Difference between revisions of "File"

From Free Pascal wiki
Jump to navigationJump to search
(Remove useless notes that file is usually a file on disk (duh), use modern XxxFile identifiers, mention FileMode, FileRec, TextRec...)
m (Fixed syntax highlighting; deleted category included in page template)
 
(12 intermediate revisions by 9 users not shown)
Line 1: Line 1:
In a [[Pascal]] program, a variable of type '''file''' may be used to read or write (or both) to the file. A file variable is (usually) associated with a normal file on a disk using the [[AssignFile]] procedure (older, equivalent name: just [[Assign]]).
+
{{File(page)}}
  
There are three types of files:
+
In a [[Pascal]] program a variable of type '''file''' may be used to read, write or both read and write to a file. A file variable is (usually) associated with a file on a disk using the [[AssignFile]] procedure (older, equivalent name: just [[Assign]]).
 +
(See [[File Handling In Pascal]] for a more object oriented approach and more examples.)
  
# The type '''file''' is an ''untyped'' binary file. ''Untyped'' means it's just a sequence of data (bytes). It may be used to store essentially anything. It can be read by '''BlockRead''' and written to by '''BlockWrite''' procedures. For example source code, see [http://www.freepascal.org/docs-html/rtl/system/blockread.html BlockRead documentation]. The word ''file'' itself is a [[Reserved word|reserved word]] in Pascal.
+
= File types =
  
# The type '''file of Xxx''', where Xxx is any simple (without references / pointers) type, is a binary file representing a sequence of Xxx values. For example, you can have files that are a sequence of integers, floating-point values, or records (as long as all record fields are still a simple type).
+
== Untyped binary ==
  
# The type [[Text|TextFile]] (older, equivalent name: Text) represents a text file. Note that this is different than just '''file of char'''. There are various comfortable read and write operations available for text files, that parse integers, floating-point values and other types. Also, line endings are properly dealt with when you use text files.
+
The type '''file''' represents an ''untyped'' binary file. ''Untyped'' means it's just a sequence of data (bytes). It may be used to store essentially anything. It can be read by '''BlockRead''' and written to by '''BlockWrite''' procedures. The word ''file'' itself is a [[Reserved word|reserved word]] in Pascal. Example:
 +
 
 +
<syntaxhighlight lang=pascal>
 +
{$mode objfpc}{$H+}
 +
var
 +
  MyFile: file;
 +
  Data: array [0..99] of Byte;
 +
begin
 +
  AssignFile(MyFile, 'a');
 +
  Reset(MyFile, 1 { size of read chunk });
 +
  try
 +
    BlockRead(MyFile, Data, SizeOf(Data));
 +
  finally
 +
    CloseFile(MyFile);
 +
  end;
 +
end.</syntaxhighlight>
 +
 
 +
For larger example code, see [http://www.freepascal.org/docs-html/rtl/system/blockread.html BlockRead documentation].
 +
 
 +
== Typed binary ==
 +
 
 +
The type [http://www.freepascal.org/docs-html/ref/refsu21.html '''file of <type>'''], where <type> is any simple type (no references/pointers) or record type, is a binary file representing a sequence of <type> values. For example, you can have files that are a sequence of integers, floating-point values, or records (as long as all record fields are also simple types).
 +
 
 +
<syntaxhighlight lang=pascal>
 +
{$mode objfpc}{$H+}
 +
var
 +
  MyFile: file of Single;
 +
  Data: Single;
 +
 
 +
begin
 +
  AssignFile(MyFile, 'singlevalues.bin');
 +
  try
 +
    Reset(MyFile);
 +
    Read(MyFile, Data);
 +
 
 +
  finally
 +
    CloseFile(MyFile);
 +
  end;
 +
end.</syntaxhighlight>
 +
 
 +
== Text file ==
 +
 
 +
The type [[Text|TextFile]] (older, equivalent name: Text) represents a text file. Note that this is different than just '''file of char'''. There are various comfortable read and write operations available for text files, that parse integers, floating-point values and other types. Also, line endings are properly dealt with when you use text files.
 +
 
 +
See [[Text|TextFile]] page for more information and examples.
 +
 
 +
= Working with files =
  
 
Working with all file types is similar:
 
Working with all file types is similar:
# Associate a file with data. ''Usually'' by calling [[AssignFile]] (older, equivalent name: [[Assign]]).
+
# Associate a file with data. ''Usually'' by calling [[AssignFile]]. An older, equivalent name is [[Assign]]. (With the advent of GUI libraries there arose the possibility of name clashes with the names of some traditional Pascal file routines. TPersistent and TForm (a TPersistent descendent) have Assign and Close methods. Because of this, confusion can arise if you write Assign or Close in TForm methods, hence AssignFile and CloseFile were introduced as aliases for the traditional Assign and Close, to prevent inadvertently calling an unintended procedure. In non-GUI code you may of course use the original Assign and Close, and save a bit of typing).
 
# Open the file by [[Reset]] (read existing file, and maybe write --- see below), [[Rewrite]] (clear contents and write), or [[Append]] (keep contents and write). In case of [[Reset]] on an untyped binary file, the global [[FileMode]] variable determines if you can only read (fmOpenRead), or both read and write (fmOpenReadWrite), to the file.
 
# Open the file by [[Reset]] (read existing file, and maybe write --- see below), [[Rewrite]] (clear contents and write), or [[Append]] (keep contents and write). In case of [[Reset]] on an untyped binary file, the global [[FileMode]] variable determines if you can only read (fmOpenRead), or both read and write (fmOpenReadWrite), to the file.
 
