How to programmatically verify intermediate certificate signature with root public key? - c++

Question

Background
I have the following chain:
Root certificate, self signed
Intermediate certificate, issued by Root
User certificate, issued by Intermediate
I am trying to verify signature of intermediate certificate using public key of issuer (Root). I am using the following code piece to implement this task:
int verify_cert_sig(X509 *issuer, X509 *cert){
EVP_PKEY *public_key = X509_get_pubkey(issuer);
if(public_key == NULL){
_log->error("unable to get public key");
return ERROR;
}
int result = X509_verify(cert, public_key);
EVP_PKEY_free(public_key);
if(result <= 0){
return ERROR;
}
return SUCCESS;
}
int verify_sig(const std::string &issuer, const std::string &cert){
X509 *issuer_x509 = load_cert("root.pem", X509_FILETYPE_PEM);
if(issuer_x509 == NULL){
_log->error("unable to load x509 cert: {}", issuer);
return ERROR;
}
X509 *cert_x509 = load_cert("intermediate.pem", X509_FILETYPE_PEM);
if(cert_x509 == NULL){
_log->error("unable to load x509 cert: {}", cert);
return ERROR;
}
auto rv = verify_cert_sig(issuer_x509, cert_x509);
X509_free(issuer_x509);
X509_free(cert_x509);
if(rv <= 0){
return ERROR;
}
return SUCCESS;
}
Issue
For reasons unknown to me, the previous snippet will return error.
However, if I use openssl cli I can verify intermediate with root:
$:openssl verify -verbose -CAfile root.pem intermediate.pem
subroot.pem: OK
Questions
What am I doing wrong? Did I miss something?
How can I verify the signature of intermediate certificate using public key of issuer?