使用openSSL从X509_Extension获取电子邮件地址（主题备用名称的一部分）

I got a iOS application trying to retrieve SAN from a certificate with openSSL. I am stuck at the part that I can get the X509_Extension correctly(I can check the data in the X509_Extension and it contains the email address, mixed in a bunch of other data). I have no idea how to generally extract the email address from the X509_Extension.I got a iOS application trying to retrieve SAN