How to get matched root CA in X509_verify_cert()

[Jerry OELoo]

Hi
I have put some(100+) root CA certificate files in a folder, and Use
x509_store_load_locations() to set to store load location.

Then Use X509_STORE_CTX_init(), X509_verify_cert() to verify a certificate,

When X509_verify_cert() return 1, I want to know which Root CA passed
verify the certificate, How can I get this information?

Thanks!