Introduction to File Operations

In C programming, file operations allow you to store and retrieve data on disk, enabling persistent data storage beyond program execution. The basic file operations in C include opening a file, reading from a file, writing to a file, and closing a file. The C standard library provides a set of functions to perform these tasks, making it easy to work with files in C.

Opening a File

To work with a file, you first need to open it using the fopen function. This function takes two arguments: the file name and the mode in which the file should be opened. The mode determines if the file is opened for reading, writing, or both.

Syntax:

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

The fopen function returns a pointer to a FILE object, which is used to access the file. If the file cannot be opened, fopen returns NULL.

File Modes

  • r - Open for reading. The file must exist.
  • w - Open for writing. Creates a new file or truncates an existing file.
  • a - Open for appending. Creates a new file if it doesn’t exist.
  • r+ - Open for both reading and writing. The file must exist.
  • w+ - Open for reading and writing. Creates a new file or truncates an existing file.
  • a+ - Open for reading and appending. Creates a new file if it doesn’t exist.

Example of Opening a File

    #include <stdio.h>

    int main() {
        FILE *file = fopen("example.txt", "r");
        if (file == NULL) {
            printf("Could not open file.\n");
            return 1;
        }
        // File opened successfully
        fclose(file);
        return 0;
    }

In this example, we try to open a file named example.txt in read mode. If the file cannot be opened, fopen returns NULL.

Reading from a File

Once a file is open, you can read from it using functions such as fgetc, fgets, or fread.

Reading a File Using fgetc

The fgetc function reads a single character from the file.

    #include <stdio.h>

    int main() {
        FILE *file = fopen("example.txt", "r");
        if (file == NULL) {
            printf("Could not open file.\n");
            return 1;
        }

        char ch;
        while ((ch = fgetc(file)) != EOF) {
            putchar(ch); // Print each character to the console
        }

        fclose(file);
        return 0;
    }

Reading a File Using fgets

The fgets function reads a line of text from the file.

    #include <stdio.h>

    int main() {
        FILE *file = fopen("example.txt", "r");
        if (file == NULL) {
            printf("Could not open file.\n");
            return 1;
        }

        char line[100];
        while (fgets(line, sizeof(line), file) != NULL) {
            printf("%s", line); // Print each line to the console
        }

        fclose(file);
        return 0;
    }

Writing to a File

You can write data to a file using functions such as fputc, fputs, or fprintf.

Writing to a File Using fputc

The fputc function writes a single character to the file.

    #include <stdio.h>

    int main() {
        FILE *file = fopen("example.txt", "w");
        if (file == NULL) {
            printf("Could not open file.\n");
            return 1;
        }

        fputc('A', file); // Write character 'A' to the file

        fclose(file);
        return 0;
    }

Writing to a File Using fputs

The fputs function writes a string to the file.

    #include <stdio.h>

    int main() {
        FILE *file = fopen("example.txt", "w");
        if (file == NULL) {
            printf("Could not open file.\n");
            return 1;
        }

        fputs("Hello, World!\n", file); // Write a line to the file

        fclose(file);
        return 0;
    }

Closing a File

After completing file operations, it’s important to close the file using the fclose function. Closing a file ensures that all data is properly written to the disk and frees up system resources.

Example of Closing a File:

    #include <stdio.h>

    int main() {
        FILE *file = fopen("example.txt", "w");
        if (file == NULL) {
            printf("Could not open file.\n");
            return 1;
        }

        fputs("Hello, World!\n", file);

        fclose(file); // Close the file
        return 0;
    }

File Operation Example

The following example demonstrates how to open a file, write data to it, read the data back, and close the file. 

    #include <stdio.h>

    int main() {
        // Open the file for writing
        FILE *file = fopen("example.txt", "w");
        if (file == NULL) {
            printf("Could not open file.\n");
            return 1;
        }

        // Write data to the file
        fputs("This is a file operation example.\n", file);

        // Close the file after writing
        fclose(file);

        // Open the file for reading
        file = fopen("example.txt", "r");
        if (file == NULL) {
            printf("Could not open file.\n");
            return 1;
        }

        // Read and display the file content
        char line[100];
        while (fgets(line, sizeof(line), file) != NULL) {
            printf("%s", line);
        }

        // Close the file after reading
        fclose(file);

        return 0;
    }

Key Points to Remember

  • File operations in C include opening, reading, writing, and closing files.
  • The fopen function is used to open a file in different modes, such as read, write, or append.
  • The fclose function closes a file, freeing system resources.
  • Common functions for reading include fgetc and fgets; for writing, fputc and fputs.

Conclusion

Understanding file operations is essential for managing persistent data in C. By learning how to open, read, write, and close files, you can create programs that store data beyond the runtime of the program. Properly using file functions like fopen, fgetc, fputs, and fclose will help you work with files efficiently and reliably.





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