# Then read/write file data by [[Read|read]], [[readln]], [[Write|write]] and [[writeln]] standard procedures.  Some Pascal Compilers also used the [[Get]] and [[Put]] routines for I/O. For untyped binary files, use [[BlockRead]] and [[BlockWrite]].
 
# Then read/write file data by [[Read|read]], [[readln]], [[Write|write]] and [[writeln]] standard procedures.  Some Pascal Compilers also used the [[Get]] and [[Put]] routines for I/O. For untyped binary files, use [[BlockRead]] and [[BlockWrite]].
Line 17: Line 64:
 
A file variable is used either for input, for output, or for both input and output (I/O). A file may be a screen and/or keyboard, a modem, a light pen, a touch screen, a mouse, an area of storage on disk, a socket for connecting to the Internet, or it could be some other device depending on how the run-time library ties the file.  In some cases, actual memory can be defined as a file; this was used in the case of a ram disk. You can customize where the file data is by initializing [http://www.freepascal.org/docs-html/rtl/sysutils/filerec.html FileRec record] (for untyped binary files) or [http://www.freepascal.org/docs-html/rtl/sysutils/textrec.html TextRec record] (for text files), instead of calling standard [[AssignFile]] ([[Assign]]) routines.
 
A file variable is used either for input, for output, or for both input and output (I/O). A file may be a screen and/or keyboard, a modem, a light pen, a touch screen, a mouse, an area of storage on disk, a socket for connecting to the Internet, or it could be some other device depending on how the run-time library ties the file.  In some cases, actual memory can be defined as a file; this was used in the case of a ram disk. You can customize where the file data is by initializing [http://www.freepascal.org/docs-html/rtl/sysutils/filerec.html FileRec record] (for untyped binary files) or [http://www.freepascal.org/docs-html/rtl/sysutils/textrec.html TextRec record] (for text files), instead of calling standard [[AssignFile]] ([[Assign]]) routines.
  
[[category:Pascal]]
+
=See also=
 +
 
 +
* [[File Handling In Pascal]]
 +
 
  
 
{{File}}
 
{{File}}

Latest revision as of 00:14, 15 February 2020

Deutsch (de) English (en) français (fr) Bahasa Indonesia (id)

In a Pascal program a variable of type file may be used to read, write or both read and write to a file. A file variable is (usually) associated with a file on a disk using the AssignFile procedure (older, equivalent name: just Assign). (See File Handling In Pascal for a more object oriented approach and more examples.)

File types

Untyped binary

The type file represents an untyped binary file. Untyped means it's just a sequence of data (bytes). It may be used to store essentially anything. It can be read by BlockRead and written to by BlockWrite procedures. The word file itself is a reserved word in Pascal. Example:

{$mode objfpc}{$H+}
var
  MyFile: file;
  Data: array [0..99] of Byte;
begin
  AssignFile(MyFile, 'a');
  Reset(MyFile, 1 { size of read chunk });
  try
    BlockRead(MyFile, Data, SizeOf(Data)); 
  finally
    CloseFile(MyFile);
  end;
end.

For larger example code, see BlockRead documentation.

Typed binary

The type file of <type>, where <type> is any simple type (no references/pointers) or record type, is a binary file representing a sequence of <type> values. For example, you can have files that are a sequence of integers, floating-point values, or records (as long as all record fields are also simple types).

{$mode objfpc}{$H+}
var
  MyFile: file of Single;
  Data: Single;

begin
  AssignFile(MyFile, 'singlevalues.bin');
  try
    Reset(MyFile);
    Read(MyFile, Data);

  finally
    CloseFile(MyFile);
  end;
end.

Text file

The type TextFile (older, equivalent name: Text) represents a text file. Note that this is different than just file of char. There are various comfortable read and write operations available for text files, that parse integers, floating-point values and other types. Also, line endings are properly dealt with when you use text files.

See TextFile page for more information and examples.

Working with files

Working with all file types is similar:

  1. Associate a file with data. Usually by calling AssignFile. An older, equivalent name is Assign. (With the advent of GUI libraries there arose the possibility of name clashes with the names of some traditional Pascal file routines. TPersistent and TForm (a TPersistent descendent) have Assign and Close methods. Because of this, confusion can arise if you write Assign or Close in TForm methods, hence AssignFile and CloseFile were introduced as aliases for the traditional Assign and Close, to prevent inadvertently calling an unintended procedure. In non-GUI code you may of course use the original Assign and Close, and save a bit of typing).
  2. Open the file by Reset (read existing file, and maybe write --- see below), Rewrite (clear contents and write), or Append (keep contents and write). In case of Reset on an untyped binary file, the global FileMode variable determines if you can only read (fmOpenRead), or both read and write (fmOpenReadWrite), to the file.
  3. Then read/write file data by read, readln, write and writeln standard procedures. Some Pascal Compilers also used the Get and Put routines for I/O. For untyped binary files, use BlockRead and BlockWrite.
  4. At the end, release the necessary file resources (and possibly flush the data to be written) by calling CloseFile (older, equivalent name: just Close).

A file variable is used either for input, for output, or for both input and output (I/O). A file may be a screen and/or keyboard, a modem, a light pen, a touch screen, a mouse, an area of storage on disk, a socket for connecting to the Internet, or it could be some other device depending on how the run-time library ties the file. In some cases, actual memory can be defined as a file; this was used in the case of a ram disk. You can customize where the file data is by initializing FileRec record (for untyped binary files) or TextRec record (for text files), instead of calling standard AssignFile (Assign) routines.

See also


File-related types, procedures and functions:

File - Text - AssignFile - CloseFile - Reset - Rewrite - Get - Put - Read - Readln - Write - Writeln