"Occasional"分段故障

"Occasional" Segmentation Fault

本文关键字：故障 Occasional 分段更新时间：2023-10-16

问题是关于grib parser的(链接到grib文件https://github.com/gifciak/grib(，(，当我执行我的代码(尽管CodeBlocks或通过控制台 - g++ main.cpp -pedantic(我会遇到错误，分段故障，但并非总是发生。

例如，当我编译10次，8次时会出现错误，而2次一切都可以正常工作，这将为我提供控制台输出和信息。

正如我所研究的那样，问题与std::copy相关，因为它可能正在尝试复制不存在的迭代器。

有人可以解释为什么会发生吗？为什么不总是崩溃或成功？

#include <iostream>
#include <vector>
#include <fstream>
#include <iterator>
#include <algorithm>
using ByteVec = std::vector<uint8_t>;

template<typename T, size_t size = sizeof(T)>
auto getReverseEndianValue(const auto & iter) {
    union {
        T result;
        char tmp[size];
    } buffer;
    auto reverseIter = std::make_reverse_iterator(std::next(iter, size));
    std::copy(reverseIter, std::next(reverseIter, size), buffer.tmp);
    return buffer.result;
}
enum Edition {
    Edition_Unknown = -1,
    Edition_GRIB1 = 1,
};
namespace section {
    class IS {
    public:
        uint32_t magicFlag;
        uint32_t size;
        Edition edition;
        static IS read(const auto & iter) {
            IS result;
            result.magicFlag = getReverseEndianValue<uint32_t>(iter);
            result.size = getReverseEndianValue<uint32_t, 3>(iter + 4);
            result.edition = (*(iter + 7) == 1 ? Edition_GRIB1 : Edition_Unknown);
            return result;
        }
    };
    class PDS {
    public:
        uint32_t size;
        uint8_t tableVersion;
        uint8_t indentificatorOfCenter;
        uint8_t numProcessID;
        uint8_t gridIndentification;
        uint8_t flagForGDSorBMS;
        uint8_t indParamAndUnit;
        uint8_t indTypeOfLevelOrLayer;
        uint16_t levelOrLayer;
        uint8_t year;
        uint8_t month;
        uint8_t day;
        uint8_t hour;
        uint8_t minute;
        uint8_t forecastTimeUnit;
        uint8_t p1;
        uint8_t p2;
        uint8_t indTimeRange;
        uint16_t averageOrAccumulate;
        uint8_t missing;
        uint8_t century;
        uint8_t subcenterId;
        uint16_t decimalScale;
        ByteVec data;
        static PDS read(const auto& iter) {
            PDS result;
            result.size = getReverseEndianValue<uint32_t, 3>(iter);
            result.tableVersion = getReverseEndianValue<uint8_t>(iter + 3);
            result.indentificatorOfCenter = getReverseEndianValue<uint8_t>(iter + 4);
            result.numProcessID = getReverseEndianValue<uint8_t>(iter + 5);
            result.gridIndentification = getReverseEndianValue<uint8_t>(iter + 6);
            result.flagForGDSorBMS = getReverseEndianValue<uint8_t>(iter + 7);
            result.indParamAndUnit = getReverseEndianValue<uint8_t>(iter + 8);
            result.indTypeOfLevelOrLayer = getReverseEndianValue<uint8_t>(iter + 9);
            result.levelOrLayer = getReverseEndianValue<uint16_t>(iter + 10);
            result.year = getReverseEndianValue<uint8_t>(iter + 12);
            result.month = getReverseEndianValue<uint8_t>(iter + 13);
            result.day = getReverseEndianValue<uint8_t>(iter + 14);
            result.hour = getReverseEndianValue<uint8_t>(iter + 15);
            result.minute = getReverseEndianValue<uint8_t>(iter + 16);
            result.forecastTimeUnit = getReverseEndianValue<uint8_t>(iter + 17);
            result.p1 = getReverseEndianValue<uint8_t>(iter + 18);
            result.p2 = getReverseEndianValue<uint8_t>(iter + 19);
            result.indTimeRange = getReverseEndianValue<uint8_t>(iter + 20);
            result.averageOrAccumulate = getReverseEndianValue<uint16_t>(iter + 21);
            result.missing = getReverseEndianValue<uint8_t>(iter + 23);
            result.century = getReverseEndianValue<uint8_t>(iter + 24);
