Which method sets the position in the current FileStream?

FileStream is used for reading and writing files on a file system, as well as other file-related operating system handles such as pipes, standard input, standard output. FileStream buffers input and output for better performance.

The FileStream class can open a file in one of two modes, either synchronously or asynchronously, with significant performance consequences for the synchronous methods (System.IO.FileStream.Read(System.Byte[],System.Int32,System.Int32) and System.IO.FileStream.Write(System.Byte[],System.Int32,System.Int32)) and the asynchronous methods (System.IO.FileStream.BeginRead(System.Byte[],System.Int32,System.Int32,System.AsyncCallback,System.Object) and System.IO.FileStream.BeginWrite(System.Byte[],System.Int32,System.Int32,System.AsyncCallback,System.Object) ). Both sets of methods will work in either mode; however, the mode will affect the performance of these methods. FileStream defaults to opening files synchronously, but provides a constructor to open files asynchronously.

When accessing files, a security check is performed when the file is created or opened. The security check is typically not done again unless the file is closed and reopened. [Note: Checking permissions when the file is first accessed minimizes the impact of the security check on application performance (since opening a file happens once, while reading and writing can happen multiple times).]

Note that if an opened file is passed to an untrusted caller, the security system can, but is not required to prevent the caller from accessing the file.

FileStream objects support random access to files using the System.IO.FileStream.Seek(System.Int64,System.IO.SeekOrigin) method, and the System.IO.Stream.CanSeek properties of FileStream instances encapsulating files are set to true . The System.IO.FileStream.Seek(System.Int64,System.IO.SeekOrigin) method allows the read/write position to be moved to any position within the file. This is done with byte offset reference point parameters. The byte offset is relative to the seek reference point, which can be the beginning, the current position, or the end of the underlying file, as represented by the three values of the SeekOrigin enumeration.

If a FileStream encapsulates a device that does not support seeking, its System.IO.FileStream.CanSeek property is false . [Note: For additional information, see System.IO.Stream.CanSeek.]

[Note: The File class provides methods for the creation of FileStream objects based on file paths. The MemoryStream class creates a stream from a byte array and functions similarly to a FileStream.]

The following example demonstrates the use of a FileStream object.using System; using System.IO; class Directory { public static void Main(String[] args) { FileStream fs = new FileStream("log.txt", FileMode.OpenOrCreate, FileAccess.Write); StreamWriter w = new StreamWriter(fs); w.BaseStream.Seek(0, SeekOrigin.End); // Set the file pointer to the end. Log ("Test1", w); Log ("Test2", w); w.Close(); // Close the writer and underlying file. fs = new FileStream("log.txt", FileMode.OpenOrCreate, FileAccess.Read); StreamReader r = new StreamReader(fs); r.BaseStream.Seek(0, SeekOrigin.Begin); DumpLog (r); } public static void Log (String logMessage, StreamWriter w) { w.Write("Log Entry : "); w.WriteLine("{0} {1}", DateTime.Now.ToLongTimeString(), DateTime.Now.ToLongDateString()); w.WriteLine(":"); w.WriteLine(":{0}", logMessage); w.WriteLine ("-------------------------------"); w.Flush(); } public static void DumpLog (StreamReader r) { while (r.Peek() > -1) { // While not at the end of the file, write to standard output. Console.WriteLine(r.ReadLine()); } r.Close(); } } Some example output is

Log Entry : 9:26:21 AM Friday, July 06, 2001

:

:Test1

-------------------------------

Log Entry : 9:26:21 AM Friday, July 06, 2001

:

:Test2

-------------------------------

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Description

Python file method seek() sets the file's current position at the offset. The whence argument is optional and defaults to 0, which means absolute file positioning, other values are 1 which means seek relative to the current position and 2 means seek relative to the file's end.

There is no return value. Note that if the file is opened for appending using either 'a' or 'a+', any seek() operations will be undone at the next write.

If the file is only opened for writing in append mode using 'a', this method is essentially a no-op, but it remains useful for files opened in append mode with reading enabled (mode 'a+').

If the file is opened in text mode using 't', only offsets returned by tell() are legal. Use of other offsets causes undefined behavior.

Note that not all file objects are seekable.

Syntax

Following is the syntax for seek() method −

fileObject.seek(offset[, whence])

Parameters

• offset − This is the position of the read/write pointer within the file.

• whence − This is optional and defaults to 0 which means absolute file positioning, other values are 1 which means seek relative to the current position and 2 means seek relative to the file's end.

Return Value

This method does not return any value.

Example

The following example shows the usage of seek() method.

Python is a great language Python is a great language #!/usr/bin/python # Open a file fo = open("foo.txt", "rw+") print "Name of the file: ", fo.name # Assuming file has following 5 lines # This is 1st line # This is 2nd line # This is 3rd line # This is 4th line # This is 5th line line = fo.readline() print "Read Line: %s" % (line) # Again set the pointer to the beginning fo.seek(0, 0) line = fo.readline() print "Read Line: %s" % (line) # Close opend file fo.close()

When we run above program, it produces following result −

Name of the file: foo.txt Read Line: Python is a great language. Read Line: Python is a great language.

python_files_io.htm

Which method sets the position in the current file stream?

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