Segmentation Fault in my solution to SPOJ - The Next Palindrome - c

Question

This is my solution to the SPOJ problem - The Next Palindrome.
I try to run it, but every time I input a no. like 9XXXXX..., it gives seg. fault, for example -
$ ./a.exe
1
99999
Palin string i++: 99999
Non-Palin String: 99999
Palin string i++: 99999
Non-Palin String: 99999
Palin string i++: 99999
Non-Palin String: 99999
Change string: 99999
9 Palin String: 99099
Palin Str j = 2
Palin Str j-- = 1
9 Palin String: 90099
Palin Str j = 1
Segmentation fault (core dumped)
My code -
#include <stdio.h>
#include <string.h>
#define MAX 1000000
int main(void)
{
unsigned int t;
scanf("%u",&t);
unsigned long int i,j,k,len,change;
char str[MAX];
while(t--){ //For every test case
scanf("%s",str);
i = 0;
change = 0;
len = strlen(str) - 1;
Recheck:
while(i <= (len/2)){ //If the input no. is a palindrome, then don't change it, else change it
if(str[i] != str[len-i]){ //Compare 1st & last digit, then 2nd & 2nd last, so on....., if not same, then...
if(str[i] > str[len-i]){ //if 1st digit > last digit,
str[len-i] = str[i]; //then make last one the same as 1st one
i++;
change++;
}
else if(str[i] < str[len-i]){ //if <, then make the last one same as 1st one,
str[len-i] = str[i]; //but also increment it's left digit,
j = (len-i) - 1; //if required, the next one too, and so on...
while(j >= 0){ //j moves from right to left (digits) in the string
if(str[j] != '9'){
str[j]++;
change++;
break;
}
else if(str[j] == '9'){
str[j] = '0';
if(j == 0){ //If the no. becomes like 9......'\0'
for(k=len+1;k>=0;k--) //and we need to increment, so shift the elements,
str[k+1] = str[k]; //and make the string: 00.......'\0
str[0] = '0'; //Next loop will make str[0] = 1 & break.
len++;
j++;
}
change++;
}
printf("\n9 Non-Palin String: %s",str);
j--;
}
if(j <= i) //if the change reaches even before from where we compared(in 1st half, that is, i),
i = j; //then change i to j, so that the no. can be checked again from j, as changes are till j.
else
i++;
}
}
else if(str[i] == str[len-i]){ //No change to a palindrome
printf("\nPalin string i++: %s",str);
i++;
}
printf("\nNon-Palin String: %s",str);
}
if(change==0){ //change == 0 means that the no. was already a palindrome
printf("\nChange string: %s",str); //so we have to find the next no. which is a palindrome
if(len%2)
j = (len/2) + 1;
else
j = len/2;
if(str[j] == '9'){ //So, if the mid char(s) is/are 9 then change the no. as we did before
while(j >= 0){
if(str[j] != '9'){
str[j]++;
change++;
break;
}
else if(str[j] == '9'){
str[j] = '0';
if(j == 0){
for(k=len+1;k>=0;k--)
str[k+1] = str[k];
str[0] = '0';
len++;
j++;
printf("\nIncrement string");
}
change++;
}
printf("\n9 Palin String: %s",str);
printf("\nPalin Str j = %lu",j);
j -= 1;
printf("\nPalin Str j-- = %lu",j);
}
printf("\nPalin to Recheck: %s",str);
i = j;
goto Recheck;
}
else{ //else, just increment the mid char/s
if(len%2){
str[j]++;
str[j-1]++;
}
else{
str[j]++;
}
}
}
printf("\n%s",str);
}
return 0;
}
I've added comments to understand the code.
For every no., 9, 99, 999, etc. it gives segmentation fault error.

Answer 1

It seems odd to write an answer to my own question, but still...
The problem with the solution is that the variable k is an unsigned int, so k is never < 0, hence the infinite loop.
Thanks to pm100 & Weather Vane for their help.

Answer 2

Not a full answer but too much for a comment.
What do you mean by "stdin input buffer"? Your typing goes into a system buffer until you press "Enter" at which point the contents are transferred to your array str by scanf(). The stream used is a predefined system stream call stdin. You can use the similar file function fscanf(stdin,"%s",str); but scanf() is more convenient (and not a very desirable method for a string anyway). However, it is very unlikely that the system buffer can hold 1 million characters, so I suggest you build the string yourself, something like this:
#include <stdio.h>
#include <conio.h>
#define MAX 1000000
int main(void)
{
char str [MAX] = ""; // initialise string to the empty string
int str_len = 0; // length (and index into) str
int c;
do {
c = getch (); // gets a char but the type returned is int
printf("%c", c);
if (c != 13)
str [str_len++] = c; // insert in array
}
while (c != 13 && str_len < MAX-1);
str [str_len] = 0; // terminate array
printf("\n%s\n", str);
printf("Length %d\n", str_len);
return 0;
}