Is there a built in swap function in C?

Question

No.
C++ builtin swap function: swap(first,second);
Check this: http://www.cplusplus.com/reference/algorithm/swap/

You can use this to swap two variable value without using third variable:

a=a^b;
b=a^b;
a=b^a;

You can also check this:

https://stackoverflow.com/questions/756750/swap-the-values-of-two-variables-without-using-third-variable

How to swap without a third variable?

Why do you not want to use a third variable? It's the fastest way on the vast majority of architectures.

The XOR swap algorithm works without a third variable, but it is problematic in two ways:

1. The variables must be distinct i.e. swap(&a, &a) will not work.
2. It is slower in general.

It may sometimes be preferable to use the XOR swap if using a third variable would cause the stack to spill, but generally you aren't in such a position to make that call.

To answer your question directly, no there is no swap function in standard C, although it would be trivial to write.

Assuming you want a C solotion, not a C++ one, you could make it a macro, at least using GCC extension to have it generic enough, something like

 #define SWAP(x,y) do {   \ 
   typeof(x) _x = x;      \
   typeof(y) _y = y;      \
   x = _y;                \
   y = _x;                \
 } while(0)

beware of tricks like invocations swap(t[i++],i); to avoid them, use the address operator &. And you'll better use a temporary (for integers, there is a famous and useless trick with exclusive-or).

^{PS: I'm using two local variables _x and _y (but I could have used one local variable only) for better readability, and perhaps also to enable more optimizations from the compiler.}

There is no such function in standard C.

(In C++ you have std::swap().)

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Maybe a macro from this question can be useful for you.