happy_1[1]_% cat kadai8-2.c
#include <stdio.h>
main()
{
    char a[2000], b[2000];
    int i, j;

    printf("Enter a word. \n");
    gets( a );/* 改行まで文字列の入力 */

    i=0;
    j=0;
    while(a[i] !=0){
         i=i+1;
    }

    b[i]=0;
    j=i-1;
    while(j>=0){
         b[j]=a[i-1-j];
         j=j-1;
    }

    printf("New word is '%s'\n", b);
}

happy_1[2]_% gcc -o kadai8-2 kadai8-2.c -lm
happy_1[3]_% kadai8-2
Enter a word. 
dog
New word is 'god'
happy_1[4]_% kadai8-2
Enter a word. 
madam
New word is 'madam'
happy_1[5]_% kadai8-2
Enter a word. 
optics
New word is 'scitpo'