How to get optional certificate in openssl

I used openssl to create SMIME.

i have a valid certificate.I tried to parse this certificate using the following function

int PKCS12_parse(PKCS12 *p12, const char *pass, EVP_PKEY **pkey, X509 **cert, STACK_OF(X509) **ca);

The certificate parsed successfully and i could able get pkey , cert values.But ca always comes out with null开发者_C百科.

How to get this STACK_OF(X509) value from a certificate. I want to use this STACK_OF(X509) value in PKCS7_sign function.

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If ca comes out as NULL, then you probably have no additional certificates in your PKCS12 structure. You do not necessarily need any additional certificates - if your certificate is signed directly by a CA certificate already known to the recipient, then you can simply supply certs = NULL to pkcs7_sign() and no additional certificates will be included in the signed message.