在C中读取File时输出错误

Question

I am trying to write a string to a file and then read the string and output the string written into the file. For example

INPUT (Input Name)
FalconHawk

OUTPUT
Hi FalconHawk! Have a great day!

My code is:

#include<stdio.h>

void main(){

char n[10],r[1000];
FILE *fptr,*fpt;

scanf("%s",n);                        //Input name

fptr=fopen("welcome.txt","w");
fprintf(fptr,"%s",n);                 //Write to file
fclose(fptr);

fpt=fopen("welcome.txt","r");
fscanf(fpt,"%s",r);                 
printf("Hi %s! Have a good day.",r);  //Output file content
fclose(fpt);
}

But because of some reason I am getting an output like

INPUT (Input Name)
FalconHawk

OUTPUT
HiHi FalconHawk! Have a great day!   //"Hi" is getting printed two times

On replacing "Hi" with "Welcome" I am getting an output like

OUTPUT
WelcomeWelcome FalconHawk! Have a great day!   //"Welcome" is getting printed two times.

What is causing this issue?

Answer 1

Your buffer is too small and there's no room for the terminating null byte, therefore, your code invokes undefined behavior. If you want to read 10 characters, then this is how you should do it

char input[11];
if (scanf("%10s", input) == 1) {
    // Safely use `input' here
}

And if you want to read an entire line of text from stdin then use fgets() instead

if (fgets(input, sizeof input, stdin) != NULL) {
    // Safely use `input' here
}

---

Strings in c always need an extra byte of space to store the terminating '\\0' , read a basic tutorial on strings in c to learn how they work and how to treat them.