The Next Palindrome : Segmentation fault [duplicate] - c

Question

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Why I am getting the segmentation fault in following c program
(2 answers)
Closed 7 years ago.
I'm trying to solve the Next Palindrome problem on SPOJ. Here is the link to the problem SPOJ
This is my code for the problem. I get correct results when I run it on my machine for the following test cases :
9 11 99 101 808 818
This is my code :
#include<stdio.h>
#include<string.h>
char k[1000004];
int find_palin(char num[])
{
int len = strlen(num);
char str1[1000004] = {NULL};
char str3[500002] = {NULL};
char str2[500002] = {NULL};
char rev[500002] ={NULL};
if(len%2==0)
{
int half = (len)/2;
int i;
int j=0;
for(i=1;i<=len-1;++i)
{
if(num[i]!='0')
break;
k[i]='0';
}
if(i>len-1)
{
k[0]=num[0];
k[len-1]=num[0];
return 0;
}
for(i=0;i<half;++i)
str1[i] = num[i];
for(i=half;i<len;++i)
str3[j++] = num[i];
for(i=0 , j=half-1;i<half;++i,--j)
{
if(str3[i]>=str1[j])
break;
else
rev[i]=str1[j];
}
if(i>=half)
{
strcat(str1,rev);
for(i=0;i<len;++i)
k[i]=str1[i];
return 0;
}
else
{
for(i=0;i<half;++i)
str3[i] = '0';
if(str1[half-1]!='9')
str1[half-1]++;
else
{
for(i=half-1;i>=0;--i)
{
if(str1[i]=='9')
str1[i] = '0';
else
{
str1[i]++;
break;
}
}
if(i<0)
{
str1[half]='0';
str1[0] = '1';
}
}
strcat(str1,str3);
find_palin(str1);
}
}
else
{
int half = (len-1)/2;
int i;
int j=0;
if(len==1)
{
if(num[0]=='9')
{
k[0] = '1';
k[1] = '1';
return 0;
}
k[0] = ++num[0];
return 0;
}
for(i=1;i<=len-1;++i)
{
if(num[i]!='0')
break;
k[i]='0';
}
if(i>len-1)
{
k[0]=num[0];
k[len-1]=num[0];
return 0;
}
for(i=0;i<half;++i)
str1[i] = num[i];
for(i=half+1;i<len;++i)
str3[j++] = num[i];
str2[0] = num[half];
for(i=0 , j=half-1;i<half;++i,--j)
{
if(str3[i]>=str1[j])
break;
else
rev[i]=str1[j];
}
if(i>=half)
{
strcat(str2 , rev);
strcat(str1 , str2);
for(i=0;i<len;++i)
k[i]=str1[i];
return 0;
}
else
{
for(i=0;i<half;++i)
str3[i] = '0';
if(str2[0]!='9')
{
str2[0]++;
strcat(str2,str3);
strcat(str1,str2);
find_palin(str1);
}
else
{
str2[0] = '0';
if(str1[half-1]!='9')
str1[half-1]++;
else
{
for(i=half-1;i>=0;--i)
{
if(str1[i]=='9')
str1[i] = '0';
else
{
str1[i]++;
break;
}
}
if(i<0)
{
str1[half]='0';
str1[0] = '1';
}
}
strcat(str2,str3);
strcat(str1,str2);
find_palin(str1);
}
}
}
}
int main()
{
char input[1000004];
int t;
scanf("%d" , &t);
int i;
for(i=0;i<t;++i)
{
scanf("%s" , &input);
find_palin(input);
printf("%s\n" , k);
}
return 0;
}
When I try to submit the code it gives Segmentation Fault. Can someone please help me as to why I'm getting this error?