            result.subcenterId = getReverseEndianValue<uint8_t>(iter + 25);
            result.decimalScale = getReverseEndianValue<uint16_t>(iter + 26);
            return result;
        }
    };
}
class GribData {
private:
    section::IS secIS;
    section::PDS secPDS;
public:
    void print() {
        std::cout
            << "### Section IS ###n"
            << "magicFlag: " << +secIS.magicFlag << "n"
            << "size: " << +secIS.size << "n"
            << "edition: " << +secIS.edition << "n"
            << "n### Section PDS ###n"
            << "size: " << +secPDS.size << "n"
            << "tableVersion: " << +secPDS.tableVersion << "n"
            << "indentificatorOfCenter: " << +secPDS.indentificatorOfCenter << "n"
            << "numProcessID: " << +secPDS.numProcessID << "n"
            << "gridIndentification: " << +secPDS.gridIndentification << "n"
            << "flagForGDSorBMS: " << +secPDS.flagForGDSorBMS << "n"
            << "indParamAndUnit: " << +secPDS.indParamAndUnit << "n"
            << "indTypeOfLevelOrLayer: " << +secPDS.indTypeOfLevelOrLayer << "n"
            << "levelOrLayer: " << +secPDS.levelOrLayer << "n"
            << "year: " << +secPDS.year << "n"
            << "month: " << +secPDS.month << "n"
            << "day: " << +secPDS.day << "n"
            << "hour: " << +secPDS.hour << "n"
            << "minute: " << +secPDS.minute << "n"
            << "forecastTimeUnit: " << +secPDS.forecastTimeUnit << "n"
            << "p1: " << +secPDS.p1 << "n"
            << "p2: " << +secPDS.p2 << "n"
            << "indTimeRange: " << +secPDS.indTimeRange << "n"
            << "averageOrAccumulate: " << +secPDS.averageOrAccumulate << "n"
            << "missing: " << +secPDS.missing << "n"
            << "century: " << +secPDS.century << "n"
            << "subcenterId: " << +secPDS.subcenterId << "n"
            << "decimalScale: " << +secPDS.decimalScale << "n";
    }
    static GribData loadData(const ByteVec& rawdata) {
        GribData result;
        constexpr char MAGIC_START[4] = { 'G', 'R', 'I', 'B' };
        constexpr char MAGIC_END[4] = { '7', '7', '7', '7' };
        auto start = std::search(rawdata.cbegin(),
            rawdata.cend(),
            std::begin(MAGIC_START),
            std::end(MAGIC_START));
        auto end = std::search(rawdata.cbegin(),
            rawdata.cend(),
            std::begin(MAGIC_END),
            std::end(MAGIC_END));
        ByteVec data(start, end + sizeof(MAGIC_END));
        result.secIS = section::IS::read(data.cbegin());
        result.secPDS = section::PDS::read(data.cbegin() + 8);
        auto size = getReverseEndianValue<uint32_t, 3>(data.cbegin() + 4);
        auto sec1 = getReverseEndianValue<uint32_t, 3>(data.cbegin() + 8);
        auto sec2 = getReverseEndianValue<uint32_t, 3>(data.cbegin() + 8 + sec1);
        auto sec3 = getReverseEndianValue<uint32_t, 3>(data.cbegin() + 8 + sec1 + sec2);
        std::cout
            << "size: " << size << "n"
            << "sec0: " << 8 << "n"
            << "sec1: " << sec1 << "n"
            << "sec2: " << sec2 << "n"
            << "sec3: " << sec3 << "n"
            << "end flag: " << sizeof(MAGIC_END) << "n"
            << "sum: " << 8 + sec1 + sec2 + sec3 + sizeof(MAGIC_END) << "nn";
        return result;
    }
    static GribData loadDataFromFile(const std::string& path) {
        std::ifstream file(path, std::ios::binary);
        ByteVec data;
        std::copy(std::istreambuf_iterator<char>(file),
            {},
            std::back_inserter(data));
        return loadData(data);
    }
};

int main() {
    auto grib = GribData::loadDataFromFile("message_2_G1.grib");
    grib.print();
}

