如何从子进程写入共享内存

Question

我正在试验子进程，因为这似乎是使不能在线程中使用的旧 C 库并行运行的唯一方法。

我的最小示例将三个整数值写入映射内存。 在子进程中，我想更改这些值。 更改最后一个（第三个）值有效，但仅更改第二个值时，此输出出现错误：

initial mapped memory in parent process:                      000000000000000000000000000000010000000000000000000000000000001000000000000000000000000000000011
mapped memory in child process before writing:                000000000000000000000000000000010000000000000000000000000000001000000000000000000000000000000011
mapped memory in child process after writing:                 0000000000000000000000000000000100000000000000000000000000011011
mapped memory in parent process after child process finished: 0000000000000000000000000000000100000000000000000000000000011011
1
27
terminate called after throwing an instance of 'std::invalid_argument'
  what():  bitset::_M_copy_from_ptr

Process finished with exit code 134 (interrupted by signal 6: SIGABRT)

最小的代码示例是这样的：

#include <iostream>
#include <vector>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <bitset>
#include <semaphore.h>

using namespace std;

int main(int /*argc*/, char* /*argv*/[])
{
    const char * shm_name = "/a_shm_name";
    const int SIZE = 4096;
    std::vector<int> v{1, 2, 3};

    int shm_fd = shm_open(shm_name, O_CREAT | O_RDWR, 0666);

    ftruncate(shm_fd, 32 * v.size());

    void * ptr0 = mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd, 0);

    //write into the memory segment
    void * ptr = ptr0;
    for(int i = 0; i < v.size(); i++)
    {
        std::string s = std::bitset<32>(v[i]).to_string();
        int count = sprintf((char*)ptr, "%s", s.c_str());
        ptr = ptr + count;
    }

    cout << "initial mapped memory in parent process:                      " << (char*)ptr0 << endl << flush;

    //fork
    int n1 = fork();
    if(n1 == 0)
    {
        //client process
        shm_fd = shm_open(shm_name, O_CREAT | O_RDWR, 0666);

        ptr0 = mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd, 0);

        cout << "mapped memory in child process before writing:                " << (char*)ptr0 << endl << flush;
        //write into the memory segment
        //sem_wait(semId);
        ptr = ptr0 + 32 * 1;
        sprintf((char*)ptr, "%s", std::bitset<32>(27).to_string().c_str());
        cout << "mapped memory in child process after writing:                 " << (char*)ptr0 << endl << flush;

        exit(0);
    }
    else
    {
        //parent process
        wait(nullptr);
    }

    //child process finished, back in the parent process
    //client process
    shm_fd = shm_open(shm_name, O_RDONLY, 0666);

    ptr = mmap(0, SIZE, PROT_READ, MAP_SHARED, shm_fd, 0);

    cout << "mapped memory in parent process after child process finished: " << (char*)ptr0 << endl << flush;
    for(int i = 0; i < v.size(); i++)
    {
        cout << std::bitset<32>(std::string((char*)ptr, 32)).to_ulong() << endl;
        ptr = ptr + 32;
    }

    munmap(ptr0, SIZE);

    shm_unlink(shm_name);

    return 0;
}

我有两个问题：

• 为什么我不能只更改第二个整数值？
• 是否有更好的技术在不同的子进程之间共享数据？

Answer 1

这根本与进程或mmap无关：具有相似结果的更简单的程序。

#include <vector>
#include <bitset>
#include <iostream>

int main() {
    using std::cout, std::endl, std::flush;

    const int SIZE = 4096;
    std::vector<int> v{1, 2, 3};

    char buffer[SIZE];
    char * ptr0 = buffer;

    //write into the memory segment
    char * ptr = ptr0;
    for(unsigned int i = 0; i < v.size(); i++)
    {
        std::string s = std::bitset<32>(v[i]).to_string();
        int count = sprintf(ptr, "%s", s.c_str());
        ptr = ptr + count;
    }

    cout << "initial memory:                                " << ptr0 << endl << flush;

    //write into the memory segment
    ptr = ptr0 + 32 * 1;
    sprintf(ptr, "%s", std::bitset<32>(27).to_string().c_str());
    cout << "mapped memory in child process after writing:  " << ptr0 << endl << flush;
}

（我将所有void*更改为char*因为向void*添加数字不是标准 C++，尽管一些编译器有一个扩展来将其视为添加到char* 。）

sprintf在实际输出结束后写入一个空终止符。 因此，当您使用它在子级中写入“第二个值”时，它会将'\\0'放入“第三个值”的第一个字节中。 此外，您实际上一直依赖((char*)ptr0)[3*32]在std::cout “第三个值”之后的'\\0'来知道何时停止打印。

而不是sprintf和operator<< with const char* ，它们都用于以空字符结尾的字符串，我建议只复制字节的函数：

std::string data = std::bitset<32>(value).to_string();
std::copy(data.begin(), data.end(), (char*) ptr);

cout.write((char*) ptr, 32);