Delphi Language Guide Delphi for Microsoft Win32 Delphi for the Microsoft. Net framework

File Input and Output
The table below lists input and output routines.
Input and output procedures and functions
Procedure or function Description
Append
Opens an existing text file for appending.
AssignFile
Assigns the name of an external file to a file variable.
BlockRead
Reads one or more records from an untyped file.
BlockWrite
Writes one or more records into an untyped file.
ChDir
Changes the current directory.
CloseFile
Closes an open file.
Eof
Returns the end-of-file status of a file.
Eoln
Returns the end-of-line status of a text file.
Erase
Erases an external file.
FilePos
Returns the current file position of a typed or untyped file.
FileSize
Returns the current size of a file; not used for text files.
Flush
Flushes the buffer of an output text file.
GetDir
Returns the current directory of a specified drive.
IOResult
Returns an integer value that is the status of the last I/O function performed.
MkDir
Creates a subdirectory.
Read
Reads one or more values from a file into one or more variables.
Readln
Does what Read does and then skips to beginning of next line in the text file.
Rename
Renames an external file.
Reset
Opens an existing file.
Rewrite
Creates and opens a new file.
RmDir
Removes an empty subdirectory.
Seek
Moves the current position of a typed or untyped file to a specified component. Not used with text files.
SeekEof
Returns the end-of-file status of a text file.
SeekEoln
Returns the end-of-line status of a text file.
SetTextBuf
Assigns an I/O buffer to a text file.
Truncate
Truncates a typed or untyped file at the current file position.
Write
Writes one or more values to a file.
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Writeln
Does the same as Write, and then writes an end-of-line marker to the text file.
A file variable is any variable whose type is a file type. There are three classes of file: typed, text, and untyped. The
syntax for declaring file types is given in File types.
Before a file variable can be used, it must be associated with an external file through a call to the AssignFile
procedure. An external file is typically a named disk file, but it can also be a device, such as the keyboard or the
display. The external file stores the information written to the file or supplies the information read from the file.
Once the association with an external file is established, the file variable must be opened to prepare it for input or
output. An existing file can be opened via the Reset procedure, and a new file can be created and opened via the
Rewrite procedure. Text files opened with Reset are read-only and text files opened with Rewrite and Append are
write-only. Typed files and untyped files always allow both reading and writing regardless of whether they were
opened with Reset or Rewrite.
Every file is a linear sequence of components, each of which has the component type (or record type) of the file.
The components are numbered starting with zero.
Files are normally accessed sequentially. That is, when a component is read using the standard procedure Read or
written using the standard procedure Write, the current file position moves to the next numerically ordered file
component. Typed files and untyped files can also be accessed randomly through the standard procedure Seek,
which moves the current file position to a specified component. The standard functions FilePos and FileSize can be
used to determine the current file position and the current file size.
When a program completes processing a file, the file must be closed using the standard procedure CloseFile. After
a file is closed, its associated external file is updated. The file variable can then be associated with another external
file.
By default, all calls to standard I/O procedures and functions are automatically checked for errors, and if an error
occurs an exception is raised (or the program is terminated if exception handling is not enabled). This automatic
checking can be turned on and off using the {$I+} and {$I} compiler directives. When I/O checking is off, that is,
when a procedure or function call is compiled in the {$I} state an I/O error doesn't cause an exception to be raised;
to check the result of an I/O operation, you must call the standard function IOResult instead.
You must call the IOResult function to clear an error, even if you aren't interested in the error. If you don't clear an
error and {$I+} is the current state, the next I/O function call will fail with the lingering IOResult error.