Dumping public key from private using libopenssl - c++

Question

Let say I want to have a code doing something like this openssl command in my c++ application.
openssl rsa -in private.pem -pubout -outform der -out ./out.pub
How can I do that?
I was look for a sample on github and came up with follow scheme.
key = PEM_read_bio_RSAPrivateKey(bio, NULL, 0, NULL);
len = i2d_RSAPublicKey(key, &bufp);
It returns different value than I'm getting from command line tool. I suppose there was no conversion from private key to public and it just saves the private key. Could any one tell me the right approach for getting pub key from private using openssl lib. I would also greatly appreciate any links to pub\priv key examples for openssl.

Answer 1

Finally I've just find proper sources inside openssl itself. Here is exactly what happens during
openssl rsa -in private.pem -pubout -outform der -out ./out.pub
I've dropped lots of checks from original code to make it smaller.
#include <openssl/pem.h>
#include <openssl/x509.h>
EVP_PKEY *load_key(const char *file)
{
BIO *key=NULL;
EVP_PKEY *pkey=NULL;
key=BIO_new(BIO_s_file());
BIO_read_filename(key,file);
pkey=PEM_read_bio_PrivateKey(key,NULL,NULL,NULL);
return pkey;
}
int _tmain(int argc, _TCHAR* argv[])
{
BIO *out=NULL;
out=BIO_new(BIO_s_file());
EVP_PKEY *pkey;
RSA *rsa=NULL;
char *infile = path_to_pem;
char *outfile = path_to_der;
pkey = load_key(infile);
if (pkey != NULL)
rsa = EVP_PKEY_get1_RSA(pkey);
EVP_PKEY_free(pkey);
BIO_write_filename(out,outfile);
i2d_RSA_PUBKEY_bio(out,rsa);
}