Is there a built in swap function in C?

Question

Is there any built in swap function in C which works without using a third variable?

Answer 1

There is no standard function in C to swap two variables.

A macro can be written this way:

#define SWAP(T, a, b) do { T tmp = a; a = b; b = tmp; } while (0)

and the macro can be called this way:

int a = 42;
int b = 2718;

SWAP(int, a, b);

Some solutions for a writing a SWAP macro should be avoided:

#define SWAP(a, b) do { a = b + a; b = a - b; a = a - b; } while (0)

when operands are of signed types an overflow can occur and signed overflow are undefined behavior.

Also a solution trying to optimize the XOR solution like this should be avoid:

#define SWAP(a, b) (a ^= b ^= a ^=b)

a is modified twice between the previous and the next sequence point, so it violates the sequence points rules and is undefined behavior.