C programming, segmentation fault core dump - c

Question

I am trying to make a program which takes in a input of "Hello" and outputs "olleH" from reversing the order of the characters. However I keep getting a segmentation fault and I don't understand why
#include <stdio.h>
#include<string.h>
int main()
{
int i;
int size;
char s[100],a[100];
printf("Enter the word you want to get reversed: ");
scanf("%s",s);
while(s[i]!='\0')
{
a[i]=s[i];
i++;
}
size=sizeof(s);
while(i<sizeof(s))
{
s[i]=a[size];
}
printf("The reversed string is : %s",s);
}

Answer 1

Another simple way to reverse string.
Try this:
while(s[++i]!='\0'); // find the size of string
while(i>=0)
a[j++] = s[--i]; // reverse the string
a[j]='\0';
printf("The reversed string is : %s",a);

Answer 2

This while loop
while(i<sizeof(s))
{
s[i]=a[size];
}
does not make sense because index i has a value that points to outside the entered string (provided that it was initially correctly initialized) and the loop is infinite because i is not changed (and was not initially initialized) in the loop and also the right hand expression of this statement
s[i]=a[size];
is always the same and again refers memory outside the array.
Take into account that neither function declared in <string.h> is used in the program. So the header may be removed.
The program can look the following way
#include <stdio.h>
#define N 100
int main()
{
char s[N], d[N];
printf( "Enter the word you want to get reversed: " );
fgets( s, N, stdin );
size_t n = 0;
while ( s[n] != '\0' && s[n] != '\n' ) n++;
for ( size_t i = 0; i != n; i++ ) d[i] = s[n-i-1];
d[n] = '\0';
printf( "The reversed string is : %s\n", d );
return 0;
}
You can reverse a string without using an auxiliary array. For example
#include <stdio.h>
#define N 100
int main()
{
char s[N];
printf( "Enter the word you want to get reversed: " );
fgets( s, N, stdin );
size_t n = 0;
while ( s[n] != '\0' && s[n] != '\n' ) n++;
s[n] = '\0';
for ( size_t i = 0; i < n / 2; i++ )
{
char c = s[i];
s[i] = s[n-i-1];
s[n-i-1] = c;
}
printf( "The reversed string is : %s\n", s );
return 0;
}

Answer 3

The problem is in this part:
size=sizeof(s);
while(i<sizeof(s))
{
s[i]=a[size];
}
sizeof(s) will be 100 whereas the string you read from input can be less than that -- which would be undefined if you access uninitialized parts of s. So, you want to use strlen() to get the actual size of the string and use it to reverse.
Notice that scanf() is unsafe as it's written (what if you input more than 100 chars?). Suggest using fgets() instead.