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GATE CSE 2021 Set 1 Q24: C library functions that always make a system call

Question

Which of the following standard C library functions will always invoke a system call when executed from a single-threaded process in a UNIX/Linux operating system?

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Answer: A, C (official GATE 2021 final answer key)

Concept used: Only operations that need the kernel (terminating, blocking) must trap; malloc can often be served from heap memory already mapped.

First step: For each function ask: can it ever complete entirely in user space?

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Source: official GATE 2021 Set 1 CS question paper and final answer key, organised by IIT Bombay. Question paper · Answer key

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