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atomic_bool值更新,其他进程看不到

c11 struct c

2trill2spill · 技术社区 · 11 年前

我有一个程序,它有两个进程与共享内存通信。在ctrl-c上,我希望两个进程都退出。我正在使用 atomic_bool 变量stop,通知进程保持循环或在设置为true时退出。然而,当 原子布尔 变量stop设置为true,则其他进程看不到更改。这意味着它仍然打印出0而不是1,但进行更改的过程显示为1。那么为什么第二个过程没有看到从false到true的更改?

Control-c无法终止进程,因此使用 killall 相反

#include <string.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>
#include <stdbool.h>
#include <stdatomic.h>
#include <sys/mman.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <errno.h>

struct shared_map
{

    atomic_bool stop;

};

struct shared_map *map;

int compare_and_swap_loop(atomic_bool target, int value)
{
    /* Loop until we can succesfully update the the value. */
    while(1)
    {
        /* Grab a snapshot of the value that need to be updated. */
        bool snapshot = atomic_load(&target);

        if(atomic_compare_exchange_weak(&target, &snapshot, value) == true)
        {
            /* We succesfully updated the value let's exit this loop and return. */
            break;
        }
    }

   printf("result: %d\n", atomic_load(&target));

    return 0;
}

static void ctrlc_handler(int sig)
{
    compare_and_swap_loop(&map->stop, true);

    return;
}

void setup_signal_handler(void)
{
    (void) signal(SIGINT, ctrlc_handler);

    return;
}

static int create_shared(void **pointer, int size)
{
    *pointer = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, -1, 0);
    if(*pointer == MAP_FAILED)
    {
        printf("mmap: %s\n", strerror(errno));
        return -1;
    }

    return 0;
}

static void loop(void)
{
    /* Set up signal handler. */
    setup_signal_handler();

    /* Check if we should stop or continue running. */
    while(atomic_load(&map->stop) == false)
    {
        sleep(2);

        printf("map->stop: %d\n", atomic_load(&map->stop));
    }

    return;
}

int main(void)
{
    int rtrn;
    pid_t pid;

    rtrn = create_shared((void **)&map, sizeof(struct shared_map));
    if(rtrn < 0)
    {
        printf("Can't create shared memory\n");
        return -1;
    }

    atomic_init(&map->stop, false);

    pid = fork();
    if(pid == 0)
    {
        loop();

        _exit(0);
    }
    else if(pid > 0)
    {
        int status;

        waitpid(pid, &status, 0);

        return 0;
    }
    else
    {
        printf("fork: %s\n", strerror(errno));
        return -1;
    }

    return 0;
}

2 回复 | 直到 11 年前

nos 11 年前

您正在将原子变量的副本传递给 compare_and_swap_loop 函数,这对您没有任何好处-您需要处理流程之间共享的相同值。

你需要这样做:

int compare_and_swap_loop(atomic_bool *target, int value)
{
   /* Loop until we can succesfully update the the value. */
   while(1)
   {
    /* Grab a snapshot of the value that need to be updated. */
       bool snapshot = atomic_load(target);

       if(atomic_compare_exchange_weak(target, &snapshot, value) == true)
       {
        /* We succesfully updated the value let's exit this loop and return. */
        break;
       }
   }

   printf("result: %d\n", atomic_load(target));

   return 0;
}

user3629249 11 年前

以下代码是在不使用“atomic_*”命令的情况下编写的但确实显示了流程中的错误。

大多数情况下,需要杀死孩子pid,而不是父母pid

以下代码显示子pid,因此很容易找到

#define _GNU_SOURCE

#include <string.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>
#include <stdbool.h>
//#include <stdatomic.h>
#include <sys/mman.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <errno.h>

struct shared_map
{
    bool stop;
};

struct shared_map *map;

int compare_and_swap_loop( bool *target )
{
    /* Loop until we can succesfully update the the value. */
    while(1)
    {
        /* Grab a snapshot of the value that need to be updated. */
        bool snapshot = *target;

        if(snapshot)
        {
            /* We succesfully updated the value let's exit this loop and return. */
            break;
        }
    }

    return 0;
}

static void ctrlc_handler(int sig)
{
    if( SIGINT==sig)
    compare_and_swap_loop(&map->stop);

    return;
}

void setup_signal_handler(void)
{
    (void) signal(SIGINT, ctrlc_handler);

    return;
}

static int create_shared(void **pointer, size_t size)
{
    *pointer = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, -1, 0);
    if(*pointer == MAP_FAILED)
    {
        perror("mmap failed");
        return -1;
    }

    return 0;
}

static void loop(void)
{
    /* Set up signal handler. */
    setup_signal_handler();

    /* Check if we should stop or continue running. */
    while(!map->stop)
    {
        sleep(2);

        printf("map->stop: %d\n", map->stop);
    }

    return;
}

int main(void)
{
    int rtrn;
    pid_t pid;

    printf( "entered Main\n");

    rtrn = create_shared((void **)&map, sizeof(struct shared_map));
    if(rtrn < 0)
    {
        printf("Can't create shared memory\n");
        return -1;
    }

    map->stop = false;

    pid = fork();
    if(pid == 0)
    { //then child
        printf( "child process\n");
        loop();

        _exit(0);
    }
    else if(pid > 0)
    { // then parent
        int status;
        printf( "parent process\n");
        printf( "child Pid: %d\n", pid);

        waitpid(pid, &status, 0);

        return 0;
    }
    else
    { // else, fork failed
        perror("fork failed");
        return -1;
    }

    return 0;
}