API Error Handling

Every API has 3 out parameters as x_return_status, x_msg_count and x_msg_data.
Using these 3 parameters, one can use below code to log or debug error in API's
dbms_output.put_line (SubStr('x_return_status = '||x_return_status, 1, 255));
dbms_output.put_line ('x_msg_count = '||TO_CHAR(x_msg_count));
dbms_output.put_line (SubStr('x_msg_data = '||x_msg_data, 1, 255));
IF x_msg_count >1 THEN
FOR I IN 1..x_msg_count
LOOP
dbms_output.put_line(I ||'.'|| SubStr(FND_MSG_PUB.Get(p_encoded =>FND_API.G_FALSE ), 1, 255));
END LOOP;
END IF;
END;

There is separate article to explain API parameters in more detail, link given below

http://www.oracleerp4u.com/2010/06/api-in-r12.html