File Handling in C Programming

File Handling in C

• Files are named locations on a disk used to store data persistently. They can hold various data types, from program instructions to images to documents. C provides functions for interacting with these files, enabling programs to read from or write to them.
• File interaction is essential for storing and retrieving information outside of the program's immediate memory.

File Types

• Text files store data as characters, suitable for human-readable text.
• Binary files store data in raw form, ideal for images, audio, and other non-text data.

Opening Files

• The fopen() function opens a file.
• It takes the file name and mode as arguments.
• Modes:
  • "r": Open for reading; file must exist.
  • "w": Open for writing; overwrites the file if it exists, or creates a new one if it doesn't.
  • "a": Open for appending; writes data to the end of the file (creates a new file if it doesn't exist).
  • "r+", "w+", "a+": Open for reading and writing; "w+" overwrites the file if it exists.

File Pointers

• FILE *fp: A file pointer is a pointer to a structure (FILE) containing file information, including position and error flags.

Reading from Files

• fscanf(): Reads formatted data from a file.
• fgetc(): Reads a single character.
• fgets(): Reads a line of text.
• fread(): Reads a block of binary data.

Writing to Files

• fprintf(): Writes formatted data to a file.
• fputc(): Writes a single character.
• fputs(): Writes a string.
• fwrite(): Writes a block of binary data.

Error Handling

• ferror(): Checks for file I/O errors.
• feof(): Checks for end-of-file.
• Crucially check the return value of fopen() for errors (non-zero values indicate errors).
• fclose(): Closes the file, releasing resources and ensuring data writes to disk.

Closing Files

• The fclose() function closes a file, releasing resources and writing buffered data to the disk. This is essential for preventing data loss and resource leaks.

File Positioning

• fseek(): Changes the file position pointer.
• ftell(): Retrieves the current file position, useful for saving and restoring positions within a file.

Example (Reading a line of text):

#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE *fp;
    char line[100];  // Crucial: Declare a buffer for the line!
    fp = fopen("myFile.txt", "r");

    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }

    while (fgets(line, sizeof(line), fp) != NULL) {
        printf("%s", line);
    }

    fclose(fp);
    return 0;
}

Additional Considerations

• Buffering: File I/O uses buffering. Data isn't immediately written/read to disk; instead, it's kept in a buffer. Flushing the buffer is needed for permanent storage, and C functions control buffering.
• File Permissions: Operating systems and file systems define permissions (read/write access) to files.
• Large Files: When dealing with very large files, memory-mapped files can improve performance.

Common Pitfalls

• Skipping fclose(): Can cause data loss or resource leaks.
• Using wrong file modes can lead to errors.
• Not checking fopen()'s return value leads to silent program failures.
• Incorrect fseek() use can damage data.

Summary

• File handling in C enables programs to interact with persistent data. Understanding file types, opening modes, related functions for reading and writing, and crucial error handling is vital for reliable programs. fopen(), fclose(), and functions like fscanf() and fprintf() are fundamental to C file operations.