Reversing String in C for loop error - c

Question

I have an array of strings and am trying to reverse each string in the array to see if that string is a palindrome. I am using a for loop to increment an int i (the index). However after the I call the reverse function, the value of i becomes some really large number and I cant figure out why this is happening.
#include <stdio.h>
#include <string.h>
void revString(char *dest, const char *source);
int main() {
const char *strs[] = {
"racecar",
"radar",
"hello",
"world"
};
int i;
char res[] = "";
for (i = 0; i < strlen(*strs); i++) {
printf("i is %d\n", i);
revString(&res[0], strs[i]); //reversing string
printf("i is now %d\n", i);
//comparing string and reversed string
if (strcmp(res, strs[i]) == 0) {
printf("Is a palindrome");
} else {
printf("Not a palindrome");
}
}
return 0;
}
void revString(char *dest, const char *source) {
printf("%s\n", source);
int len = strlen(source);
printf("%d\n", len);
const char *p;
char s;
for (p = (source + (len - 1)); p >= source; p--) {
s = *p;
*(dest) = s;
dest += 1;
}
*dest = '\0';
}
This is the output showing the value of i before and after the revString function is called.
i is 0
i is now 1667588961
Illegal instruction: 4

Answer 1

There are multiple problems in your code:
You pass a destination array char res[] = ""; that is much too small for the strings you want to reverse. It's size is 1. This causes buffer overflow, resulting in undefined behavior.
Use char res[20]; instead.
You enumerate the array of string with an incorrect upper bound. Use this instead:
for (i = 0; i < sizeof(strs) / sizeof(*strs); i++)
The termination test for the loop in revString() is incorrect too: decrementing p when is equal to source has undefined behavior, although it is unlikely to have an consequences. You can simplify this function this way:
void revString(char *dest, const char *source) {
size_t len = strlen(source);
for (size_t i = 0; i < len; i++) {
dest[i] = source[len - i - 1];
}
dest[len] = '\0';
}
Here is the resulting code:
#include <stdio.h>
#include <string.h>
void revString(char *dest, const char *source) {
size_t len = strlen(source);
for (size_t i = 0; i < len; i++) {
dest[i] = source[len - i - 1];
}
dest[len] = '\0';
}
int main(void) {
const char *strs[] = { "racecar", "radar", "hello", "world" };
char res[20];
for (size_t i = 0; i < sizeof(strs) / sizeof(*strs); i++) {
revString(res, strs[i]);
//comparing string and reversed string
if (strcmp(res, strs[i]) == 0) {
printf("Is a palindrome\n");
} else {
printf("Not a palindrome\n");
}
}
return 0;
}

Answer 2

Here is Final Code with some change
#include <stdio.h>
#include <string.h>
void revString(char* dest, const char* source);
int main(){
const char* strs[] = {
"racecar",
"radar",
"hello",
"world"
};
static int i;
char res[] = "";
int length = (int) sizeof(strs)/sizeof(char*);
for(i = 0; i < length; i++)
{
printf("i is %d\n", i);
revString(&res[0], strs[i]); //reversing string
printf("i is now %d\n", i);
//comparing string and reversed string
if(strcmp(res, strs[i]) == 0){
printf("Is a palindrome");
}else{
printf("Not a palindrome");
}
}
return 0;
}
void revString(char* dest, const char* source){
printf("%s\n", source);
int len = (int) strlen(source);
printf("%d\n", len);
const char* p;
char s;
for(p = (source + (len - 1)); p >= source; p--){
s = *p;
*(dest) = s;
dest += 1;
}
*dest = '\0';
}
Change 1 :-
int i; to static int i; (Reason:- i is local variable you are calling
function so when function call the value of i will remove and after
that it will assign garbage value.)
change 2 :-
strlen(*strs) to length of array (because strlen(*strs) will give the
length of first string)