File Handling in C

File handling in C enables programs to create, read, and write to files on disk. This allows data persistence beyond the program's runtime, facilitating the storage of large amounts of data.

1. Opening and Closing Files

Before performing any operations on a file, it must be opened using fopen(). After operations are complete, the file must be closed using fclose() to free resources.

a. Opening a File (fopen())

fopen() is used to open a file and returns a pointer to the file. If the file cannot be opened, it returns NULL. It takes two arguments:

• File name (as a string).
• Mode (as a string), which determines how the file is opened (read, write, append, etc.).

Syntax

FILE *fopen(const char *filename, const char *mode);

File Opening Modes

• "r": Open a file for reading (file must exist).
• "w": Open a file for writing (creates file if it doesn’t exist, overwrites existing file).
• "a": Open a file for appending (creates file if it doesn’t exist).
• "r+": Open a file for both reading and writing.
• "w+": Open a file for reading and writing (overwrites existing file).
• "a+": Open a file for reading and appending.

Example: Opening a File

#include <stdio.h>
 
int main() {
    FILE *file;
    file = fopen("example.txt", "r");  // Open file for reading
 
    if (file == NULL) {
        printf("File could not be opened\n");
        return 1;
    }
 
    // Perform file operations...
 
    fclose(file);  // Close the file
    return 0;
}

b. Closing a File (fclose())

fclose() is used to close an open file. It ensures that any data written to the file is saved and that system resources are released.

Syntax

int fclose(FILE *stream);

Example: Closing a File

fclose(file);

2. Reading and Writing to Files

C provides various functions to read from and write to files. These functions can handle formatted and unformatted data.

a. Reading from a File (fscanf())

fscanf() reads data from a file in a formatted manner, similar to how scanf() works for console input.

Syntax

int fscanf(FILE *stream, const char *format, ...);

Example: Reading from a File

#include <stdio.h>
 
int main() {
    FILE *file;
    int num;
    file = fopen("example.txt", "r");
 
    if (file == NULL) {
        printf("File could not be opened\n");
        return 1;
    }
 
    fscanf(file, "%d", &num);  // Read an integer from the file
    printf("Number from file: %d\n", num);
 
    fclose(file);
    return 0;
}

b. Writing to a File (fprintf())

fprintf() writes formatted output to a file, similar to how printf() works for console output.

Syntax

int fprintf(FILE *stream, const char *format, ...);

Example: Writing to a File

#include <stdio.h>
 
int main() {
    FILE *file;
    file = fopen("example.txt", "w");
 
    if (file == NULL) {
        printf("File could not be opened\n");
        return 1;
    }
 
    fprintf(file, "Hello, world!\n");  // Write a string to the file
 
    fclose(file);
    return 0;
}

3. File Handling Functions in Detail

a. fopen()

fopen() is used to open a file. It takes two arguments: the file name and the mode (e.g., read, write).

Example

FILE *file = fopen("data.txt", "r");

b. fclose()

fclose() is used to close an open file, ensuring that any data in memory is written to disk and that resources are freed.

Example

fclose(file);

c. fscanf()

fscanf() is used to read data from a file in a formatted manner. It takes a file pointer and a format string as arguments.

Example

int value;
fscanf(file, "%d", &value);  // Reads an integer from the file

d. fprintf()

fprintf() is used to write formatted data to a file. It takes a file pointer and a format string as arguments.

Example

fprintf(file, "%s", "Data to write");  // Writes a string to the file

Diagram for File Handling in C

4. Error Handling

When working with files, it’s crucial to handle errors, such as failure to open a file or unexpected end-of-file conditions. Always check if the file pointer returned by fopen() is NULL.

Example: Error Handling

FILE *file = fopen("nonexistent.txt", "r");
if (file == NULL) {
    printf("File could not be opened\n");
}

Summary of File Handling Functions

• fopen(): Opens a file in specified mode (r, w, a, etc.).
• fclose(): Closes an open file, releasing resources.
• fscanf(): Reads formatted input from a file.
• fprintf(): Writes formatted output to a file.

Best Practices:

• Always check if fopen() returns NULL before performing file operations.
• Always call fclose() to release resources after file operations.
• Use proper format specifiers when using fscanf() and fprintf() to avoid data corruption.