Question about creating digital signature using OpenSSL‏(pkcs7)

Question

I'm using OpenSSL to create digital signature fo my application but I'm getting one problem.

BIO *in = NULL, *out = NULL, *tbio = NULL;
X509 *scert = NULL;
EVP_PKEY *skey = NULL;
PKCS7 *p7 = NULL;
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**p7 = PKCS7_sign(scert, skey, NULL, in, flags);**

I don't know how can we get digital signature from PKCS7 object to put it into specific variable. Could you please help me to resolve this problem? Thank you very much.