_prctl(0x3001 /* ARCH_??? */, 0x7ffd3c958fc0) = -1 EINVAL (Invalid argument)
     0.002302 arch_prctl(ARCH_SET_FS, 0x7f264e40a540) = 0
# clone is fork()
     0.001086 clone(strace: Process #CHILD## attached
child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f264e40a810) = #CHILD##
# parent establishes SIGALRM handler
[pid @PARENT@]      0.000791 rt_sigaction(SIGALRM, {sa_handler=0x564709a5859c, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f264e25e470},  <unfinished ...>
# child establishes SIGUSR1 handler
[pid #CHILD##]      0.000087 rt_sigaction(SIGUSR1, {sa_handler=0x564709a58605, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f264e25e470},  <unfinished ...>
[pid @PARENT@]      0.000132 <... rt_sigaction resumed> NULL, 8) = 0
[pid #CHILD##]      0.000056 <... rt_sigaction resumed> NULL, 8) = 0
# child establishes SIGUSR2 handler
[pid #CHILD##]      0.000072 rt_sigaction(SIGUSR2, {sa_handler=0x564709a58605, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f264e25e470}, NULL, 8) = 0
# child establishes SIGALRM handler
[pid #CHILD##]      0.000141 rt_sigaction(SIGALRM, {sa_handler=0x564709a58605, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f264e25e470}, NULL, 8) = 0
# parent establishes SIGCHLD handler
[pid @PARENT@]      0.000389 rt_sigaction(SIGCHLD, {sa_handler=0x564709a5859c, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f264e25e470},  <unfinished ...>
# child blocks SIGUSR2
[pid #CHILD##]      0.000132 rt_sigprocmask(SIG_BLOCK, [USR2], NULL, 8) = 0
# child waits for signal
[pid #CHILD##]      0.000204 pause( <unfinished ...>
[pid @PARENT@]      0.000837 <... rt_sigaction resumed> NULL, 8) = 0
# parent waits for signal
[pid @PARENT@]      0.000133 pause()       = ? ERESTARTNOHAND (To be restarted if no handler)
# parent receives SIGALRM
[pid @PARENT@]      3.000317 --- SIGALRM {si_signo=SIGALRM, si_code=SI_KERNEL} ---
# parent sends SIGUSR2 and SIGUSR1 to child
[pid @PARENT@]      0.000106 kill(#CHILD##, SIGUSR2) = 0
[pid @PARENT@]      0.000116 kill(#CHILD##, SIGUSR1) = 0
[pid #CHILD##]      0.000144 <... pause resumed> ) = ? ERESTARTNOHAND (To be restarted if no handler)
# child receives SIGUSR1 (SIGUSR2 is blocked)
[pid #CHILD##]      0.000166 --- SIGUSR1 {si_signo=SIGUSR1, si_code=SI_USER, si_pid=@PARENT@, si_uid=1000} ---
# parent waits for signal again. As this "pause" was called from a signal handler triggered by SIGALRM, this signal is now blocked.
[pid @PARENT@]      0.000227 pause( <unfinished ...>
# child waits for signal. As this "pause" was called from a signal handler triggered by SIGUSR1, this signal is now blocked.
[pid #CHILD##]      0.000566 pause()       = ? ERESTARTNOHAND (To be restarted if no handler)
# child receives SIGALRM
[pid #CHILD##]      9.997742 --- SIGALRM {si_signo=SIGALRM, si_code=SI_KERNEL} ---
# child unblocks SIGUSR2
[pid #CHILD##]      0.000102 rt_sigprocmask(SIG_UNBLOCK, [USR2], NULL, 8) = 0
# child now receives SIGUSR2 which was already sent by the parent
[pid #CHILD##]      0.000122 --- SIGUSR2 {si_signo=SIGUSR2, si_code=SI_USER, si_pid=@PARENT@, si_uid=1000} ---
# child exits
[pid #CHILD##]      0.000183 exit_group(0) = ?
[pid #CHILD##]      0.000204 +++ exited with 0 +++
     0.000081 <... pause resumed> )     = ? ERESTARTNOHAND (To be restarted if no handler)
# parent receives SIGCHLD from child
     0.000056 --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=#CHILD##, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
Parent done
# parent exits
     0.000563 exit_group(0)             = ?
     0.000301 +++ exited with 0 +++

我建议从信号处理程序中设置标志，并在主函数中进行决策和等待。像这样

static volatile sig_atomic_t childSigUsr1 = 0;
static volatile sig_atomic_t childSigUsr2 = 0;
static volatile sig_atomic_t childSigAlrm = 0;

void chld_signal(int signum) {
    if (signum == SIGALRM) {
        childSigAlrm = 1;
    } else if (signum == SIGUSR1) {
        childSigUsr1 = 1;
    } else if (signum == SIGUSR2) {
        childSigUsr2 = 1;
    }
}

int main() {
    chld = fork();
    struct sigaction action;

    if (chld == -1) {
        fprintf(stderr, "Failed to create child process.\n");
        return EXIT_FAILURE;
    } else if (chld == 0) {
        init_signal(&action, &chld_signal);
        bind_signal(&action, 3, SIGUSR1, SIGUSR2, SIGALRM);        

        sigset_t sig;
        init_sigset(&sig, SIGUSR2);

        sigprocmask(SIG_BLOCK, &sig, NULL);
        alarm(13);
        while(1) {
            pause();
            if(childSigUsr1) {
                childSigUsr1 = 0;
                printf("Received SIGUSR1\n");
            }
            if(childSigUsr2) {
                childSigUsr2 = 0;
                printf("Child done\n");
                exit(EXIT_SUCCESS);
            }
            if(childSigAlrm) {
                sigset_t sig;
                init_sigset(&sig, SIGUSR2);

                sigprocmask(SIG_UNBLOCK, &sig, NULL);
            }
        }
    }
    init_signal(&action, &par_signal);
    bind_signal(&action, 2, SIGALRM, SIGCHLD);

    while(1) {
        alarm(3);
        pause();
        /* handle and reset signal flags similar to child */
    }
}