What is a system call?

Question

BlackTom AI · Accepted Answer

When approaching the concept of a system call, we evaluate what each option claims about how user programs interact with the operating system.
Option 1 suggests: 'A function written by the user that directly manipulates kernel memory for faster execution.' This is inaccurate because user programs do not directly manipulate kernel memory; doing so would break protection and is not how system calls operate. Access to kernel memory is restricted for safety and stability, and system calls provide a controlled interface instead.
Option 2 claims: 'A hardware instruction that allows applications to bypass the operating system and access devices directly.' In reality, there is no standard mechanism for applications to bypass the OS; devices are accessed through system calls and device drivers managed by the kernel. This option describes unauthorized privileged access, which system calls are designed to prevent.
Option 3 states: 'A request to the operating system kernel to perform privileged operations that user programs cannot do directly.' This matches the canonical definition of a system call: a controlled entry into the kernel from user space to perform operations that require higher privileges, such as file I/O, process management, or inter-process communication.
Option 4 asserts: 'A library function that executes in user mode but pretends to be the kernel.' While libraries can wrap system calls or provide abstractions, they do not execute in kernel space and cannot impersonate the kernel. Real kernel actions happen through actual system calls into the kernel, not through user-space libraries pretending to be kernel code.
In summary, Option 3 provides the precise mechanism by which user-space programs request privileged operations from the kernel, aligning with the standard OS design: user mode to kernel mode transitions via system calls. The other options describe incorrect or misleading mechanisms that would violate protection boundaries and OS architecture.

What is a system call? 单项选择题

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