segmentation fault in use of strrchr function

Question

Here is my program which reverses a string. I am trying to use function strrchr at the end because I am entering the string James Bond on standard input. So I want to see if strrchr function gives a pointer to value character B of the above string.

#include<stdio.h>
#include<string.h>
int main ()
{
    char rt[100];
    char temp,*op;

    printf("enter a string\n");
    scanf("%s",rt);

    printf("the string you entered is %s\n",rt);

    int i, k, length;
    i=0;
    k=strlen(rt);

    printf("strlen gives =%d\n",k);

    length=k;
    length--;

    for(;i<k/2;i++)
    {
        temp=rt[i];
        rt[i]=rt[length-i];
        rt[length-i]=temp;
    }

    printf("the reverse string is %s\n",rt);

    op=strrchr(rt,'B');
    printf("strrchr gives %c\n",*op);
}

Now when I run above I get

./a.out 
enter a string
James Bond
the string you entered is James
strlen gives =5
the reverse string is semaJ
Segmentation fault

What can be the reason for this. Is above use of strrchr wrong?

Answer 1

Try this:

#include<stdio.h>
#include<string.h>
int main ()
{
    char rt[100];
    char temp,*op;
    printf("enter a string\n");
    fgets(rt, 100, stdin);
    printf("the string you entered is %s\n",rt);
    int i,k,length;
    i=0;k=strlen(rt);
    printf("strlen gives =%d\n",k);
    length=k;
    length--;
    for(;i<k/2;i++)
    {
        temp=rt[i];
        rt[i]=rt[length-i];
        rt[length-i]=temp;
    }
    printf("the reverse string is %s\n",rt);
    op=strrchr(rt,'B');
    printf("strrchr gives %c\n",*op);
    return 0;
}

scanf %s only takes non-white space chars. So James Bond is not completely read. fgets works well and should be preferred for user input.

For more info regarding scanf, gets and fgets, see this.

Answer 2

Before dereferencing op with the * unary operator, you have to check whether it's NULL (this will be the case when the character is not found):

op = strrchr(rt, 'B');

if (op != NULL)
  printf("strrchr gives %c\n", *op);

Answer 3

From the fine manual:

Upon successful completion, strrchr() shall return a pointer to the byte or a null pointer if c does not occur in the string.

Since there is no 'B' in rt when you call strrchr(rt, 'B'), you get a NULL pointer back and then you try dereference it in your printf call. Hence your segfault.