// example 3

<'
struct  packet_s {  // structs are more or less like classes or structures in C, 
                    //I will talk more about them later on
   
   length : uint;      
   // This is a regular field that will be given a random value within the 
   //relevant constraints when a new packet is
   // created. 
   keep length >= 10 and length <=20; // length of the packet in bytes

   %header1 : byte;           
   // The percent sign indicates that this field is a real part 
   //of the packet, which will be sent (physical field).
   // The  length field for example is a virtual field and 
   //will not be sent. After the packet is created,   
   // you can translate it into bits using a method of every 
   // struct called pack() (which is a lot like
   // the serialize() method that C++ provides for writing 
   //data structures into a file).
   // pack() translates all the real, physical fields 
   //into bits and then concatenates them.

   keep header1 != 0; // header1 can get any random value except 0


   %header2 : byte;
   keep header1 <  128 => header2 != 0; 
   // The Value of header2 depends on the value of header1. 
   // If header1 is smaller than 128
   // header2 must not be  equal to zero. If header1 is 
   // equal or greater than 128 header2 can get any value.
   // Note that header1 is assigned a  random value (generated) 
   // before header2 because header1 is defined above header2.

   %data : list of byte; // data is a list of bytes   
   keep data.size() == length-2; 
   // size() is a method of every list. One of the best things 
   // about Specman is the large
   // number of predefined methods for list  objects. 
   // When a list is randomly generated  
   // the size of the list is also random. The constraint 
   // assures that the length of the packet
   // including the two header bytes will in fact be  equal 
   // to the  field length
};
'>