# Thread: How is this possible?

1. ## How is this possible?

I'm trying to implement a b-tree and have hit a perplexing roadblock. Here's a section of code, it is pretty simple and the whole purpose is debugging/to examine the nodes:
Code:
```195 |___while (cur) {
196 |___|___fprintf(stderr,"\t%s ->%p %p\n", cur->val,cur->ls,cur->rs);
197 |___|___ray[i] = cur->ls;
198 |___|___para[i++] = cur->val;
199 |___|___if (!cur->next) ray[i] = cur->rs;
200 |___|___cur = cur->next;
201 |___}```
So I get a seg fault at line 200. The only possible reason AFAIK would be if cur is NULL. Which I do not see how that could happen based on the while condition, so I checked it out in GDB:

Breakpoint 4, topdown (n=0x602250, from=0x0) at b-tree2.c:199
199 if (!(cur->next)) ray[i] = cur->rs;
(gdb) print cur
\$5 = (key *) 0x6022b0
(gdb) print cur->next
\$6 = (struct _key *) 0x0
(gdb) step
200 cur = cur->next;
(gdb) print cur
\$7 = (key *) 0x0
(gdb) print cur->next
Cannot access memory at address 0x10

So of course the next thing is the seg fault. But how can the address in cur change from 0x6022b0 to NULL because of this instruction:

if (!(cur->next)) ray[i] = cur->rs;

????????????????

2. Hey, I bet the size of one of those arrays is to blame...heh heh.

....yep

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