Unit 14: File Structure

1. Categories of Files

A file is a collection of related data stored on a secondary storage device. In C, files are treated as a stream of bytes. They are primarily categorized based on how the data is stored and accessed.

• Sequential Access Files: Data must be accessed in the order it was written. To read the 10th record, you must pass through the first nine.
• Random (Direct) Access Files: Data can be accessed from any location instantly using specific pointer manipulation functions.

2. Opening and Closing Files

Before performing any operation on a file, it must be opened, and after the work is done, it must be closed to free resources.

FILE Pointer

In C, all file operations use a special structure called FILE defined in stdio.h. We declare a pointer of this type to track the file.

FILE *fp;

fopen() Function

Used to open a file. It returns the address of the file if successful, otherwise NULL.

fp = fopen("filename.txt", "mode");

fclose() Function

Used to close the file. It ensures all data is properly written (flushed) from the buffer to the disk.

fclose(fp);

3. File Opening Modes

Modes specify the purpose for which the file is being opened.

4. Text vs. Binary Files

C distinguishes between two types of file formats:

• Text Files (.txt): Store data as a sequence of characters (ASCII). They are human-readable but less efficient for numeric data. Special characters like newline (\n) may be translated.
• Binary Files (.dat, .bin): Store data exactly as it appears in memory (0s and 1s). They are not human-readable but are highly efficient and faster for large datasets. No character translation occurs.

5. Reading, Writing, and Appending

Various functions are used to transfer data between the program and the file.

Formatted I/O

• fprintf(): fprintf(fp, "Format", variables); - Writes formatted data to a file.
• fscanf(): fscanf(fp, "Format", &variables); - Reads formatted data from a file.

Character I/O

• fputc() / putc(): Writes a single character to a file.
• fgetc() / getc(): Reads a single character from a file. Returns EOF at the end of the file.

Block I/O (Binary)

• fwrite(): Writes a block of memory (like a structure) to a file.
• fread(): Reads a block of memory from a file.

6. Creating Header Files

A header file is a file with a .h extension that contains C function declarations and macro definitions. It allows you to share code across multiple source files.

1. Write your functions and definitions in a file (e.g., mymath.h).
2. Include it in your main program using double quotes: #include "mymath.h".

7. Preprocessor Directives and Macros

The preprocessor is a tool that processes the source code before it is passed to the compiler. Directives start with a # symbol.

Common Directives

• #include: Includes the content of a header file.
• #define: Used to create symbolic constants or Macros.
• #undef: Undefines an existing macro.
• #ifdef / #ifndef: Conditional compilation directives used to include or exclude code blocks.

Macros

Definition: A macro is a fragment of code which has been given a name. Whenever the name is used, it is replaced by the contents of the macro.

Example: #define PI 3.14 (Object-like macro) or #define SQUARE(x) ((x)*(x)) (Function-like macro).

8. Exam Focus Enhancements