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Date: Wed, 22 Jul 2015 19:20:34 -0700 (PDT)
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Subject: Returning a string from C
From: NiGHTS <nights@unku.us>
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Xref: news.eternal-september.org comp.lang.ada:26998
Date: 2015-07-22T19:20:34-07:00
List-Id: <comp.lang.ada>

I am having some difficulty with returning a string from a C function.

Say I have the following C function:

    void Return_Hello (char *inout_Test_String ) {
        strcpy( inout_Test_String, "Hello World" );
    }

This function will use the string memory generated on the Ada side and return a simple string "Hello World". Here is how I would interface with it:

    procedure Return_Hello ( Test_String : in out chars_ptr );
    pragma Import (C, Return_Hello, "Return_Hello");

And this is how I would use it.

    use Interfaces.C;
    use Interfaces.C.Strings;

    declare

        Test_String      : String (1 .. 100);
        Test_String_Char : chars_ptr := New_String (Test_String);

    begin
    
        Return_Hello ( Test_String_Char );
        Test_String := Value ( Test_String_Char ) ;
        Ada.Text_IO.Put_Line ( Test_String );
        Free (Test_String_Char);
  
    end;

When I run the program I get a runtime error "length check failed" on the line "Test_String := Value ( Test_String_Char );".

What am I doing wrong here?