Choose the incorrect statement about system calls.

Question

BlackTom AI · Accepted Answer

System calls are kernel-level entry points that let user-space processes request privileged services; user programs do not contain the kernel’s implementation of those services.

1) Relevant knowledge points and concepts
- System calls run in kernel mode; their implementations live in the operating system kernel, not in user binaries.
- C library (libc) provides wrapper functions (thin user-space stubs) that invoke a system call via a software trap/syscall instruction or through a small assembly helper.
- User-space applications can invoke system calls either through these libc wrappers or by issuing the syscall/trap directly.
- Linking (static or dynamic) may include libc wrapper code in the executable, but never the kernel’s implementation of the system call.

2) Step-by-step reasoning
- Consider calling write() from a C program: the program calls the libc write() function; that libc function packages arguments and executes the syscall instruction (or calls into a vdso helper). The actual file-write logic executes inside the kernel after the trap.
- The C compiler and linker can include the wrapper code (if static linking) or rely on dynamic linking to provide it at run time, but they cannot embed the kernel’s body because the kernel code resides and runs in kernel space, separate from user binaries.

3) Intermediate “calculation” steps (conceptual)
- Program source -> compiled into object code -> linked with libc wrappers -> executable contains user-space wrappers (if statically linked) or references to shared libc (if dynamically linked).
- Kernel syscall implementation remains only in kernel image; user executable contains at most the wrapper and a syscall instruction.

4) Why the provided answer is correct
- The statement "The C compiler will embed the entire body of a system call function inside the program binary (executable)" is false because the full system call implementation is in kernel space; only a thin user-space wrapper or trap sequence is included in the program, not the kernel’s code.

5) Why the other options are correct (briefly)
- "C library functions can make system calls": true — libc wrappers invoke syscalls.
- "User applications written in C can make system calls": true — either via libc or direct syscall.
- "System calls are process requests for services that only the kernel has the privilege to do": true — that is the definition of a syscall.

6) Conclusion
Therefore the incorrect statement is: "The C compiler will embed the entire body of a system call function inside the program binary (executable)". This matches the provided answer.

Choose the incorrect statement about system calls.单项选择题

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