#include <stdio.h>

int main() {
    FILE *fp;

    fp = fopen("write.txt", "w");   // open in write mode
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    fprintf(fp, "This is a text file.\n");
    fprintf(fp, "File handling in C.\n");

    fclose(fp);   // always close after use

    printf("Data written successfully!");
    return 0;
}

#include <stdio.h>

int main() {
    FILE *fp;
    char line[100];

    fp = fopen("write.txt", "r");   // open the file we just wrote
    if (fp == NULL) {
        printf("File not found!\n");
        return 1;
    }

    printf("Contents of File:\n");

    while (fgets(line, sizeof(line), fp)) {
        printf("%s", line);
    }

    fclose(fp);
    return 0;
}

Contents of File:
This is a text file.
File handling in C.

#include <stdio.h>

int main() {
    FILE *fp = fopen("write.txt", "a");
    if (fp == NULL) {
        printf("Error!\n");
        return 1;
    }

    fprintf(fp, "Appended line.\n");
    fclose(fp);

    printf("Data appended successfully!");
    return 0;
}

#include <stdio.h>

int main() {
    FILE *fp = fopen("write.txt", "r");
    if (fp == NULL) return 1;

    char ch;
    while ((ch = fgetc(fp)) != EOF) {
        putchar(ch);   // print each character
    }

    fclose(fp);
    return 0;
}