Your first C++ TANGO device class

Intended audience: beginner developers, Programming language: c++

The code given in this chapter as example has been generated using POGO. Pogo is a code generator for Tango device server. See POGO home page for more information about POGO. The following examples briefly describe how to write device class with commands which receives and return different kind of Tango data types and also how to write device attributes The device class implements 5 commands and 3 attributes. The commands are :

  • The command DevSimple deals with simple Tango data type
  • The command DevString deals with Tango strings
  • DevArray receive and return an array of simple Tango data type
  • DevStrArray which does not receive any data but which returns an array of strings
  • DevStruct which also does not receive data but which returns one of the two Tango composed types (DevVarDoubleStringArray)

For all these commands, the default behavior of the state machine (command always allowed) is acceptable. The attributes are :

  • A spectrum type attribute of the Tango string type called StrAttr
  • A readable attribute of the Tango::DevLong type called LongRdAttr. This attribute is linked with the following writable attribute
  • A writable attribute also of the Tango::DevLong type called LongWrAttr.

Since release 9, a Tango device also supports pipe. This is an advanced feature reserved for some specific cases. Therefore, there is no device pipe example in this Getting started chapter.

The commands and attributes code

For each command called DevXxxx, pogo generates in the device class a method named dev_xxx which will be executed when the command is requested by a client. In this chapter, the name of the device class is DocDs

The DevSimple command

This method receives a Tango::DevFloat type and also returns a data of the Tango::DevFloat type which is simply the double of the input value. The code for the method executed by this command is the following:

 1   Tango::DevFloat DocDs::dev_simple(Tango::DevFloat argin)
 2   {
 3           Tango::DevFloat argout ;
 4           DEBUG_STREAM << "DocDs::dev_simple(): entering... !" << endl;
 5 
 6           //      Add your own code to control device here
 7 
 8           argout = argin * 2;
 9           return argout;
10   }

This method is fairly simple. The received data is passed to the method as its argument. It is

doubled at line 8 and the method simply returns the result.

The DevArray command

This method receives a data of the Tango::DevVarLongArray type and also returns a data of the Tango::DevVarLongArray type. Each element of the array is doubled. The code for the method executed by the command is the following :

 1   Tango::DevVarLongArray *DocDs::dev_array(const Tango::DevVarLongArray *argin)
 2   {
 3           //      POGO has generated a method core with argout allocation.
 4           //      If you would like to use a static reference without copying,
 5           //      See "TANGO Device Server Programmer's Manual"
 6           //              (chapter x.x)
 7           //------------------------------------------------------------
 8           Tango::DevVarLongArray  *argout  = new Tango::DevVarLongArray();
 9 
10           DEBUG_STREAM << "DocDs::dev_array(): entering... !" << endl;
11 
12           //      Add your own code to control device here
13 
14           long argin_length = argin->length();
15           argout->length(argin_length);
16           for (int i = 0;i < argin_length;i++)
17                   (*argout)[i] = (*argin)[i] * 2;
18 
19           return argout;
20   }

The argout data array is created at line 8. Its length is set at line 15 from the input argument length. The array is populated at line 16,17 and returned. This method allocates memory for the argout array. This memory is freed by the Tango core classes after the data have been sent to the caller (no delete is needed). It is also possible to return data from a statically allocated array without copying. Look at chapter [Data exchange] for all the details.

The DevString command

This method receives a data of the Tango::DevString type and also returns a data of the Tango::DevString type. The command simply displays the content of the input string and returns a hard-coded string. The code for the method executed by the command is the following :

 1   Tango::DevString DocDs::dev_string(Tango::DevString argin)
 2   {
 3           //      POGO has generated a method core with argout allocation.
 4           //      If you would like to use a static reference without copying,
 5           //      See "TANGO Device Server Programmer's Manual"
 6           //              (chapter x.x)
 7           //------------------------------------------------------------
 8           Tango::DevString        argout;
 9           DEBUG_STREAM << "DocDs::dev_string(): entering... !" << endl;
10 
11           //      Add your own code to control device here
12 
13           cout << "the received string is " << argin << endl;
14 
15           string str("Am I a good Tango dancer ?");
16           argout = new char[str.size() + 1];
17           strcpy(argout,str.c_str());
18 
19           return argout;
20   }

The argout string is created at line 8. Internally, this method is using a standard C++ string. Memory for the returned data is allocated at line 16 and is initialized at line 17. This method allocates memory for the argout string. This memory is freed by the Tango core classes after the data have been sent to the caller (no delete is needed). It is also possible to return data from a statically allocated string without copying. Look at chapter [Data exchange] for all the details.

The DevStrArray command

This method does not receive input data but returns an array of strings (Tango::DevVarStringArray type). The code for the method executed by this command is the following:

 1   Tango::DevVarStringArray *DocDs::dev_str_array()
 2   {
 3           //      POGO has generated a method core with argout allocation.
 4           //      If you would like to use a static reference without copying,
 5           //      See "TANGO Device Server Programmer's Manual"
 6           //              (chapter x.x)
 7           //------------------------------------------------------------
 8           Tango::DevVarStringArray        *argout  = new Tango::DevVarStringArray();
 9 
10           DEBUG_STREAM << "DocDs::dev_str_array(): entering... !" << endl;
11 
12           //      Add your own code to control device here
13 
14           argout->length(3);
15           (*argout)[0] = CORBA::string_dup("Rumba");
16           (*argout)[1] = CORBA::string_dup("Waltz");
17           string str("Jerck");
18           (*argout)[2] = CORBA::string_dup(str.c_str());
19           return argout;
20   }

The argout data array is created at line 8. Its length is set at line 14. The array is populated at line 15,16 and 18. The last array element is initialized from a standard C++ string created at line 17. Note the usage of the string_dup function of the CORBA namespace. This is necessary for strings array due to the CORBA memory allocation schema.