Answer 1

The way to use large arrays is to allocate memory on the heap not on the stack. I have done this in your program to show you. Instead of doing that in main I lazily moved input[] to be a global var. You can't do that in find_palin because of the recursion, and anyway global variables are frowned on.
I also changed all your return 0 statements to goto so that a common clean-up can be done. It is not elegant, but I didn't want to change the structure of your code.
I also tweaked a few other things, such as checking the input was valid, and using a single #define from which all other sizes specified are derived.
#include<stdio.h>
#include<string.h>
#define ARRSIZE 1000004 // don't hard code stuff
char k[ARRSIZE];
char input[ARRSIZE]; // move out of main
void find_palin(char num[]) // change return type to void
{
int len = strlen(num);
char *str1 = calloc(ARRSIZE, sizeof(*str1)); // allocate and zero
char *str2 = calloc(ARRSIZE/2, sizeof(*str2));
char *str3 = calloc(ARRSIZE/2, sizeof(*str3));
char *rev = calloc(ARRSIZE/2, sizeof(*rev));
if (str1 == NULL || str2 == NULL || str3 == NULL || rev == NULL)
exit (1); // check memory allocations
if(len%2==0)
{
int half = (len)/2;
int i;
int j=0;
for(i=1;i<=len-1;++i)
{
if(num[i]!='0')
break;
k[i]='0';
}
if(i>len-1)
{
k[0]=num[0];
k[len-1]=num[0];
goto endfunc; // replace all return statements
}
for(i=0;i<half;++i)
str1[i] = num[i];
for(i=half;i<len;++i)
str3[j++] = num[i];
for(i=0 , j=half-1;i<half;++i,--j)
{
if(str3[i]>=str1[j])
break;
else
rev[i]=str1[j];
}
if(i>=half)
{
strcat(str1,rev);
for(i=0;i<len;++i)
k[i]=str1[i];
goto endfunc;
}
else
{
for(i=0;i<half;++i)
str3[i] = '0';
if(str1[half-1]!='9')
str1[half-1]++;
else
{
for(i=half-1;i>=0;--i)
{
if(str1[i]=='9')
str1[i] = '0';
else
{
str1[i]++;
break;
}
}
if(i<0)
{
str1[half]='0';
str1[0] = '1';
}
}
strcat(str1,str3);
find_palin(str1);
}
}
else
{
int half = (len-1)/2;
int i;
int j=0;
if(len==1)
{
if(num[0]=='9')
{
k[0] = '1';
k[1] = '1';
goto endfunc;
}
k[0] = ++num[0];
goto endfunc;
}
for(i=1;i<=len-1;++i)
{
if(num[i]!='0')
break;
k[i]='0';
}
if(i>len-1)
{
k[0]=num[0];
k[len-1]=num[0];
goto endfunc;
}
for(i=0;i<half;++i)
str1[i] = num[i];
for(i=half+1;i<len;++i)
str3[j++] = num[i];
str2[0] = num[half];
for(i=0 , j=half-1;i<half;++i,--j)
{
if(str3[i]>=str1[j])
break;
else
rev[i]=str1[j];
}
if(i>=half)
{
strcat(str2 , rev);
strcat(str1 , str2);
for(i=0;i<len;++i)
k[i]=str1[i];
goto endfunc;
}
else
{
for(i=0;i<half;++i)
str3[i] = '0';
if(str2[0]!='9')
{
str2[0]++;
strcat(str2,str3);
strcat(str1,str2);
find_palin(str1);
}
else
{
str2[0] = '0';
if(str1[half-1]!='9')
str1[half-1]++;
else
{
for(i=half-1;i>=0;--i)
{
if(str1[i]=='9')
str1[i] = '0';
else
{
str1[i]++;
break;
}
}
if(i<0)
{
str1[half]='0';
str1[0] = '1';
}
}
strcat(str2,str3);
strcat(str1,str2);
find_palin(str1);
}
}
}
endfunc: // free the memory
free(rev);
free(str3);
free(str2);
free(str1);
}
int main()
{
int t;
int i;
if (1 != scanf("%d" , &t)) // check garbage input
exit (1);
for(i=0;i<t;++i)
{
if (1 != scanf("%s" , input)) // remove &
exit (1); // check garbage input
find_palin(input);
printf("%s\n" , k);
}
return 0;
}