First problem: don't rely on `short` and `long` sizes being respectively 16 and 32 bit.

Use `stdint.h` include which defines `uint16_t` and `uint32_t`

Second problem: protect your term with parentheses because you could pass some expression to the macros.

For instance `REVERSE_SHORT(4000<<3)` would yield 0 because of operator precedence. What you cannot avoid is the multiple computation of `n` expression (maybe the optimizer can help but that's not guaranteed)

To sum it up I would write them as:

    #include <stdint.h>
    #define REVERSE_UINT16(n) ((uint16_t) ((((n) & 0xFF) << 8) | \
                                                (((n) & 0xFF00) >> 8)))
    #define REVERSE_UINT32(n) ((uint32_t) ((((n) & 0xFF) << 24) | \
                                              (((n) & 0xFF00) << 8) | \
                                              (((n) & 0xFF0000) >> 8) | \
                                              (((n) & 0xFF000000) >> 24)))