circular_buffer 和 managed_mapped_file 分段错误

Question

我正在使用 boost 1.73.0，并尝试使用 circular_buffer 和 manage_mapped_file 将字符串存储在磁盘上持久保存的循环缓冲区中。

我对 create/open circular_buffer 执行以下操作：

boost::interprocess::managed_mapped_file mmf(boost::interprocess::open_or_create, "./circ_buffer.bin", 10u << 10);
typedef boost::interprocess::allocator<std::string, boost::interprocess::managed_mapped_file::segment_manager> string_allocator;
typedef boost::circular_buffer<std::string, string_allocator> circ_buf;
circ_buf* instance = mmf.find_or_construct<circ_buf>("some_name")(10, mmf.get_segment_manager());

这很好用，我可以像这样在 circular_buffer 中放置字符串：

for(int idx = 0 ; idx < 15; idx++) {
    std::string v = "mystring1-" + std::to_string(idx);
    instance->push_back(v);
}

查看原始文件（即使它是二进制文件）我确实看到了其中的字符串，所以看起来 circular_buffer 确实被保留了。

但是，如果我尝试在另一个进程中加载 circular_buffer，如第一个代码片段所示，并像这样读取第一个元素：

instance->front()

我遇到了分段错误。我知道最后我需要围绕内存访问进行同步，但这不应该是上面示例中的问题，因为在任何给定时间只有一个进程正在访问文件。

有趣的是，如果我在分配器中用 char 替换 std::string，我不会得到分段错误。我做错了什么？

Rgds 克劳斯

Answer 1

boost::interprocess::managed_mapped_file mmf(boost::interprocess::open_or_create, "./circ_buffer.bin", 10u << 10);
typedef boost::interprocess::allocator<std::string, boost::interprocess::managed_mapped_file::segment_manager> string_allocator;
typedef boost::circular_buffer<std::string, string_allocator> circ_buf;
circ_buf* instance = mmf.find_or_construct<circ_buf>("some_name")(10, mmf.get_segment_manager());

您在共享内存中分配 string 个对象。但是，string 自己进行分配，而您从未告诉它使用共享内存分配器。

您可能会在数据中看到短字符串的原因是 SSO

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#include <boost/circular_buffer.hpp>
#include <boost/interprocess/managed_mapped_file.hpp>
#include <boost/interprocess/containers/string.hpp>
#include <iostream>
#include <iomanip>
namespace bip = boost::interprocess;

namespace Shared {
    using Mem = bip::managed_mapped_file;
    using Segment = Mem::segment_manager;

    template <typename T> using Alloc = bip::allocator<T, Segment>;
    template <typename T> using Buffer = boost::circular_buffer<T, Alloc<T> >;

    using String = bip::basic_string<char, std::char_traits<char>, Alloc<char> >;
    using StringBuf = Buffer<String>;
}

int main() {
    using namespace Shared;
    Mem mmf(bip::open_or_create, "./circ_buffer.bin", 10U << 10);

    auto& buf = *mmf.find_or_construct<StringBuf>("some_name")(10, mmf.get_segment_manager());

    for (auto& s : buf) {
        std::cout << "Existing " << std::quoted(std::string_view(s)) << "\n";
    }

    for (char const* init : {"foo", "bar", 
            "some pretty long string to make sure we don't fall into SSO territory"
            "some pretty long string to make sure we don't fall into SSO territory"
            "some pretty long string to make sure we don't fall into SSO territory"
            "some pretty long string to make sure we don't fall into SSO territory"
        })
    {
        buf.push_back(String(init, mmf.get_segment_manager()));
    }
}

在第二个运行打印：

Existing "foo"
Existing "bar"
Existing "some pretty long string to make sure we don't fall into SSO territorysome pretty long string to make sure we don't fall into SSO territorysome pretty long string to make sure we don't fall into SSO territorysome pretty long string to make sure we don't fall into SSO territory"

circular_buffer 和 managed_mapped_file 分段错误

circular_buffer and managed_mapped_file segmentation fault

c++

boost

memory-mapped-files

interprocess

circular-buffer

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