Segmentation Fault while Splitting an Array containing IP Address

Question

After splitting the IP address, I want to pick out the last element i.e 15 and increment it but after getting 192 in buff when it goes to 1st case, i get segmentation fault!

Why it can't go further? What may be the problem?

int main(int argc, char** argv) 
{

char str[] = "192.168.10.15"; 
char str1[12];
char *str2, *str3, *str4, *str5;
unsigned char bytes[4];
int i = 0;

int lastelem; 

char* buff = (char *)malloc(20);
buff = strtok(str,".");


while (buff != NULL)
{
   printf("%s\n",buff);

   switch(i)
   {
       case 0: str2=buff;
       break;

       case 1: str3=buff;
       break;

       case 2: str4=buff;
       break;

       case 3: str5=buff;
       break;
   } 

   lastelem=atoi(str5);
   sprintf(str5, "%d",lastelem);

   bytes[i] = (unsigned char)atoi(buff);
   buff = strtok(NULL,".");
   i++;
}
return 0;
}

Please [see why not to cast](http://stackoverflow.com/q/605845/2173917) the return value of `malloc()` and family in `C`. — Sourav Ghosh, Aug 06 '15 at 07:29
@SouravGhosh: one day, your message about not casting `malloc()` *will* get through! — Bathsheba, Aug 06 '15 at 07:32
This `char* buff = (char *)malloc(20); buff = strtok(str,".");` leaks `20` bytes of memory. Also you cannot copy an array per assignment. — alk, Aug 06 '15 at 07:41
In C you cannot copy all elements of an array at once using the `=` operator. — alk, Aug 06 '15 at 08:27
@Bathsheba I admit having somewhat given up on that one. But Sourav Ghosh has taken over with a remarkable fervour :p — Quentin, Aug 06 '15 at 08:33

Sourav Ghosh · Accepted Answer · 2015-08-06T07:42:30.647

4

In your code, in the very first iteration, (or, for that case, on any other iteration, except when case 3 is hit)

 lastelem=atoi(str5);

str5 is uninitialized., so it's invoking undefined behaviour.

You should use str5 only after it has been assigned. You can use a flag, if you want, to mark the hit on case 3: or, put the code under the case block itself.

Also, you don't need to allocate memory to buff, as you're overwriting it with the returned pointer from strtok(). That creates a memory leak.

edited Aug 06 '15 at 07:42

answered Aug 06 '15 at 07:31

Sourav Ghosh

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1

@DeepJyotiGharphalia easiest way, move all the `str5` related code under case 3... – Sourav Ghosh Aug 06 '15 at 07:38

score 2 · Answer 2 · answered Aug 06 '15 at 07:40

2

Move your str5 related stuff, inside case3

case 3: str5 = buff;
lastelem = atoi(str5);
sprintf(str5, "%d", lastelem);
break;

Also, no need to allocate memory to buff, you are not copying something in it, you are just pointing something, this will cause a memory leak in your code.

answered Aug 06 '15 at 07:40

Nishant

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Just change this line `char* buff = (char *)malloc(20);` to char* buff = NULL; – Nishant Aug 06 '15 at 08:21

Segmentation Fault while Splitting an Array containing IP Address

2 Answers2