Implement generic swap macro in C [duplicate]

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This works well only with integers.

For floats it will fail (e.g. try running it with a very large float and a very small one).

I would suggest something as follows:

#define swap(x,y) do \ 
   { unsigned char swap_temp[sizeof(x) == sizeof(y) ? (signed)sizeof(x) : -1]; \
     memcpy(swap_temp,&y,sizeof(x)); \
     memcpy(&y,&x,       sizeof(x)); \
     memcpy(&x,swap_temp,sizeof(x)); \
    } while(0)

memcpy is pretty optimized when the amount to copy is known at compilation time.
Also, there’s no need to manually pass a type name or use compiler specific extensions.

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