Dev Notes

Software Development Resources by David Egan.

Using C++ Vectors in C Functions

C, C++,
David Egan

You can use C utilities in C++ code. For example, you may be using openssl to perform hashing algorithms.

In this case, the openssl code is C rather than C++. It can’t operate on vectors of bytes - instead it uses basic C style data structures like const void * - an array of void pointers.

Example: SHA256_update()

The SHA256_update() function is part of the openssl library. The signature of this function is: int SHA256_Update(SHA256_CTX *c, const void *preimageBytes, size_t len);.

It serves to fill the bytes of c with the hash of preimageBytes.

Note that 2nd parameter is a const void *. This means that the caller must provide a pointer to data rather than a std::vector or a vector iterator.

The caller can either pass or a pointer to the first element in the vector: &v[0].

Code Example

// main.cpp
#include <iostream>
#include <vector>
#include <iomanip>
#include <openssl/sha.h>
#include <openssl/ripemd.h>

#define HASH_LEN 32

int sha256(const uint8_t preimageBytes[], size_t len, std::vector<uint8_t>& res)
	SHA256_CTX sha256;
	SHA256_Update(&sha256, preimageBytes, len); 
	int ret = SHA256_Final(, &sha256);
	return ret;

int main()
	std::vector<uint8_t> preimageBytes{0xde, 0xad, 0xbe, 0xef};
	std::vector<uint8_t> hash(HASH_LEN);
	sha256(, preimageBytes.size(), hash);
	for (auto& el : hash)
		std::cout << std::hex << std::setfill('0') << std::setw(2) << (int)el;
	std::cout << '\n';

	return 0;

To compile:

g++ -W -Wall -std=c++17 -g -o prog main.cpp -lssl -lcrypto

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