FILE STRUCTURE:
*Indexes made it possible to keep a list of keys and pointers in a smaller file that could be searched more quickly.
*Simple indexes became difficult to manage for dynamic files in which the set of keys changes.
*The simplest organization for a file is sequential.
*A sequential file is a sequence of records.
*The records may or may not be kept in sorted order in the sequence.
*Although file records are typically of type structure, a file record may also be declared to be of type integer, float, character, or any other C type.
*Trees grew unevenly as records were added and deleted, resulting in long searches requiring multiple disk accesses to find a record. Hence an elegant, self-adjusting binary tree structure called an AVL tree was developed for data in memory.
*Even with a balanced binary tree, dozens of accesses were required to find a record in moderate-sized files.
*A method was needed to keep a tree balanced when each node of the tree was not a single record, as in a binary tree, but a file block containing hundreds of records.
*AVL trees grow from top down as records are added, B-Trees grow from the bottom up.
*B-Trees provided excellent access performance but, a file could not be accessed sequentially with efficiency.
*To further reduce the number of disk accesses, hashing was introduced for files that do not change size greatly over time.
*Extendible, dynamic hashing was introduced for volatile, dynamic files which change.
The open function must be supplied with :
- The name of the physical file
- The access mode
- For new files, the protection mode
The value returned by the open is the fd, and is assigned to the file variable.
Function to open a file:
- fd = open(filename,flags[,pmode]);
- fd-file descriptor
UNIX and PC-DOS:
For handle level access, the logical file is declared as an int.
The handle is also known as a file descriptor.
Prototypes:
- int open (const char* Filename, int Access);
- int open (const char* Filename, int Access, int Protection);
Example:
int Input;
Input = open ("Daily.txt", O_RDONLY);
The following flags can be bitwise ored together for the access mode:
- O_RDONLY : Read only.
- O_WRONLY : Write only.
- O_RDWR : Read or write
- O_CREAT : Create file if it does not exist.
- O_EXCL : If the file exists, truncate it to a length of zero, destroying itscontents. (used only with O_CREAT).
- O_APPEND : Append every write operation to the end of the file.
- O_TRUNC : Delete any prior file contents
Pmode- protection mode
The security status of a file, defining who is allowed to access a file, and which access modes are allowed.
To disassociate a logical program file from a physical system file.
- Closing a file frees system resources for reuse
- .Data may not be actually written to the physical file until a logical file is closed.
- A program should close a file when it is no longer needed.
- The C++ close function is used to close a file for handle level access.
- The handle close function must be supplied with (as an argument):
The handle of the logical file
The value returned by the close is 0 if the close succeeds, and -1 if the close fails..
Prototypes:
EXAMPLE:
