PKCS7
	STACK of X509_ATTRIBUTES
		ASN1_OBJECT
		STACK of ASN1_TYPE

So it is

p7.xa[].obj
p7.xa[].data[]

get_obj_by_nid(STACK , nid)
get_num_by_nid(STACK , nid)
get_data_by_nid(STACK , nid, index)

X509_ATTRIBUTE *X509_ATTRIBUTE_new(void );
void		X509_ATTRIBUTE_free(X509_ATTRIBUTE *a);

X509_ATTRIBUTE *X509_ATTRIBUTE_create_by_NID(X509_ATTRIBUTE **ex,
			int nid, STACK *value);

X509_ATTRIBUTE *X509_ATTRIBUTE_create_by_OBJ(X509_ATTRIBUTE **ex,
			int nid, STACK *value);

int		X509_ATTRIBUTE_set_object(X509_ATTRIBUTE *ex,ASN1_OBJECT *obj);
int		X509_ATTRIBUTE_add_data(X509_ATTRIBUTE *ex, int index,
			ASN1_TYPE *value);

ASN1_OBJECT *	X509_ATTRIBUTE_get_object(X509_ATTRIBUTE *ex);
int 		X509_ATTRIBUTE_get_num(X509_ATTRIBUTE *ne);
ASN1_TYPE *	X509_ATTRIBUTE_get_data(X509_ATTRIBUTE *ne,int index);

ASN1_TYPE *	X509_ATTRIBUTE_get_data_by_NID(X509_ATTRIBUTE *ne,
			ASN1_OBJECT *obj);

X509_ATTRUBUTE *PKCS7_get_s_att_by_NID(PKCS7 *p7,int nid);
X509_ATTRUBUTE *PKCS7_get_u_att_by_NID(PKCS7 *p7,int nid);