Question at position 8 A System call is a: Library function that uses user/ kernel stubs to carry out the service task by running its code in the kernel modePrivileged instruction that run in the kernel modeKernel function that my be called only within the kernelClear my selection

Question

Question at position 8 A System call is a: Library function that uses user/ kernel stubs to carry out the service task by running its code in the kernel modePrivileged instruction that run in the kernel modeKernel function that my be called only within the kernelClear my selection

BlackTom AI · Accepted Answer

The question asks: A System call is a: 
Option 1: Library function that uses user/ kernel stubs to carry out the service task by running its code in the kernel mode
Option 2: Privileged instruction that run in the kernel mode
Option 3: Kernel function that my be called only within the kernel

Option 1 analysis: This choice describes a system call interface where user-space requests a service from the kernel, typically implemented via stubs in the C library that trap into the kernel to execute privileged code. The essence is that a system call is invoked from user space and handled in kernel space, often through a transition mechanism using stubs. This aligns with the standard understanding of how system calls are mediated, making this option plausible as the correct one.

Option 2 analysis: A privileged instruction that runs in the kernel mode refers to an instruction that can only execute when the processor is in kernel mode, such as specific privileged machine instructions. System calls themselves are not single privileged instructions; rather, they are a mechanism or interface for requesting kernel services. Therefore this option mischaracterizes what a system call is, even though kernel mode is involved in executing the service.

Option 3 analysis: A kernel function that may be called only within the kernel would be an internal kernel routine or API, not a system call exposed to user applications. System calls are the gateway from user space to kernel space, whereas kernel-only functions are inaccessible directly from user space. This option conflates kernel-internal calls with the user-facing system call mechanism, so it is inaccurate.

In summary, the first option best captures the concept of a system call as the interface that enables user-space programs to request services by passing through user-space stubs to the kernel, where the actual service executes, typically via a transition to kernel mode. The second option confuses the notion with a single privileged instruction, and the third option describes an internal kernel function rather than the user-visible system call interface.

Question at position 8 A System call is a: Library function that uses user/ kernel stubs to carry out the service task by running its code in the kernel modePrivileged instruction that run in the kernel modeKernel function that my be called only within the kernelClear my selection单项选择题

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