difference between #if defined(WIN32) and #ifdef(WIN32)

If you use #ifdef syntax, remove the brackets.

The difference between the two is that #ifdef can only use a single condition,
while #if defined(NAME) can do compound conditionals.

For example in your case:

#if defined(WIN32) && !defined(UNIX)
/* Do windows stuff */
#elif defined(UNIX) && !defined(WIN32)
/* Do linux stuff */
#else
/* Error, both can't be defined or undefined same time */
#endif

#ifdef FOO

and

#if defined(FOO)

are the same,

but to do several things at once, you can use defined, like

#if defined(FOO) || defined(BAR)

#ifdef checks whether a macro by that name has been defined, #if evaluates the expression and checks for a true value

#define FOO 1
#define BAR 0

#ifdef FOO
#ifdef BAR
/* this will be compiled */
#endif
#endif

#if BAR
/* this won't */
#endif

#if FOO || BAR
/* this will */
#endif