I want to store a character string in block of memory space created by malloc and then modify the same space to store a much larger string

Question

Here is the code

#include<stdio.h>
#include<stdlib.h>
#include<string.h> 

int
main()
{ 

int n,i;

printf("Enter the size or the number of chracters that you want to enter inside the string.\n");
scanf("%d",&n);

char *p = (char*)malloc((n+1)*sizeof(char));
if(p==NULL)
 {
 printf("Memory allocation fails..");
 exit(0);
 }
 
puts("Enter string");
for(i=0;i<(n+1);i++)
scanf("%c",p+i);

*(p+i)= '\0';

printf("String entered %s",p);

fflush(stdin);

printf("\nEnter new size\n");
scanf(" %d",&n); 

p = realloc(p,(n+1)*sizeof(char));

puts("Enter new string");
scanf("%d", &n);

 for(i=0;i<(n+1);i++)
 scanf("%c",p+i);
 
 *(p+i)= '\0'; 
 
 printf("New string \n%s",p);
 
 free(p);
 
return 0;
}

I used an online compiler (https://www.onlinegdb.com/online_c_compiler).

The question that I want to ask. When I enter more elements than the size that I initialized for the string during runtime. It does not take the next input and prints the result. And when I enter less than the size limit, it does not go to the next part. Instead, it takes multiple inputs to move towards the next part and print the results.

When I enter string within the size limit.

When I enter string more than the size limit.

When I try to enter string less than the size limit.

Can anybody please tell me what I am doing wrong?

`*(p+i)= '\0';` is bad when `i == n+1`. Use `for(i=0;i<(n /* +1 */);i++)`. — chux - Reinstate Monica, Jan 09 '22 at 07:11
Does this answer your question? [scanf() leaves the newline character in the buffer](https://stackoverflow.com/questions/5240789/scanf-leaves-the-newline-character-in-the-buffer) — anatolyg, Jan 09 '22 at 07:14

Ole Morud · Accepted Answer · 2022-01-09T20:27:27.887

The loop

for(i=0;i<(n+1);i++)
    scanf("%c",p+i);

will keep running until you've entered all characters. That's why you need to write a until the array is filled in the last screenshot. If you want the line to be read when enter is pressed you can use fgets

Regarding the other screenshots, as per C standard, it is undefined behavior to use fflush(stdin). Thats why the input buffer doesn't get flushed. You can use

int c;
while((c = getchar()) != '\n' && c != EOF); //Discard

to flush stdin

I want to store a character string in block of memory space created by malloc and then modify the same space to store a much larger string

1 Answers1