CODEWITHSUNDEEP
c++17
Naydachi Kamikadze
Naydachi Kamikadze

Reputation: 107

How convert std::vector<std::byte> to C-style raw data(unsigned char**)?

It's examle how convert std::vector<std::byte> to raw data for C-style function. It's perfect work.

void SomeCApi(unsigned char* buffer, unsigned int size)
{
    for (unsigned char index = 0; index < size; ++index) {
        buffer[index] = index;
    }
}
 
int main()
{
    std::vector<std::byte> buffer{ 100 };
    SomeCApi(reinterpret_cast<unsigned char*>(&buffer[0]), buffer.size());
 
    for (const auto& element : buffer) { PrintByte(element); }
}

But how I can provide my bytes vector to const unsigned char**?? Me need it for function SomeCApi(const unsigned char** buffer, unsigned int size). My try reinterpret_cast<const unsigned char**>(&bytes[0]), but it's not work. I get exception "Access violation reading location".

MSVC 19, c++17(latest). My case:

unsigned char* sig;
sig = (unsigned char*)alloca(signature_len);
std::vector<std::byte> bytes(signature_len);

ec_sig = d2i_ECDSA_SIG(NULL, reinterpret_cast<const unsigned char**>(&bytes[0]), signature_len); //It's throw Access violation reading location
//ec_sig = d2i_ECDSA_SIG(NULL, (const unsigned char**)&sig, signature_len); //It's perfect work with C-Style raw data and C-style cast.
if (ec_sig == NULL)
    std::cout << "BAD" << std::endl;

Upvotes: 0

Views: 564

Answers (1)

Adrian Maire
Adrian Maire

Reputation: 14815

To provide a vector<char> to a C function accepting const char** you can access the internal buffer and return a pointer to it:

#include <iostream>
#include <vector>

void someCApi(const char** c, size_t len)
{
    for (size_t i=0; i<len; ++i)
    {
        std::cout << static_cast<int>((*c)[i]) << std::endl;
    }
}

auto main() -> int
{
    std::vector<char> buff{1,3,5,7,11,13,17,19};
    const char* buffPtr = buff.data();
    someCApi(&buffPtr, buff.size());

    return 0;
}

https://onlinegdb.com/B1BSNKyrv

Simple interface to get the pointer to array:

template <typename T>
class pptr
{
    const T* buff=nullptr;
public:
    ppta(std::vector<T>& v): buff(v.data()){};
    const T** operator()(){return &buff;}
};

//Usage
someCApi(pptr(buff)(), buff.size());

Upvotes: 1

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