Question: I usually code in C++, but I'm working on a project in C and I came across a printf with the following syntax: 

```
printf( 0, "%d\n", num);

```

I've looked around and can't find an explanation of what the first 0 in the printf does. Can someone please explain it to me? Thanks.

Answer: 
It is [undefined behavior](http://en.wikipedia.org/wiki/Undefined_behavior); I guess you might have a segmentation violation.
```
printf( 0, "%d\n", num);

```

invokes undefined behavior because the first argument of `printf` has to be a pointer to a string.