GATE CSE 2021 Set 1 | Question: 14

Question

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

• A. $\textsf{exit}$
• B. $\textsf{malloc}$
• C. $\textsf{sleep}$
• D. $\textsf{strlen}$

Comments

@arjun sir and others please help that “sleep()” is a system call or function call?

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either the question is wrong or given options(as the final answer is wrong) because sleep is not a c standard lib function. it belongs to a header file <unistd.h>( or <windows.h> ) present inside c posix os api which works on posix conformant systems like linux etc.interesting point to note is c posix lib is a superset of c standard lib . only exit() is a function which belongs to c standard lib and requires system call because it will ultimately change the state of a process (along with deallocation of many resources).

also as stated in some comments the same doubt exist for malloc implementation.

Answer 1

System calls are used to get some service from operating system which generally requires some higher level of privilege. 

This question uses two important words “always” and “standard C library functions”.
Let’s check options

1. exit- This is a function defined in standard C library and it always invokes system call every time, flushes the streams, and terminates the caller.
2. malloc – This is a function defined in standard C library and it does not always invoke the system call. When a process is created, certain amount of heap memory is already allocated to it, when required to expand or shrink that memory, it internally uses sbrk/brk system call on Unix/Linux. i.e., not every malloc call needs a system call but if the current allocated size is not enough, it’ll do a system call to get more memory. 
3. sleep- This is not even  standard C library function, it is a POSIX standard C library function. Unix and Windows uses different header files for it. Now as question has said the following standard C library function, let’s consider it as that way. Yes it always invokes the system call .
4. strlen – This is a function defined in standard C library and doesn’t require any system call to perform its function of calculating the string length. 

Answer : A,C

Comments

https://media.geeksforgeeks.org/wp-content/uploads/20231017212555/Types-of-System-Calls-(3)-(2).png

Answer 2

Answer should be sleep, as it is a blocking system call the OS would consider the process as blocked.

The exit call in Linux is also a system call.

malloc is a library call and not always a system call

Comments

always is the key term

sleep and exit  are sys calls.

malloc  libary call ,but may invoke sbrk() sys call sometimes

strlen library call

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When a process needs memory, some room is created by moving the upper bound of the heap forward, using the brk() or sbrk() system calls. Because a system call is expensive in terms of CPU usage, a better strategy is to call brk() to grab a large chunk of memory and then split it as needed to get smaller chunks. This is exactly what malloc() does. It aggregates a lot of smaller malloc() requests into fewer large brk() calls. Doing so yields a significant performance improvement. The malloc() call itself is much less expensive than brk(), because it is a library call, not a system call. Symmetric behavior is adopted when memory is freed by the process. Memory blocks are not immediately returned to the system, which would require a new brk() call with a negative argument. Instead, the C library aggregates them until a sufficiently large, contiguous chunk can be freed at once.

Reference : Advanced Memory Allocation | Linux Journal

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Yes this to confirm further.  This is a chart given in Galvin.

Answer 3

The Correct answer is A,B,C.

As exit , malloc, sleep invole system calls but strlen doesnt.

Comments

I have already confirmed in the GO keys used in Pragys app :)

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“will always” – normally there is no need to add this “always” in the given sentence. So, this is a very intentional addition and the question asker would have wanted to check the knowledge of how malloc works – is it a graduate level question? I dont think so. “can cause a system call” should have been a better question for GATE level.

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@arjun sir there is one more ques of hamming code where they gave d4 twice,don’t you think it should be marks to all?

Answer 4

Exit is the correct ans

This is a function defined in standard C library and it always invokes system call every time, flushes the streams, and terminates the caller.