Which system call returns the process identifier of a terminated child?

Question

BlackTom AI · Accepted Answer

The wait system call returns the process identifier (PID) of a terminated child, so the correct answer is wait.

1) Relevant knowledge points and concepts
- Process creation: fork() creates a child; fork() returns the child's PID in the parent and 0 in the child.
- Process termination: exit(status) causes a process to terminate and makes its exit status available to the parent.
- Reaping a child: wait(&status) (and variants like waitpid) allow a parent to collect a terminated child’s exit status and return the child’s PID. getpid() (not listed) returns the caller’s PID; there is no standard syscall named get that returns a child PID.

2) Step-by-step reasoning
- A parent needs a mechanism to learn which child terminated and to obtain its exit status.
- Calling wait(&status) blocks until one of the parent’s children terminates (unless called with non-blocking options).
- When a child terminates, the kernel records its exit status and wait() returns the PID of that child and stores its status in the provided status pointer.

3) Example and intermediate steps
- Code example: pid_t childpid = wait(&status);
  - Kernel finds terminated child, copies status into status, and returns that child’s PID.
  - If no child exists or an error occurs, wait() returns -1 and errno is set.
- Variant: waitpid(-1, &status, 0) behaves similarly and returns the PID of the reaped child.

4) Why the correct answer is correct
- wait is explicitly defined to return the PID of the terminated child and to provide its exit status. That is its principal purpose in process control APIs.

5) Why other options are incorrect
- exit: terminates the calling process; it does not return a child PID to the parent.
- fork: creates a child and returns the child’s PID to the parent at creation time, not the PID of a terminated child.
- get: not a standard syscall for retrieving a child PID; likely a mistaken reference to getpid(), which returns the caller’s own PID, not a child’s.

6) Conclusion
The wait system call is the correct choice; it returns the PID of a terminated child, matching the provided answer: wait.

Which system call returns the process identifier of a terminated child?单项选择题

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