Assigning a pointer returned from a function

Question

I have a program which looks like this:

EVP_PKEY *generate_RSA_key_and_uuid(unsigned char uuid[16])
{
    EVP_PKEY        *key_p;
    key_p = EVP_PKEY_new();
    return key_p;  
}

int build_and_save_csr(int dn_entries, X509_REQ *req_p, EVP_PKEY *priv_key_p, char *passphrase)
{   
    priv_key_p = generate_RSA_key_and_uuid(uuid);
}

int makecsr(X509_REQ *req_p, EVP_PKEY *priv_key_p, char *passphrase)
{

    if (build_and_save_csr(2, req_p, priv_key_p, passphrase) != 0) {
        fprintf(stderr, "Could not create csr file / private key...\n");
        exit(1);
    }

    return 0;
}

int main(int argc, char *argv[])
{

    EVP_PKEY        *priv_key_p;
    X509_REQ        *req_p;

    makecsr(req_p, priv_key_p, passphrase);
    return 0;
}

After this has run, *priv_key_p should contain the value returned from *generate_RSA_key_and_uuid.

In gdb, I can see that at the end of generate_RSA_and_uuid that key_p contains the correct value but after priv_key_p = generate_RSA_key_and_uuid(uuid); has executed, priv_key_p is empty.

Does anybody know how to properly assign the value to the pointer?

Answer 1

You need an extra level of indirection, otherwise you're just modifying local copies of the pointers and nothing gets returned:

int build_and_save_csr(int dn_entries, X509_REQ *req_p, EVP_PKEY **priv_key_p, char *passphrase)
{   
    *priv_key_p = generate_RSA_key_and_uuid(uuid);
}

int makecsr(X509_REQ *req_p, EVP_PKEY **priv_key_p, char *passphrase)
{
    if (build_and_save_csr(2, req_p, priv_key_p, passphrase) != 0) {
        fprintf(stderr, "Could not create csr file / private key...\n");
        exit(1);
    }
    return 0;
}

int main(int argc, char *argv[])
{
    EVP_PKEY        *priv_key_p;
    X509_REQ        *req_p;

    makecsr(req_p, &priv_key_p, passphrase);
    return 0;
}

Alternatively you can use the function result (as you do already in generate_RSA_key_and_uuid()) to return the pointer (perhaps return NULL on error) - this gets rid of the need for the extra level of indirection.