File Types

• A common technique for implementing file types is to include the type as part of the file name (see Fig. 4).
  • The name is split into two parts -a name and an extension, usually separated by a period character.
  • In this way, the user and the OS can tell from the name alone what the type of a file is.

  Figure 4: Common file types.

  
• The system uses the extension to indicate the type of the file and the type of operations that can be done on that file. Only a file with a $.com, .exe$, or $.bat$ extension can be executed, for instance.
• Application programs also use extensions to indicate file types in which they are interested. For example, assemblers expect source files to have an $.asm$ extension, and the Microsoft Word word processor expects its files to end with a $.doc$ extension.
• Because these extensions are not supported by the OS, they can be considered as ``hints'' to the applications that operate on them.
• Many file systems support names as long as 255 characters. Some file systems distinguish between upper and lower case letters (Case (in)sensitivity).
• Windows 95 and Windows 98 both use the MS-DOS file system, and thus inherit many of its properties, such as how file names are constructed. In addition, Windows NT and Windows 2000 support the MS-DOS file system and thus also inherit its properties. However, the latter two systems also have a native file system (NTFS) that has different properties (such as file names in Unicode).
• Consider the Mac OS X OS. In this system, each file has a type, such as TEXT (for text file) or APPL (for application).
  • Each file also has a creator attribute containing the name of the program that created it.
  • This attribute is set by the OS during the $create ()$ call, so its use is enforced and supported by the system.
• The UNIX system uses a crude magic number stored at the beginning of some files to indicate roughly the type of the file -executable program, batchfile (or shell script), PostScript file, and so on.
  • Not all files have magic numbers, so system features cannot be based solely on this information.
  • UNIX does not record the name of the creating program, either. UNIX does allow file-name-extension hints, but these extensions are neither enforced nor depended on by the OS (interpreted by tools); they are meant mostly to aid users in determining the type of contents of the file.
  • Extensions can be used or ignored by a given application, but that is up to the application's programmer.
  • In contrast, Windows is aware of the extensions and assigns meaning to them. Users (or processes) can register extensions with the operating system (Interpreted by OS).
• UNIX also has character and block special files (Device Files).
  • Character special files are related to input/output and used to model serial I/O devices such as terminals, printers, and networks.
  • Block special files are used to model disks.
• Other files are binary files, which just means that they are not ASCII files. Usually, they have some internal structure known to programs that use them (see Fig. 5).

  Figure 5: (a) An executable file. (b) An archive.

  
• Every OS must recognize at least one file type; its own executable file. A simple executable binary file taken from a version of UNIX is seen in Fig. 5a .
  • Although technically the file is just a sequence of bytes, the operating system will only execute a file if it has the proper format.
  • It has five sections: header, text, data, relocation bits, and symbol table.
  • The header starts with a so-called magic number, identifying the file as an executable file (to prevent the accidental execution of a file not in this format).
  • Then come the sizes of the various pieces of the file, the address at which execution starts, and some flag bits.
  • Beyond this header, executable files are typically divided into subsections (the text and data of the program itself).
  • Try the following commands:

    readelf -S exe_file
    objdump -h exe_file
    

• Second example of a binary file is an archive, also from UNIX (see Fig. 5b).
  • It consists of a collection of library procedures (modules) compiled but not linked.
  • Each one is prefaced by a header telling its name, creation date, owner, protection code, and size.