Is there a built-in way to swap two variables in C?

You need to define it yourself.

C doesn’t have templates.
If such function does exist it would look like void swap(void* a, void* b, size_t length), but unlike std::swap, it’s not type-safe.
And there’s no hint such function could be inlined, which is important if swapping is frequent (in C99 there’s inline keyword).
We could also define a macro like
```
 #define SWAP(a,b,type) {type ttttttttt=a;a=b;b=ttttttttt;}
```
but it shadows the ttttttttt variable, and you need to repeat the type of a. (In gcc there’s typeof(a) to solve this, but you still cannot SWAP(ttttttttt,anything_else);.)
And writing a swap in place isn’t that difficult either — it’s just 3 simple lines of code!