C++ SHA256 is different from command line SHA256 - c++

Question

I'm trying to compute a SHA256 hash of the string iEk21fuwZApXlz93750dmW22pw389dPwOkm198sOkJEn37DjqZ32lpRu76xmw288xSQ9
When I run my C++ code, I get a string that's not even a valid SHA256 hash. However, when I run echo -n iEk21fuwZApXlz93750dmW22pw389dPwOkm198sOkJEn37DjqZ32lpRu76xmw288xSQ9 | openssl sha256, I get the correct hash. Here's my C++ code:
#include <iostream>
#include <time.h>
#include <sstream>
#include <string>
#include <iomanip>
#include <typeinfo>
#include <openssl/sha.h>
#include <cstdio>
#include <cstring>
std::string hash256(std::string string) {
unsigned char digest[SHA256_DIGEST_LENGTH];
SHA256_CTX ctx;
SHA256_Init(&ctx);
SHA256_Update(&ctx, string.c_str(), std::strlen(string.c_str()));
SHA256_Final(digest, &ctx);
char mdString[SHA256_DIGEST_LENGTH*2+1];
for (int i = 0; i < SHA256_DIGEST_LENGTH; i++)
std::sprintf(&mdString[i*2], "%02x", (unsigned int)digest[i]);
return std::string(mdString);
}
int main(int argc, char *argv[])
{
const char *hash = hash256("iEk21fuwZApXlz93750dmW22pw389dPwOkm198sOkJEn37DjqZ32lpRu76xmw288xSQ9").c_str();
std::cout << hash << std::endl;
return 0;
}
Another thing to note: When I run my code in an online compiler, such as Coliru, I get the correct hash. I am compiling with G++ on Cygwin with OpenSSL version OpenSSL 1.0.1g 7 Apr 2014

Answer 1

As pointed out by #Alan Stokes, you have Undefined Behavior due to a dangling reference to the internal structure of the string. Change your declaration of hash in main:
std::string hash = hash256("...");