Which of these will result in the parent's waiting for any child process to finish and exit?

Question

BlackTom AI · Accepted Answer

wait and waitpid both return the child PID (>0) when a child terminates and -1 when there are no more children (or on error), so looping while the return is >0 causes the parent to block and reap each child until none remain.

1. Relevant knowledge points and concepts
- wait(&status) blocks until any child process terminates and returns its PID (>0), storing termination info in status.
- wait(NULL) is the same except it ignores the child's status.
- waitpid(-1, &status, 0) with pid = -1 and options = 0 behaves like wait(): it waits for any child, returns the child PID (>0), and fills status.
- All these functions return -1 when there are no child processes left to wait for (or on error). A loop that continues while the return value is >0 will therefore wait for every child, one after another.

2. Step-by-step reasoning
- Evaluate while (wait(NULL) > 0) { ... }:
  - First iteration: wait blocks until one child exits, returns its pid (a positive integer), loop body executes.
  - Repeat until all children have exited. When none remain, wait returns -1, condition fails, loop ends.
- Evaluate while ((pid = wait(&status)) > 0) { ... }:
  - Same sequence, but pid captured and status available for inspection; loop stops when wait returns -1.
- Evaluate while ( waitpid(-1, &status, 0) > 0 ) { ... }:
  - waitpid with pid = -1 and options 0 waits for any child (identical semantics to wait), returns child pid (>0), and stops when -1 is returned.

3. Intermediate details
- No arithmetic calculations are needed; the key is the return semantics: positive PID -> continue; -1 -> stop.
- Using NULL vs &status only affects whether termination status is stored, not the waiting behavior.

4. Why the correct answer is correct
- Each listed loop will block and reap any child processes one by one until none remain. Therefore the statement “parent waits for any child process to finish and exit” is true for all three variants.

5. Why the other options are not the sole best answer
- Each individual code snippet is correct on its own, so selecting any single snippet is incomplete because the option “all of these” correctly captures that every listed snippet achieves the described behavior.

6. Conclusion
All three code snippets cause the parent to wait for any child to finish and exit. Hence the correct choice is: all of these. This matches the provided answer.

Which of these will result in the parent's waiting for any child process to finish and exit?单项选择题

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