The Crystal standard library includes some pre-defined annotations:

Link

Tells the compiler how to link a C library. This is explained in the lib section.

Extern

Marking a Crystal struct with this annotation makes it possible to use it in lib declarations:

@[Extern]
struct MyStruct
end
lib MyLib
  fun my_func(s : MyStruct) # OK (gives an error without the Extern annotation)
end

You can also make a struct behave like a C union (this can be pretty unsafe):

# A struct to easily convert between Int32 codepoints and Chars
@[Extern(union: true)]
struct Int32OrChar
  property int = 0
  property char = '\0'
end
s = Int32OrChar.new
s.char = 'A'
s.int # => 65
s.int = 66
s.char # => 'B'

ThreadLocal

The @[ThreadLocal] annotation can be applied to class variables and C external variables. It makes them be thread local.

class DontUseThis
  # One for each thread
  @[ThreadLocal]
  @@values = [] of Int32
end

ThreadLocal is used in the standard library to implement the runtime and shouldn’t be needed or used outside it.

Packed

Marks a C struct as packed, which prevents the automatic insertion of padding bytes between fields. This is typically only needed if the C library explicitly uses packed structs.

AlwaysInline

Gives a hint to the compiler to always inline a method:

@[AlwaysInline]
def foo
  1
end

NoInline

Tells the compiler to never inline a method call. This has no effect if the method yields, since functions which yield are always inlined.

@[NoInline]
def foo
  1
end

ReturnsTwice

Marks a method or lib fun as returning twice. The C setjmp is an example of such a function.

Raises

Marks a method or lib fun as potentially raising an exception. This is explained in the callbacks section.

CallConvention

Indicates the call convention of a lib fun. For example:

lib LibFoo
  @[CallConvention("X86_StdCall")]
  fun foo : Int32
end

The list of valid call conventions is:

• C (the default)
• Fast
• Cold
• WebKit_JS
• AnyReg
• X86_StdCall
• X86_FastCall

They are explained here.

Flags

Marks an enum as a “flags enum”, which changes the behaviour of some of its methods, like to_s.