Ada: Serial port, String to Stream_element_array - serial-port

Question

when 'B' |'b' =>
cons.Put_Line("Enter text - less than 20 chars: ");
cons.Get_Line(Item => st,
Last => m);
--buffer_ser'Write(st,m);
ser.Write(Port => S_Port,
Buffer => buffer_ser);
Defined as:
package cons renames gnat.IO;
package ser renames gnat.Serial_Communications;
S_Port : gnat.Serial_Communications.Serial_Port;
buffer_ser: ada.Streams.Stream_Element_Array(1..20);
x : Ada.Streams.Stream_Element_Offset;
m : Integer;
st : string(1..20) := (others => ASCII.NUL);
ComPort : GNAT.Serial_Communications.Port_Name(1..5);
Basically, I needed a very portable application, to operate an old device that overlays a video stream based on serial commands. I decided to try out Ada as I'd been learning it for something else.
How do I convert to a Stream_element_array(1..20) from a String(1..20) in Ada? I've tried the obvious answers, and I'm just totally stuck!
Edited as requested for the person who down voted me...

Answer 1

First we need an MCVE.
with gnat.IO;
with ada.Streams;
with gnat.Serial_Communications;
procedure MCVE is
package cons renames gnat.IO;
package ser renames gnat.Serial_Communications;
S_Port : gnat.Serial_Communications.Serial_Port;
buffer_ser: ada.Streams.Stream_Element_Array(1..20);
x : Ada.Streams.Stream_Element_Offset;
m : Integer;
st : string(1..20) := (others => ASCII.NUL);
ComPort : GNAT.Serial_Communications.Port_Name(1..5);
begin
cons.Put_Line("Enter text - less than 20 chars: ");
cons.Get_Line(Item => st,
Last => m);
--buffer_ser'Write(st,m);
ser.Write(Port => S_Port,
Buffer => buffer_ser);
end MCVE;
it compiles successfully and fails with the error
raised GNAT.SERIAL_COMMUNICATIONS.SERIAL_ERROR : write: port not
opened
A little reading on Streams shows that the answer to the actual question:
How do I convert to a Stream_element_array(1..20)
is : normally, you don't. Streams take care of that for you.
A couple of modifications, respectively:
make S_Port aliased so we can take its access (to allow redirectable stream Writes) and delete unnecessary intermediate variables
actually open the serial port as a Stream
write the string directly to the stream
look like
S_Port : aliased gnat.Serial_Communications.Serial_Port;
-- buffer_ser: ada.Streams.Stream_Element_Array(1..20);
-- x : Ada.Streams.Stream_Element_Offset;
...
gnat.Serial_Communications.Open(S_Port,ComPort);
String'Write(S_Port'access, st(1..m));
--buffer_ser'Write(st,m);
--ser.Write(Port => S_Port, Buffer => buffer_ser);
But using fixed length strings when they aren't appropriate is a bad idea. Let's declare the string to be of the right length and simplify further. This requires a function form of Get_Line, which the Gnat.IO package doesn't provide, so let's use the more portable Ada.Text_IO instead. (And initialise the serial port name, and actually use the renamed packages...)
When we're done, we have something like
with Ada.Text_IO;
with ada.Streams;
with gnat.Serial_Communications;
procedure MCVE is
package cons renames Ada.Text_IO;
package ser renames gnat.Serial_Communications;
S_Port : aliased ser.Serial_Port;
ComPort : ser.Port_Name := "COM1";
begin
ser.Open(S_Port,ComPort);
cons.Put_Line("Enter text : ");
declare
st : String := cons.Get_Line;
begin
String'Write(S_Port'access, st);
end;
end MCVE;