这是预期的结果，因为我已经从控制台复制了

尺寸：4538SEC0：8第1节：28Sec2：178Sec3：4320末端标志：4总和：4538###部分是###魔术：1196575042尺寸：1191186874版：1###部分PDS ###尺寸：28桌面：2凹陷处的中心：7NumProcessid：81网格化：37FlagForGDSORBMS：128indparamandunit：33Indtypeoflevelorlayer：100Levelorlayer：850年：15月份：3日：10小时：0分钟：0预测时间诺：1P1：0P2：0Indtimerange：10平均水平：0缺少：0世纪：21子中心：0小数：1

首先，使用g++ main.cpp -pedantic并不是很有用，因为您尚未启用任何警告。将-Wall -Wextra添加到编译器标志和-g，以便您进行调试。

使用-fsanitize=undefined编译显示使用有效指针的NULL指针引起的运行时错误：

/usr/include/c++/8/bits/stl_algobase.h:368:23: runtime error: null pointer passed as argument 2, which is declared to never be null
Segmentation fault (core dumped)

这意味着您的程序有一个错误。

用-D_GLIBCXX_DEBUG编译将为std::vector添加其他检查，这告诉您问题：

/usr/include/c++/8/debug/safe_iterator.h:374:
Error: attempt to advance a past-the-end iterator 4 steps, which falls 
outside its valid range.
Objects involved in the operation:
    iterator @ 0x0x7fffb09ceb90 {
      type = __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<unsigned char const*, std::__cxx1998::vector<unsigned char, std::allocator<unsigned char> > >, std::__debug::vector<unsigned char, std::allocator<unsigned char> > > (constant iterator);
      state = past-the-end;
      references sequence with type 'std::__debug::vector<unsigned char, std::allocator<unsigned char> >' @ 0x0x7fffb09cf050
    }
Aborted (core dumped)

您应该在调试器下运行该程序，以查看此无效的迭代器增量发生的位置。在GDB中运行该程序，然后使用其up命令向上移动堆栈，显示错误来自此处，在loadData中：

    constexpr char MAGIC_START[4] = { 'G', 'R', 'I', 'B' };
    constexpr char MAGIC_END[4] = { '7', '7', '7', '7' };
    auto start = std::search(rawdata.cbegin(),
        rawdata.cend(),
        std::begin(MAGIC_START),
        std::end(MAGIC_START));
    auto end = std::search(rawdata.cbegin(),
        rawdata.cend(),
        std::begin(MAGIC_END),
        std::end(MAGIC_END));
    ByteVec data(start, end + sizeof(MAGIC_END));
                        ^^^^^^^^^^^^^^^^^^^^^^^

考虑当rawdata不包含MAGIC_START字符，而是包含MAGIC_END字符时会发生什么。start和end是否形成有效的迭代器范围？

考虑当rawdata不包含MAGIC_END字符时会发生什么。end + sizeof(MAGIC_END)是否有效？

您不应按照预期的是对std::search的两个呼叫。您应该通过测试start == rawdata.end()还是end == rawdata.end()添加一些错误检查。如果两个是正确的，则出了问题(rawdata字符串中的不良输入(。

您还应该学习如何使用调试器，并了解编译器为检测错误提供的其他工具(例如，GCC的-fsanitize=undefined和-D_GLIBCXX_DEBUG选项应用于帮助确认存在错误，并使用GDB来查找。这些错误发生的地方(。