c++ - 给定一个填充 unsigned char** 的 C 函数，如何在没有中间拷贝的情况下用数据填充 std::vector

Question

C 函数是:

int i2d_PrivateKey(EVP_PKEY *a, unsigned char **pp);

int i2d_X509(X509 *a, unsigned char **ppout);

我已经编写了这样的代码来复制到 std::vector 中:

// populate PrivateKey
std::vector<uint8_t> PrivateKey;
EVP_PKEY *privatekey = NULL;
int size = i2d_PrivateKey(privatekey, NULL);
if (size > 0)
{
    PrivateKey.reserve(size);
    uint8_t* ptr = &PrivateKey[0];
    i2d_PrivateKey(privatekey, &ptr);
std::cout << "PrivateKey size=" << PrivateKey.size() << '\n';
}

PrivateKey.size() 返回零，因此我知道该 vector 尚未填充。但是，我知道 size 是一个正整数，所以执行 if (size block 中的代码。

如果 ptr 是 PrivateKey 数组的起始地址，那么这段代码不应该工作吗？

虽然这段代码使用了openssl，但我认为它更像是一个一般的指针问题。如果我创建一个临时 uint8_t 数组，那么它可以工作，但我宁愿直接复制到 vector 中并节省临时拷贝的开销。

代码如下:

#include <openssl/ssl.h>
#include <openssl/pem.h>
#include <openssl/ossl_typ.h>
#include <openssl/x509.h>
#include <openssl/x509v3.h>
#include <openssl/pkcs12.h>
#include <openssl/err.h>

#include <iostream>
#include <vector>
#include <string>
#include <cstdint>


int main()
{
    std::vector<uint8_t> input;  // contains pkcs12 data
    std::string Password = "password";
    std::vector<uint8_t> Certificate;
    std::vector<uint8_t> PrivateKey;
    OpenSSL_add_all_algorithms();
    OpenSSL_add_all_ciphers();
    OpenSSL_add_all_digests();
    ERR_load_crypto_strings();

    PKCS12* p12_cert = NULL;
    const uint8_t* p1 = &input[0];
    if(d2i_PKCS12(&p12_cert, &p1, input.size()) != NULL) {
        EVP_PKEY *privatekey = NULL;
        X509 *x509_cert = NULL;
        // additional certs, last arg is CA which we don't care about
        if (PKCS12_parse(p12_cert, Password.c_str(), &privatekey, &x509_cert, NULL))
        {
            // populate m_privateKey
            int size = i2d_PrivateKey(privatekey, NULL);
    std::cout << "privatekey size=" << size << '\n';
            if (size > 0)
            {
                PrivateKey.reserve(size);
                uint8_t* ptr = &PrivateKey[0];
                i2d_PrivateKey(privatekey, &ptr);
        std::cout << "PrivateKey size=" << PrivateKey.size() << '\n';
            }
            // populate certificate
            size = i2d_X509(x509_cert, NULL);
            std::cout << "certificate size=" << size << '\n';
            if(size > 0)
            {
                Certificate.reserve(size);
                uint8_t* ptr = &Certificate[0];
                int ret = i2d_X509(x509_cert, &ptr);
        std::cout << "ret=" << ret <<'\n';
        std::cout << "cert size=" << Certificate.size() << '\n';
            }
        }
        PKCS12_free(p12_cert);
    }
}

更新，可以使用下面的代码来合并 Arron 的修复:

#include <openssl/ssl.h>
#include <openssl/pem.h>
#include <openssl/ossl_typ.h>
#include <openssl/x509.h>
#include <openssl/x509v3.h>
#include <openssl/pkcs12.h>
#include <openssl/err.h>

#include <iostream>
#include <fstream>
#include <iterator>
#include <iomanip>
#include <vector>
#include <string>
#include <cstdint>

using namespace std;

std::vector<uint8_t>& File2Buffer(const std::string path,
                            std::vector<uint8_t>& buffer) {
    fstream fs(path, ios::in | ios::binary);
    if (fs) {
        // Don't skip new lines
        fs.unsetf(ios::skipws);

        fs.seekg(0, ios::end);
        size_t size = static_cast<size_t>(fs.tellg());
        fs.seekg(0, ios::beg);

        buffer.reserve(size);
        buffer.insert(buffer.begin(),
            istream_iterator<uint8_t>(fs),
            istream_iterator<uint8_t>());
    }
    return buffer;
}


int main(int argc, char* argv[])
{
    if (argc != 3) {
        cout << "Usage: " << argv[0] << " <pkcs12 file> " << "<password>\n";
        exit(0);
    }

    std::vector<uint8_t> input;
    File2Buffer(argv[1], input);
    std::string Password = argv[2];
    std::vector<uint8_t> Certificate;
    std::vector<uint8_t> PrivateKey;
    OpenSSL_add_all_algorithms();
    OpenSSL_add_all_ciphers();
    OpenSSL_add_all_digests();
    ERR_load_crypto_strings();

    PKCS12* p12_cert = NULL;
    const uint8_t* p1 = &input[0];
    if(d2i_PKCS12(&p12_cert, &p1, input.size()) != NULL) {
        EVP_PKEY *privatekey = NULL;
        X509 *x509_cert = NULL;
        // additional certs, last arg is CA which we don't care about
        if (PKCS12_parse(p12_cert, Password.c_str(), &privatekey, &x509_cert, NULL))
        {
            // populate m_privateKey
            int size = i2d_PrivateKey(privatekey, NULL);
    std::cout << "privatekey size=" << size << '\n';
            if (size > 0)
            {
                PrivateKey.resize(size);
                uint8_t* ptr = &PrivateKey[0];
                i2d_PrivateKey(privatekey, &ptr);
        std::cout << "PrivateKey size=" << PrivateKey.size() << '\n';
            }
            // populate certificate
            size = i2d_X509(x509_cert, NULL);
            std::cout << "certificate size=" << size << '\n';
            if(size > 0)
            {
                Certificate.resize(size);
                uint8_t* ptr = &Certificate[0];
                int ret = i2d_X509(x509_cert, &ptr);
        std::cout << "ret=" << ret <<'\n';
        std::cout << "cert size=" << Certificate.size() << '\n';
            }
        }
        PKCS12_free(p12_cert);
    }

    // test it out:
    if (Certificate.size() > 0) {
        cout << "Certificate size=" << Certificate.size() << '\n';
        for (auto& ch : Certificate) {
            cout << hex << ch << " ";
        }
    }
}

Answer 1

使用调整大小而不是保留。 reserve 的问题在于，如果您执行分配(如 PrivateKey[5] = 5)，并调用 PrivateKey.size()，大小仍将保持为 0。(实际上，Reserve 可以与 std::copy 中的 back_inserter 配对使用)，但在您的情况下，您应该调整大小。