The DevStruct command

This method does not receive input data but returns a structure of the Tango::DevVarDoubleStringArray type. This type is a composed type with an array of double and an array of strings. The code for the method executed by this command is the following:

 1   Tango::DevVarDoubleStringArray *DocDs::dev_struct()
 2   {
 3           //      POGO has generated a method core with argout allocation.
 4           //      If you would like to use a static reference without copying,
 5           //      See "TANGO Device Server Programmer's Manual"
 6           //              (chapter x.x)
 7           //------------------------------------------------------------
 8           Tango::DevVarDoubleStringArray  *argout  = new Tango::DevVarDoubleStringArray();
 9 
10           DEBUG_STREAM << "DocDs::dev_struct(): entering... !" << endl;
11 
12           //      Add your own code to control device here
13 
14           argout->dvalue.length(3);
15           argout->dvalue[0] = 0.0;
16           argout->dvalue[1] = 11.11;
17           argout->dvalue[2] = 22.22;
18 
19           argout->svalue.length(2);
20           argout->svalue[0] = CORBA::string_dup("Be Bop");
21           string str("Smurf");
22           argout->svalue[1] = CORBA::string_dup(str.c_str());
23 
24           return argout;
25   }

The argout data structure is created at line 8. The length of the double array in the output structure is set at line 14. The array is populated between lines 15 and 17. The length of the string array in the output structure is set at line 19. This string array is populated between lines 20 an 22 from a hard-coded string and from a standard C++ string. This method allocates memory for the argout data. This memory is freed by the Tango core classes after the data have been sent to the caller (no delete is needed). Note the usage of the string_dup function of the CORBA namespace. This is necessary for strings array due to the CORBA memory allocation schema.

The three attributes

Some data have been added to the definition of the device class in order to store attributes value. These data are (part of the class definition) :

1  protected :
2          //      Add your own data members here
3          //-----------------------------------------
4          Tango::DevString        attr_str_array[5];
5          Tango::DevLong          attr_rd;
6          Tango::DevLong          attr_wr;

One data has been created for each attribute. As the StrAttr attribute is of type spectrum with a maximum X dimension of 5, an array of length 5 has been reserved.

Several methods are necessary to implement these attributes. One method to read the hardware which is common to all readable attributes plus one read method for each readable attribute and one write method for each writable attribute. The code for these methods is the following :

 1  void DocDs::read_attr_hardware(vector<long> &attr_list)
 2  {
 3      DEBUG_STREAM << "DocDs::read_attr_hardware(vector<long> &attr_list) entering... "<< endl;
 4  // Add your own code here
 5 
 6      string att_name;
 7      for (long i = 0;i < attr_list.size();i++)
 8      {
 9          att_name = dev_attr->get_attr_by_ind(attr_list[i]).get_name();
10 
11         if (att_name == "LongRdAttr")
12         {
13             attr_rd = 5;
14         }
15     }
16  }
17 
18  void DocDs::read_LongRdAttr(Tango::Attribute &attr)
19  {
20      DEBUG_STREAM << "DocDs::read_LongRdAttr(Tango::Attribute &attr) entering... "<< endl;
21 
22      attr.set_value(&attr_rd);
23  }
24 
25  void DocDs::read_LongWrAttr(Tango::Attribute &attr)
26  {
27      DEBUG_STREAM << "DocDs::read_LongWrAttr(Tango::Attribute &attr) entering... "<< endl;
28 
29      attr.set_value(&attr_wr);
30  }
31 
32  void DocDs::write_LongWrAttr(Tango::WAttribute &attr)
33  {
34      DEBUG_STREAM << "DocDs::write_LongWrAttr(Tango::WAttribute &attr) entering... "<< endl;
35 
36      attr.get_write_value(attr_wr);
37      DEBUG_STREAM << "Value to be written = " << attr_wr << endl;
38  }
39 
40  void DocDs::read_StrAttr(Tango::Attribute &attr)
41  {
42      DEBUG_STREAM << "DocDs::read_StrAttr(Tango::Attribute &attr) entering... "<< endl;
43 
44      attr_str_array[0] = const_cast<char *>("Rock");
45      attr_str_array[1] = const_cast<char *>("Samba");
46 
47      attr_set_value(attr_str_array, 2);
48  }

The read_attr_hardware() method is executed once when a client execute the read_attributes CORBA request whatever the number of attribute to be read is. The rule of this method is to read the hardware and to store the read values somewhere in the device object. In our example, only the LongRdAttr attribute internal value is set by this method at line 13. The method read_LongRdAttr() is executed by the read_attributes CORBA call when the LongRdAttr attribute is read but after the read_attr_hardware() method has been executed. Its rule is to set the attribute value in the TANGO core classes object representing the attribute. This is done at line 22. The method read_LongWrAttr() will be executed when the LongWrAttr attribute is read (after the read_attr_hardware() method). The attribute value is set at line 29. In the same manner, the method called read_StrAttr() will be executed when the attribute StrAttr is read. Its value is initialized in this method at line 44 and 45 with the string_dup Tango function. There are several ways to code spectrum or image attribute of the DevString data type. A HowTo related to this topic is available on the Tango control system Web site. The write_LongWrAttr() method is executed when the LongWrAttr attribute value is set by a client. The new attribute value coming from the client is stored in the object data at line 36.

Pogo also generates a file called DocDsStateMachine.cpp (for a Tango device server class called DocDs). This file is used to store methods coding the device state machine. By default a allways allowed state machine is provided. For more information about coding the state machine, refer to the chapter Writing a device server.