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Can applications call kernel functions?
Kernel functions cannot be simply invoked from applications that live in user space. System calls are the only functions in user space that can request kernel services. To call kernel functions directly, if you are interested in kernel programming, you must implement a kernel module.
How system call is accessed from the user space?
Accessing the System Call from User-Space Generally, the C library provides support for system calls. User applications can pull in function prototypes from the standard headers and link with the C library to use your system call (or the library routine that, in turn, uses your syscall call).
How to invoking kernel calls in GNU / Linux?
Invoking specific kernel functions (system calls) is a natural part of application development on GNU/Linux. But what about going in the other direction, kernel space calling user space?
Is there a function call from user space to kernel space?
No ordinary function call from user space to kernel space is listed, only syscall (adding new syscall is not easy) and upcall (call in inverse direction). One of easiest interface is ioctl; but you can’t start to use ioctl before creating procfs, sysfs or similiar file.
How to load kernel modules from user space?
Applications of the usermode-helper API in the kernel One of the most straightforward applications of the usermode-helper API is loading kernel modules from kernel space. The function request_module encapsulates the functionality of the usermode-helper API and provides a simple interface.
How does the kernel do the work for You?
Based on these argument the kernel will do the required work for you. Call the system call using the syscall assembly instruction. This is exact place where the programs hand-over the work to the kernel. The process then waits for the system call to return.