# Handling Operating System Variation

When writing cross-platform applications or libraries, it is often necessary to allow for differences between operating systems. The variable `Sys.KERNEL` can be used to handle such cases. There are several functions in the `Sys` module intended to make this easier, such as `isunix`, `islinux`, `isapple`, `isbsd`, `isfreebsd`, and `iswindows`. These may be used as follows:

    if Sys.iswindows()
        windows_specific_thing(a)
    end

Note that `islinux`, `isapple`, and `isfreebsd` are mutually exclusive subsets of `isunix`. Additionally, there is a macro `@static` which makes it possible to use these functions to conditionally hide invalid code, as demonstrated in the following examples.

Simple blocks:

    ccall((@static Sys.iswindows() ? :_fopen : :fopen), ...)

Complex blocks:

    @static if Sys.islinux()
        linux_specific_thing(a)
    else
        generic_thing(a)
    end

When chaining conditionals (including `if`/`elseif`/`end`), the `@static` must be repeated for each level (parentheses optional, but recommended for readability):

    @static Sys.iswindows() ? :a : (@static Sys.isapple() ? :b : :c)

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