C Write to Files: Storing Data Permanently

Writing to a file involves three distinct steps: Opening the file in a writable mode, Sending the data through a stream, and Closing the file to flush the buffer and save changes.

1. The Primary Writing Functions

C provides different functions depending on whether you want to write formatted text, simple strings, or single characters.

fprintf(): Works exactly like printf(), but writes to a file instead of the console. Great for formatted data (integers, floats).
fputs(): Writes a complete string to the file. It is faster than fprintf() for plain text.
fputc(): Writes a single character to the file.

2. Implementation Example

Here is how you would write structured data into a text file named data.txt:

#include <stdio.h>

int main() {
    FILE *fptr;
    int score = 95;
    char name[] = "John Doe";

    // Open for writing ("w")
    fptr = fopen("data.txt", "w");

    if (fptr != NULL) {
        // Writing a formatted string
        fprintf(fptr, "Student: %s\nScore: %d", name, score);

        // Closing is mandatory to ensure data is saved
        fclose(fptr);
        printf("Data written successfully.");
    } else {
        printf("Unable to create file.");
    }

    return 0;
}

3. The Difference Between "w" and "a"

Understanding the file mode is critical. Choosing the wrong one can result in losing existing data.

Mode	Action	Risk Level
`"w"` (Write)	Deletes old content and starts fresh.	High (Data loss)
`"a"` (Append)	Keeps old content; adds new data to the end.	Low (Safe)

4. Buffering and fclose()

When you write to a file, C doesn't always send the data to the disk immediately. It stores it in a small "buffer" in RAM to improve speed. fclose() performs a "flush," forcing any remaining data in the buffer to be written to the physical storage device.

5. Writing Large Data (fwrite)

If you need to write an entire array or a struct to a file at once, professional C developers use fwrite(). This writes data in binary format, which is much faster and takes up less space than text.

// Writing an array of integers in one go
int numbers[] = {1, 2, 3, 4, 5};
fwrite(numbers, sizeof(int), 5, fptr);

Pro Tip: Always use \n at the end of your fprintf or fputs calls if you want the next piece of data to appear on a new line. Unlike puts() in the console, fputs() does not add a newline automatically!