File I/O in C Language

Introduction

File Input/Output (I/O) is a fundamental concept in the C programming language that allows you to read from and write to external files. This is essential for data storage, processing, and retrieval in various applications.

Opening a File

To open a file in C, you use the fopen() function, which is defined in the stdio.h header file. The fopen() function requires two parameters: the name of the file and the mode in which the file should be opened.

FILE *filePointer;
filePointer = fopen("example.txt", "r");

In this example, filePointer is a pointer to the file, "example.txt" is the name of the file, and "r" stands for the read mode.

File Modes

Different modes available for opening a file include:

• "r" - Open for reading. The file must exist.
• "w" - Open for writing. If the file exists, its contents are overwritten. If the file does not exist, it is created.
• "a" - Open for appending. Data is added to the end of the file. If the file does not exist, it is created.
• "r+" - Open for both reading and writing. The file must exist.
• "w+" - Open for both reading and writing. If the file exists, its contents are overwritten. If the file does not exist, it is created.
• "a+" - Open for both reading and appending. Data is added to the end of the file. If the file does not exist, it is created.

Closing a File

After you are done with a file, it is a good practice to close it using the fclose() function. This frees up resources used by the file.

fclose(filePointer);

Writing to a File

To write data to a file, you can use functions like fprintf(), fputs(), or fwrite().

FILE *filePointer = fopen("example.txt", "w");
if (filePointer != NULL) {
    fprintf(filePointer, "Hello, World!\n");
    fclose(filePointer);
}

Reading from a File

To read data from a file, you can use functions like fscanf(), fgets(), or fread().

FILE *filePointer = fopen("example.txt", "r");
char buffer[100];
if (filePointer != NULL) {
    while (fgets(buffer, 100, filePointer) != NULL) {
        printf("%s", buffer);
    }
    fclose(filePointer);
}

Error Handling

It is important to handle errors while performing file I/O operations. You can use the perror() function to print a descriptive error message if a file operation fails.

FILE *filePointer = fopen("nonexistent.txt", "r");
if (filePointer == NULL) {
    perror("Error opening file");
}

Example Program

Here is a complete example program that demonstrates how to open a file, write to it, read from it, and handle errors:

#include <stdio.h>

int main() {
    FILE *filePointer;
    
    // Open file for writing
    filePointer = fopen("example.txt", "w");
    if (filePointer == NULL) {
        perror("Error opening file for writing");
        return 1;
    }
    fprintf(filePointer, "Hello, World!\n");
    fclose(filePointer);
    
    // Open file for reading
    filePointer = fopen("example.txt", "r");
    if (filePointer == NULL) {
        perror("Error opening file for reading");
        return 1;
    }
    char buffer[100];
    while (fgets(buffer, 100, filePointer) != NULL) {
        printf("%s", buffer);
    }
    fclose(filePointer);
    
    return 0;
}