If a process invokes the exec system call:

Question

BlackTom AI · Accepted Answer

The exec family of system calls replaces the calling process's memory image with a new program. When exec is invoked successfully, the OS loads the specified executable into the current process’s address space, sets up the new program’s stack (arguments and environment), and begins execution at the new program’s entry point — the process identity (PID) remains the same.

Relevant concepts:
- exec/execve/execl/etc. semantics: replace current process image with a new program.
- fork vs exec: fork creates a new process; exec replaces the program within an existing process.
- Behavior on success vs failure: exec does not return on success; it returns -1 and sets errno only if it fails.
- Resources: open file descriptors remain open across exec unless marked close-on-exec; credentials and PID remain unchanged; signal dispositions are reset for some signals.

Step-by-step reasoning:
1. The process calls exec with a path and argv/envp.
2. Kernel checks permissions and locates the executable file.
3. The kernel loads the executable (text, data, bss) into the calling process’s address space, unmaps the old image, and prepares the stack with new argv/envp.
4. The kernel sets the instruction pointer to the program’s entry point and transfers control to the new program.
5. The exec call does not return if successful; execution continues under the new program image using the same PID.

Why the correct answer is correct:
- This matches the formal definition: exec replaces the current process image with the binary specified. The process becomes that program while retaining its PID and many kernel-level attributes.

Why other options are incorrect:
- “the executable executes as a new process”: incorrect—creating a new process requires fork (or clone); exec alone does not create a new PID.
- “the exec is ignored by default”: false—exec actively replaces the process image or fails and returns an error.
- “none of the mentioned are possible”: false because the replacement behavior is exactly what exec does.

Conclusion: The correct answer is that the binary specified in the exec parameter will replace the program in the current process. This matches the provided answer.

If a process invokes the exec system call:单项选择题

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