小编典典

无法读取共享内存

linux

我试图在共享内存上发布一些随机的东西;出于某些奇怪的原因,阅读器没有选择发件人写的东西

#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>
#include <sys/types.h>
#include <cstdio>

class SHM {
    volatile char* _ptr;
public:
    SHM() {
        const auto handle = shm_open("myTest", O_RDWR|O_CREAT, 0666);
        const auto size =  4 * 1024 * 1024;
        if (-1 == ftruncate(handle, size)) {
            throw;
        }
        _ptr = (volatile char*)mmap(0,size , PROT_READ | PROT_WRITE, MAP_SHARED, handle, 0);

        if(_ptr == MAP_FAILED){
            throw;
        }

                int rc = fchmod(handle, 0666);
                if (rc == -1) {
            throw;
                }
    }

    bool read(uint64_t& magic, uint64_t& time) {
        const uint64_t newVal = *(uint64_t*)_ptr;
        if (newVal != magic) {
            magic = newVal;
            printf("value changed!!!\n");
            time = *(uint64_t*)(_ptr + sizeof(magic));
            return true;
        }
        //printf("old value: %lu\n", newVal);
        return false;
    }

    void publish(const uint64_t time) {
        __sync_fetch_and_add((uint64_t*)_ptr, time);
        __sync_synchronize();
        *(uint64_t*)(_ptr + sizeof(uint64_t)) = time;
    }
};

这是发件人:

#include <ctime>
#include <unistd.h>
#include <cstdlib>
#include <cstdint>
#include "shm.h"

int main() {
    SHM shm;
    timespec t;
    for (auto i = 0; i < 10000; i++) {
        if (0 == clock_gettime(CLOCK_REALTIME, &t)) {
            const uint64_t v = t.tv_sec * 1000 * 1000 * 1000 + t.tv_nsec;
            shm.publish(v);
            printf("published %lu\n", v);
            usleep(100);
        }
    }
}

这是读者:

#include <iostream>
#include "shm.h"

int main() {
    SHM shm;
    uint64_t magic = 0;
    uint64_t t = 0;
    while (true) {
        if (shm.read(magic, t)) {
            printf("%lu, %lu\n", magic, t);
        }
    }
}

如果重新启动阅读器,则阅读器确实能够读取发送方已写入的最后一个值。

但是,如果我先启动阅读器,然后再启动发送器,则阅读器不会拾取发送器写入的所有值。

为了使这个更奇怪,如果我在SHM :: read()中取消对printf语句的注释,那么读者有时可以使用。

任何的想法?

GCC版本:

g++ (GCC) 7.2.1 20170829 (Red Hat 7.2.1-1)

阅读 379

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2020-06-07

共1个答案

小编典典

我发现了几个问题,但是,我不确定它们是否可以解决您的问题。

  1. nameshm_open应开始/携带使用。
  2. readpublish演员一定不能丢弃volatile。例如:const uint64_t newVal = *(uint64_t volatile*)_ptr;。更好的是,丢弃volatile并使用std::atomic

尽管涉及不同的过程,但是仍然存在相同的对象被多个执行线程访问的情况,并且这些线程中的至少一个会修改共享对象。


我进行了上述更改。使用std::atomic固定的:

class SHM {
    void* _ptr;
public:
    SHM() {
        const auto handle = shm_open("/myTest", O_RDWR|O_CREAT, 0666);
        const auto size =  4 * 1024 * 1024;
        if (-1 == ftruncate(handle, size))
            throw;

        _ptr = mmap(0,size , PROT_READ | PROT_WRITE, MAP_SHARED, handle, 0);

        if(_ptr == MAP_FAILED)
            throw;
    }

    bool read(uint64_t& magic, uint64_t& time) {
        auto p = static_cast<std::atomic<uint64_t>*>(_ptr);
        const uint64_t newVal = p[0];
        if (newVal != magic) {
            magic = newVal;
            printf("value changed!!!\n");
            time = p[1];
            return true;
        }
        return false;
    }

    void publish(const uint64_t time) {
        auto p = static_cast<std::atomic<uint64_t>*>(_ptr);
        p[0] += time;
        p[1] = time;
    }
};

void sender() {
    SHM shm;
    timespec t;
    for (auto i = 0; i < 10000; i++) {
        if (0 == clock_gettime(CLOCK_REALTIME, &t)) {
            const uint64_t v = t.tv_sec * 1000 * 1000 * 1000 + t.tv_nsec;
            shm.publish(v);
            printf("published %lu\n", v);
            usleep(100);
        }
    }
}

void reader() {
    SHM shm;
    uint64_t magic = 0;
    uint64_t t = 0;
    while (true) {
        if (shm.read(magic, t)) {
            printf("%lu, %lu\n", magic, t);
        }
    }
}

int main(int ac, char**) {
    if(ac > 1)
        reader();
    else
        sender();
}

有了std::atomic您,您可以拥有更多控制权。例如:

struct Data {
    std::atomic<uint64_t> time;
    std::atomic<uint64_t> generation;
};

// ...

    bool read(uint64_t& generation, uint64_t& time) {
        auto data = static_cast<Data*>(_ptr);

        auto new_generation = data->generation.load(std::memory_order_acquire); // 1. Syncronizes with (2).
        if(generation == new_generation)
            return false;

        generation = new_generation;
        time = data->time.load(std::memory_order_relaxed);
        printf("value changed!!!\n");
        return true;
    }

    void publish(const uint64_t time) {
        auto data = static_cast<Data*>(_ptr);

        data->time.store(time, std::memory_order_relaxed);
        data->generation.fetch_add(time, std::memory_order_release);  // 2. (1) Synchronises with this store.
    }
2020-06-07