6.Īt this point, the processor is executing the kernel system call function with kernel privileges. The system services trap handler dereferences a system call table using the service call number and transfers control to the function bound to the table. The trap is an instruction that causes transition from the current user space execution to the function in the exception vector table, with the associated increase in privileges. The service wrapper call pushes the calling arguments on to the stack, loads a register ( eax for IA-32 platforms) with the service number for the call, and issues a service trap (or software interrupt depending on the target processor architecture). The previously allocated numbers do not change or get reassigned because doing so could break existing applications or libraries, that is, it would break backward compatibility 3. The assignment of numbers for each service call is done by the maintainer of the operating systems. 2.Įach service call is assigned an operating system service identifier in the form of an integer. 1.Įach service call has a user space library wrapper providing a function for each call.

The general design pattern used for service calls is as follows (using Linux as an example). The operating system services are exposed through function wrappers provided as part of the POSIX/C Libraries.

What is a software wrapper