# 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)
elseif Sys.isapple()
apple_specific_thing(a)
else
generic_thing(a)
end

When nesting conditionals, the @static must be repeated for each level (parentheses optional, but recommended for readability):

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