[SOLVED]File Read using Block Read - Segmentation fault

[wujiyan]

I have a Class procedure named LoadData which read records to dynamic array(1 based) of records written to a file. Initially array size is set to 2 and each time a record is read  , array size is incremented using the SetLength procedure.

When I am running this code, after reading the records, following error is returned .
'Segmentation fault (core dumped)'

I am wondering why is this happening. Please help ..

Code: [Select]

type
   TPerson = record
      name : String[20];
      age : Integer;
      gender : String[10];
      phNum : String[15];
   end;

type
   TPersonDetails = class
      names : array of Tperson;
      count : Integer;
      procedure SaveData;
      procedure LoadData;
      procedure Display;
   end;

procedure TPersonDetails.SaveData;
   var
      f : file of TPerson;
      i : Integer;
   begin
      {$i+}
      AssignFile(f,'/home/user1/pers.bat');
      ReWrite(f);
      {$i+}
      try
         for i := 1 to count - 1 do begin
            BlockWrite(f,names[i],SizeOf(names[i]));
         end;
      finally
         WriteLn('File Write Successfull');
         CloseFile(f);
      end;
   end;

procedure TPersonDetails.LoadData;
   var
      i : Integer;
      f : file of TPerson;
   begin
      i := 2;
      SetLength(names,i);
      count := 1;
      {$i-}
      AssignFile(f,'/home/user1/pers.bat');
      Reset(f);
      {$i+}
      try
         while not EOF(f) do begin
            BlockRead(f,names[count],SizeOf(names[count]));
            SetLength(names,i+1);
            count := count + 1;
         end;
      finally
         CloseFile(f);
         WriteLn('Read successfully');
      end;
   end;   

After calling TPersonDetails.LoadData

Read successfully
Segmentation fault (core dumped)

[Cyrax]

Index of dynamic arrays always starts with 0 (zero). You need to change your program so it will take this in the mind.

[wujiyan]

Index of dynamic arrays always starts with 0 (zero). You need to change your program so it will take this in the mind.

Thanks, but I am setting the array length to 2, and using the Ist Index and in the while loop I am incrementing the length.

[Cyrax]

Index of dynamic arrays always starts with 0 (zero). You need to change your program so it will take this in the mind.

Thanks, but I am setting the array length to 2, and using the Ist Index and in the while loop I am incrementing the length.

See FPC documentation about dynamic arrays : http://www.freepascal.org/docs-html/ref/refsu18.html#x42-480003.3.1

You need to set its length to 3, if you want to start with index 1.
With length 2, minimum index is 0 and maximum 1.

[wujiyan]

See FPC documentation about dynamic arrays : http://www.freepascal.org/docs-html/ref/refsu18.html#x42-480003.3.1

You need to set its length to 3, if you want to start with index 1.
With length 2, minimum index is 0 and maximum 1.

Thanks for the link. I will go through the docs..

[Laksen]

You need Rewrite(f,1); and Reset(f,1); when using blockread with a byte sized argument. You should also use file instead of file of X in that case

[wujiyan]

You need Rewrite(f,1); and Reset(f,1); when using blockread with a byte sized argument. You should also use file instead of file of X in that case

Thanks a mil, Laksen. I followed your instructions and made the changes and my flleload procedure is working like charm ..

modified procedure

Code: [Select]

procedure TPersonDetails.LoadData;
   var
      i : Integer;
      f : file;
   begin
      i := 2;
      FillChar(names,SizeOf(names),#0);
      SetLength(names,i);
      count := 1;
      {$i-}
      AssignFile(f,'/home/bladerunner/cpgms/pascalFiles/nnn.bat');
      Reset(f,1);
      {$i+}
      try
         while not EOF(f) do begin
            BlockRead(f,names[count],SizeOf(names[count]));
            SetLength(names,i+1);
            i := i + 1;
            count := count + 1;
         end;
      finally
         SetLength(names,count- 1);
         count := count - 1;
         CloseFile(f);
      end;
   end; 


[eny]

You need Rewrite(f,1); and Reset(f,1); when using blockread with a byte sized argument. You should also use file instead of file of X in that case

Or use file of TPerson and read/write (and not blockread/blockwrite).

[wujiyan]

Or use file of TPerson and read/write (and not blockread/blockwrite).

Thanks Eny, I will try this as well ..