逆向角度看cpp（一）#

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#include <iostream>
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#include <string>
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#include <cstring>
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#include <cstdint>
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#include <cstddef>
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#include <iomanip>
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// 防止常见编译器轻易内联这些函数，方便初次观察调用关系
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#if defined(_MSC_VER)
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#define NOINLINE __declspec(noinline)
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#elif defined(__GNUC__) || defined(__clang__)
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#define NOINLINE __attribute__((noinline))
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#else
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#define NOINLINE
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#endif
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union Payload
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{
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    std::int32_t number;
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    float value;
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    char name[16];
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};
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struct Request
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{
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    std::uint32_t type;
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    std::uint32_t length;
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    Payload payload;
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};
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std::uint32_t getRequestType(const Request *req)
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{
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    return req->type;
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}
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std::uint32_t getRequestLength(const Request *req)
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{
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    return req->length;
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}
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Payload getRequestPayload(const Request *req)
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{
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    return req->payload;
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}
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std::int32_t getPayloadLength(const Request *req)
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{
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    return req->payload.number;
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}
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std::int32_t processRequest(const Request *req)
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{
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    Payload p = getRequestPayload(req);
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    if (req->type == 1)
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    {
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        p.number += 100;
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        return p.number;
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    }
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    else if (req->type == 2)
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    {
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        if (req->payload.value > 10.0f)
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        {
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            return 200;
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        }
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        return 201;
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    }
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    return -1;
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}
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// c风格长度
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std::size_t c_len(const char *s)
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{
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    return std::strlen(s);
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}
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// 字符串比较
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NOINLINE bool isAdmin(const char *s)
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{
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    return std::strcmp(s, "admin") == 0;
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}
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int main()
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{
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    std::cout << '\n'
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              << "string相关";
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    std::string a = "nihao";
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    std::string b = "i am mj,nice to meet you.mjmjmjmjmjmj!";
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    std::cout << "short_size: " << a.size() << '\n';
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    std::cout << "long_size: " << b.size() << '\n';
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    std::cout << "sizeof(std::string) = "
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              << sizeof(std::string) << '\n';
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    std::cout << "short_str->object: " << static_cast<const void *>(&a) << '\n';
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    std::cout << "short_str data: " << static_cast<const void *>(a.data()) << '\n';
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    std::cout << "long_str->object: " << static_cast<const void *>(&b) << '\n';
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    std::cout << "long_str data: " << static_cast<const void *>(b.data()) << '\n';
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    // 观察c字符串
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    std::cout << "===============观察c字符串===============" << '\n';
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    std::cout << "isAdmin(admin) = "
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              << isAdmin("admin") << '\n';
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    std::cout << "strlen(hello) = "
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              << c_len("hello") << '\n';
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    // 观察结构体字段偏移
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    std::cout << "sizeof(Request) = " << sizeof(Request) << '\n';
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    std::cout << "offsetof(type) = " << offsetof(Request, type) << '\n';
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    std::cout << "offsetof(length) = "
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              << offsetof(Request, length) << '\n';
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    std::cout << "offsetof(payload) = "
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              << offsetof(Request, payload) << '\n';
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    // 联合体
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    Request req{};
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    req.length = sizeof(req.payload.number);
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    req.type = 1;
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    req.payload.number = 25;
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    std::cout << "request type = "
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              << getRequestType(&req) << '\n';
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    std::cout << "request length = "
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              << getRequestLength(&req) << '\n';
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    std::cout << "payload code = "
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              << getPayloadLength(&req) << '\n';
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    std::cout << "process result = "
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              << processRequest(&req) << '\n';
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    std::cout << '\n'
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              << "观察联合体的底层字节" << '\n';
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    const auto *bytes = reinterpret_cast<const unsigned char *>(&req.payload);
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    std::cout << "payload bytes = ";
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    for (int i = 0; i < sizeof(req.payload); i++)
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    {
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        // 在字节输出循环中
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        std::cout << std::hex
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                  << std::setw(2) << std::setfill('0')
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                  << static_cast<unsigned int>(bytes[i]) << ' ';
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    }
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    std::cout << std::dec << '\n';
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    // D. 改为使用 value 成员，不读取 code 成员
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    req.type = 2;
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    req.payload.value = 12.5f;
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    std::cout << "payload bytes = ";
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    for (int i = 0; i < sizeof(req.payload); i++)
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    {
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        // 在字节输出循环中
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        std::cout << std::hex
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                  << std::setw(2) << std::setfill('0')
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                  << static_cast<unsigned int>(bytes[i]) << ' ';
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    }
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    std::cout << std::dec << '\n';
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    std::cout << "process float result = "
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              << processRequest(&req) << '\n';
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    return 0;
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}

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PS D:\Data structure\cpp> g++ -O0 -std=c++17 -g -fno-inline lab.cpp -o lab.exe
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PS D:\Data structure\cpp> ./lab.exe
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string相关short_size: 5
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long_size: 38
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sizeof(std::string) = 32
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下面四行体现了sso，long_Str的data和对象地址相隔甚远
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short_str->object: 0xfb859ff600
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short_str data: 0xfb859ff610
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long_str->object: 0xfb859ff5e0
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long_str data: 0x2b422e87d00
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===============观察c字符串===============
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isAdmin(admin) = 1
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strlen(hello) = 5
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下面体现了结构体内存布局与对齐
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sizeof(Request) = 24
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offsetof(type) = 0
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offsetof(length) = 4
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offsetof(payload) = 8
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request type = 1
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request length = 4
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payload code = 25
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下面是修改结构体数据
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process result = 125
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观察联合体的底层字节
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payload bytes = 19 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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修改成float后，可以发现确实是公用一块内存，16字节。
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payload bytes = 00 00 48 41 00 00 00 00 00 00 00 00 00 00 00 00
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process float result = 200