Question

How do the available system calls in modern versions of the Linux Kernel (2.6.30+) compare with the system calls available in the earliest versions of UNIX? How has the system call interface been expanded, and why? Focus on major changes or extensions in functionality.

Answer

System Calls

Overview

A system call is a mean by which programs in the user space can access kernel services. Systems calls vary from operating system to operating system, although the underlying concepts tends to be the same. In general, a process is not supposed to be able to access the kernel directly. It cannot access kernel memory and it can't call kernel functions. The hardware of the CPU prevents this (called "protected mode"). System calls are an exception to this rule. For example, older x86 processors used an interrupt mechanism to go from user-space to kernel-space, but newer processor (PentiumII+) provided instructions that optimize this transition (using sysenter and sysexit instructions).

The Linux operating system (2.6.30+) contains hundreds of system calls. They are roughly grouped into 5 major categories: Process Control, File Management, Device Management, Information Maintenance and Comminution.

System call numbers

Each system calls are identified by their numbers. For example, the number of the sys_read is 3. These numbers are located in the file /arch/i386/include/asm/unistd.h. You can find the association between numbers and names in the sys_call_table, for example in arch/i386/kernel/entry.S.

Linux system call interface (SCI)

Using the interrupt method The process fills the registers with the appropriate values and then calls a special instruction which then go to a previously defined location in the kernel. Under x86 CPUs, this is done using the interrupt 0x80. Once the interrupt is used, the hardware knows that once you jump to this location, you are not running in restricted user mode anymore but as the operating system kernel, or kernel mode. You are now allowed to do whatever you want. The location in the kernel a process can jump is called 'system_call'. The procedure at that location checks the system call number, specifying to kernel which service the process requested. The table of system calls (sys_call_table) is accessed to determine the address of the kernel function to call. The function is then called and returned. After it returns, the system call procedure does a few system checks and then return back to the process.

References

Here is the original manual --Lmundt 18:29, 7 October 2010 (UTC) http://cm.bell-labs.com/cm/cs/who/dmr/1stEdman.html

The Linux KErnel Module Programming Guide by http://www.faqs.org/docs/kernel/x931.html