Abstract


Better Security

Since privileged services are carried out by the kernel, kernel can enforce security implementation. This prevents programs from doing malicious stuff.

System Call Mechanism


Step 1-3: Calling Process (进程) pushes the arguments for the parameters of the system call to its Stack Segment

Step 4(the start of Library Call): An Instruction is triggered to trigger the corresponding Library Call, the same instruction is used to trigger other library calls

Step 5 Library Call puts system call interrupt number in a place where Kernel expects it, such as a Register

Step 6: Execute Trap Interrupt (陷入) to enter the Kernel Mode and start execution at a fixed address within the kernel

Step 7: The kernel codes following the trap interrupt examines system call interrupt number, dispatch the correct Interrupt Handler via Interrupt Vector Table

Step 8: The desired Interrupt Handler starts running

Step 9: After the interrupt handler finishes, control is returned to the library call at the Instruction following the Trap Interrupt (陷入)

Step 10: Then, library call returns, and we are back to the user program

Step 11: To finish the job, the process needs to remove the library call related data like the arguments we pushed to the stack segment from its stack segment by incrementing the Stack Pointer

System call & process management

Control is passed back to the Kernel when a system call is made by the Process (进程). Kernel uses this opportunity to perform its Process Management. If the process hogs to the CPU and doesn’t make any system call, we have Preemptive Scheduling to handle this.

Tip

The system calls made by a process can be traced by strace.

System Call is ISA-dependent


Solution: abstraction!

We have Abstraction Barrier built on top of these System Call (系统调用) in the form of Library Call that follows a standardised interface like POSIX by wrapping the Assembly Instruction of different ISAs. Specific ISA Instruction is generated during Compilation automatically.

This makes it possible for user programs like OS System Program to make system call that requests privileged services from the kernel regardless of the underlying ISA - user program is communicating with library call directly and communicating with system call indirectly!

Unix-like systems use libc and Windows uses ntdll.dll.

Important

Program should always check the results of Library Call to see if an error has occurred.

Linux System Call


Windows System Call


References