.PAS File Format – Standard library

The System unit

Every program uses System without naming it. It holds the standard procedures and functions, which the compiler knows and which are not ordinary routines (they may take any type, change their arguments or be evaluated at compile time). Standard Pascal (ISO 7185) has the first group; Turbo Pascal, Delphi and Free Pascal have the rest.

The System unit also declares the standard types (the Types page), the constants MaxInt, MaxLongInt, the variables Input, Output, ErrOutput, ExitCode, ErrorAddr and RandSeed, and the class TObject with IInterface, IUnknown and TInterfacedObject.

Main units of Delphi and Free Pascal

Free Pascal calls its runtime library the RTL and its class library the FCL; the units for graphics, forms and controls are Lazarus’s LCL. Delphi’s are the RTL and the VCL or FireMonkey.

Turbo Pascal units

Free Pascal has its own Crt, Dos, Graph and Objects units for compatibility.

Input and output

var
  F: TextFile;
  Line: string;
begin
  Assign(F, 'data.txt');
  Reset(F);
  while not Eof(F) do
  begin
    ReadLn(F, Line);
    WriteLn(Length(Line):6, ' ', Line);       { :6 is a field width }
  end;
  Close(F);
  WriteLn(Pi:10:4);                           { width 10, 4 decimals: '    3.1416' }
end;

Write and WriteLn take any number of arguments of simple types and strings, each optionally followed by :width and, for reals, :decimals. Read and ReadLn read the same kinds from the input. A first argument that is a file variable names the file; with none, Output and Input (the console) are used. These are not ordinary routines, which is why they take the : forms.

The names the viewer colours

The file viewer on this site colours these names by themselves (without help from the context). A name that matches is coloured in the stated way unless it follows a dot (a member) or, for a type, is followed by a colon (a variable or parameter of that name).