To define system calls open a module and use the def_syscall macro. Pass in the system call name, the return type and its arguments. For example:

module YourSyscalls
  Syscall.def_syscall pipe2, Int32, pipefd : Int32[2]*, flags : Int32
  Syscall.def_syscall write, Int32, fd : Int32, buf : UInt8*, count : LibC::SizeT
  Syscall.def_syscall read, Int32, fd : Int32, buf : UInt8*, count : LibC::SizeT
  Syscall.def_syscall close, Int32, fd : Int32
end

They can then be called as methods from the Syscall module:

YourSyscalls.write(1, "Hello!\n".to_unsafe, 7)