Programmatic way to get x509 to be signed digest

I have an X509 object with the usual public components filled in (subject
validity issuer algorithm public key), I want to serialize the data to be
hashed and signed.

Is i2d_X509() the proper way? 

It doesn't seem to work. I sign and add the signature to the x509 object,
but it doesn't verify. I wonder if i2d_x509 is adding some extra content at
the end where the signature should be.

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