C Write Files

To write into a file, you first open it in write mode ("w") or append mode ("a"), and then use a function to send text into it. The most common function for this is fprintf(), which works almost exactly like the printf() you already know, except it sends the text into a file instead of the screen.

Writing with fprintf()

The only difference between printf() and fprintf() is that fprintf() takes one extra piece of information at the front: the file handle you got from fopen(). Everything after that is the same format string and values you are used to.

#include <stdio.h>

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

    // Send text into the file instead of the screen
    fprintf(fptr, "Hello from C!\n");
    fprintf(fptr, "Writing files is easy.\n");

    fclose(fptr);
    printf("Text saved to greeting.txt\n");
    return 0;
}

Because fprintf() supports format specifiers, you can mix variables and text together just like with printf(). This makes it perfect for saving numbers, names, and calculated results into a file.

#include <stdio.h>

int main() {
    char name[] = "Ada";
    int score = 95;

    FILE *fptr = fopen("result.txt", "w");
    if (fptr == NULL) {
        return 1;
    }

    // %s inserts the name, %d inserts the number
    fprintf(fptr, "Player: %s\n", name);
    fprintf(fptr, "Score: %d\n", score);

    fclose(fptr);
    return 0;
}

Write Mode vs Append Mode

• Opening with "w" erases whatever was already in the file before writing your new text.
• Opening with "a" keeps the old content and adds your new text to the very end.
• Use "a" when you want to build up a log or add records over many runs of your program.