Resize specific cells of array in c - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
char *readline()
{
char s[256],*ds;
printf("Give string: ");
gets(s);
ds = (char *) calloc(strlen(s)+1,sizeof(char));
strcpy(ds,s);
return ds;
}
main()
{
char *s1,*s2;
int a,b;
s1=readline();
printf("Start:");
scanf("%d",&a);
printf("End:");
s2=(char*)realloc(s1,b-a);
puts(s2);
return 0;
}
I have this code and I want to resize specific cells of the s1 array which are determined from the variables a and b.
E.g. If I give the string "Hello" and the integers a=2 and b=4 then I want the program to print "ell".

Answer 1

Don't use gets use fgets instead.
With fgets you would use if( fgets( s,256, stdin)!= NULL ){ /*Got input in s*/ }.
You have taken \n as input also. s[strcspn(s,"\n")]=0 will overwrite \n with \0.
Casting return value of calloc is not needed since, void* will be implicitly converted to char*. calloc may return NULL - you didn't include check in there.
ds = calloc(strlen(s)+1,sizeof(char));
if( NULL == ds ){
perror("calloc: error");
exit(EXIT_FAILURE);
}
In main() value of b is indeterminate. You didn't take input in b.
if( scanf("%d",&b)!= 1){
fprintf(stderr,"Error in input");
exit(EXIT_FAILURE);
}
Then you realloc-ed. But not clear why. If you thought that shrinking the memory to 4-2 or 2 Bytes will help you get "ell" then you are wrong in many ways. It won't.
what you can do is - allocate memory for that in s2.
if(b<a){
fprintf(stderr,"End must be greater than beginnning\n");
exit(EXIT_FAILURE);
}
s2 = malloc(b-a+1);
if(!s2){
perror("calloc: error");
exit(EXIT_FAILURE);
}
memcpy(s2,s1+a,b-a);
s2[b-a]=0;
puts(s2);
This will print the relevant "ell" just you wanted. There is one thing left for you - put the code snippets in the code.

Answer 2

Since you are open to other solutions:
If there is another way then no problem.
I would point you to a simple solution, where you terminate the original string at b and skip required number of characters a. (Note: in C char array starts from index 0);
This is the idea:
#include <stdio.h>
#include <string.h>
char *splice(char *str, int s, int e)
{
str[e]=0;
return (str+s-1);
}
int main()
{
char *after;
int a=2;
int b=4;
char original[] = { 'H', 'e', 'l' , 'l' , 'o', 0 };
after = splice(original,a,b);
printf("My new string = %s", after);
return 0;
}
Output:
My new string = ell