write_into_file.txt

#include <stdio.h>
#include <stdlib.h>
int main() {
    char s[10001]; // 10k letters, 1 for null char
    // read spaces also in the string (the whole line)
    // no & in scanf before string (char arrays) names
    scanf("%[^\n]", s);
    FILE *f = fopen("file.txt", "w");
    if (f == NULL) {
        printf("Error opening file!\n");
        exit(1);
    }
    fprintf(f, "%s\n", s);
    fclose(f);
}