File Types: Public and Private File Types

Public file types are also known as popular or contentious types because competing applications might want to be associated with these file types. Characteristics of public file types include:

• They are typically defined by standards bodies, and/or are promoted by their defining organizations as interchange formats.
• They are often exchanged between computers and users for diverse purposes.
• They need to be supported on many different platforms.
• Applications from multiple vendors are likely to handle them.

Some examples of file types that are considered public are the image file types .png, .gif, .jpg, and .bmp, and the audio types .wav, .mp3, and .au.

Unlike public file types, private or proprietary file types typically have a format that is implemented and understood by only one application or vendor. As a result, private file types are typically not prone to conflicts between applications. Some file types can start as private file types but later become public file types.

Note  Windows does not differentiate between public and private file types. The distinction is relevant only in making decisions about your choice of file type registration.

File Operation: The OS can provide system calls to create, write, read, reposition, delete, and truncate files.

• Creating a file. Two steps are necessary to create a file.
  1. Space in the file system must be found for the file.
  2. An entry for the new file must be made in the directory.
• Writing a file. To write a file, we make a system call specifying both the name of the file and the information to be written to the file. The system must keep a write pointer to the location in the file where the next write is to take place. The write pointer must be updated whenever a write occurs.
• Reading a file. To read from a file, we use a system call that specifies the name of the file and where (in memory) the next block of the file should be put. The system needs to keep a read pointer to the location in the file where the next read is to take place.
  • Because a process is usually either reading from or writing to a file, the current operation location can be kept as a per-process current-file-position pointer.
  • Both the read and write operations use this same pointer, saving space and reducing system complexity.
• Repositioning within a file. The directory is searched for the appropriate entry, and the current-file-position pointer is repositioned to a given value. Repositioning within a file need not involve any actual I/O. This file operation is also known as a file seek.
• Deleting a file. To delete a file, we search the directory for the named file. Having found the associated directory entry, we release all file space, so that it can be reused by other files, and erase the directory entry.
• Truncating a file. The user may want to erase the contents of a file but keep its attributes. Rather than forcing the user to delete the file and then recreate it, this function allows all attributes to remain unchanged (except for file length) but lets the file be reset to length zero and its file space released.

File Attributes: File attributes are metadata values stored by the file system on disk and are used by the system and are available to developers via various file I/O APIs. For a list of related APIs and topics, see the See Also section. A file or directory that is an archive file or directory.

File Types: Public and Private File Types

Public file types are also known as popular or contentious types because competing applications might want to be associated with these file types. Characteristics of public file types include:

• They are typically defined by standards bodies, and/or are promoted by their defining organizations as interchange formats.
• They are often exchanged between computers and users for diverse purposes.
• They need to be supported on many different platforms.
• Applications from multiple vendors are likely to handle them.

Some examples of file types that are considered public are the image file types .png, .gif, .jpg, and .bmp, and the audio types .wav, .mp3, and .au.

Unlike public file types, private or proprietary file types typically have a format that is implemented and understood by only one application or vendor. As a result, private file types are typically not prone to conflicts between applications. Some file types can start as private file types but later become public file types.

Note  Windows does not differentiate between public and private file types. The distinction is relevant only in making decisions about your choice of file type registration.

File Operation: The OS can provide system calls to create, write, read, reposition, delete, and truncate files.

• Creating a file. Two steps are necessary to create a file.
  1. Space in the file system must be found for the file.
  2. An entry for the new file must be made in the directory.
• Writing a file. To write a file, we make a system call specifying both the name of the file and the information to be written to the file. The system must keep a write pointer to the location in the file where the next write is to take place. The write pointer must be updated whenever a write occurs.
• Reading a file. To read from a file, we use a system call that specifies the name of the file and where (in memory) the next block of the file should be put. The system needs to keep a read pointer to the location in the file where the next read is to take place.
  • Because a process is usually either reading from or writing to a file, the current operation location can be kept as a per-process current-file-position pointer.
  • Both the read and write operations use this same pointer, saving space and reducing system complexity.
• Repositioning within a file. The directory is searched for the appropriate entry, and the current-file-position pointer is repositioned to a given value. Repositioning within a file need not involve any actual I/O. This file operation is also known as a file seek.
• Deleting a file. To delete a file, we search the directory for the named file. Having found the associated directory entry, we release all file space, so that it can be reused by other files, and erase the directory entry.
• Truncating a file. The user may want to erase the contents of a file but keep its attributes. Rather than forcing the user to delete the file and then recreate it, this function allows all attributes to remain unchanged (except for file length) but lets the file be reset to length zero and its file space released.

File Attributes: File attributes are metadata values stored by the file system on disk and are used by the system and are available to developers via various file I/O APIs. For a list of related APIs and topics, see the See Also section. A file or directory that is an archive file or directory.