strcmp returning only 0 (palindrom algo bug) - c

Question

Hi I learned in class about the string.h library and specifically about the strcmp function that compares between to strings .
and if the first string appears first in the dictionary it will return a number bigger than 0 , if the second string is bigger than first than it will return a number smaller than 0 , and if they are equil its supposed to return a 0.
ive used it like this :
strcmp(strArr , strrev(strArr));
Feel free to educate me.
The code :
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_LENGTH 100
#define PALINDROM_TRUE 0//because in strcmp if both strings compared are equile the strcmp will return 0.
int main(void)
{
char strArr[MAX_LENGTH];
printf("Enter string (max length 100 chars): ");
fgets(strArr , MAX_LENGTH , stdin);
int pali = strcmp(strArr , strrev(strArr));
if(pali == PALINDROM_TRUE)
{
printf("Palindrom\n");
}
else
{
printf("Not Palindrom\n");
}
system("PAUSE");
return 0;
}
my problem is that the code below when I enter i.e "abc" it prints to the screen Palindrom which it should print Not Palindrom , it never prints Not Palindrom

Answer 1

The reason for this is your call to strrev(). strrev() function works in-place. In other words, strrev works on the same buffer(i.e. strArr array) as the original array, and does not allocate a new space for the reversed string. As a result, once you call strrev() with strArr, the string in strArr is reversed, and all you are doing is comparing the two identical strings: the one in strArr, and again, the one in strArr, both of which are now reversed. (Note that the term both may be misguiding. Actually, there is just a single buffer and both arguments to strcmp point to that one.)
One way to correct this issue is to allocate a second array strArr2 and copy the string into it. Then, reverse the string in strArr2, and call strcmp with strArr and strArr2. Below is how this solution would look like in code.
...
char strArr2[MAX_LENGTH];
...
strcpy(strArr2, strArr);
strrev(strArr2);
int pali = strcmp(strArr , strArr2);
if(pali == PALINDROM_TRUE)
{
printf("Palindrom\n");
}
else
{
printf("Not Palindrom\n");
}
...
Note that you may also have to check the end of strArr for any trailing whitespace characters, due to your specific method of taking input. Any such trailing whitespace character could render your reversing and comparing strategy useless by affecting the result of the comparison.
If you do not have that much space, or if MAX_SPACE is too large a value, another way would be to use a custom comparison function for checking if a string is palindrome without having to use strcmp or strrev.

Answer 2

int pali = strcmp(strArr , strrev(strArr));
This line is obviously the issue.
The call to strrev(strArr) reverses strArr so afterwards when strcmp is called, do you expect strArr to be different? But it's the same variable...
You must create another char array to store the reversed string.
EDIT:
#include <stdio.h>
#include <string.h>
#define MAX_LENGTH 100
int main(void)
{
char str[MAX_LENGTH];
char rev[MAX_LENGTH];
puts("Enter string (max length 100 chars): ");
scanf("%s", str);
strcpy(rev, str);
strrev(rev);
if(strcmp(str, rev) == 0) {
puts("Palindrom");
} else {
puts("Not Palindrom");
}
}

Answer 3

ive added
strArr[strcspn(strArr, "\n")] = 0;
after
fgets(strArr , MAX_LENGTH , stdin);
And it worked , its basicly rewriting the (null) at the end. which was messing up the reverse to include null (0) at the start of the string.
Thanks everyone.
Finsihed Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_LENGTH 100
#define PALINDROM_TRUE 0//because in strcmp if both strings compared are equile the strcmp will return 0.
int main(void)
{
char strArr[MAX_LENGTH], strArrRev[MAX_LENGTH];
printf("Enter string (max length 100 chars): ");
fgets(strArr , MAX_LENGTH , stdin);
strArr[strcspn(strArr, "\n")] = 0;
strcpy(strArrRev, strArr);
int pali = strcmp(strArr , strrev(strArrRev));
if(pali == PALINDROM_TRUE)
{
printf("Palindrom\n");
}
else
{
printf("Not Palindrom\n");
}
system("PAUSE");
return 0;
}