What are the PID values at lines A, B, C, D? Let parent be pid 100 and child be pid 101.

Question

BlackTom AI · Accepted Answer

fork() returns 0 in the child and the child's PID in the parent; getpid() returns the calling process’s own PID. Using that core behavior gives the PID values at A–D.

1) Relevant knowledge points and concepts
- fork(): called once, returns twice: returns 0 in the child process, and returns the child’s PID (a positive integer) to the parent.
- getpid(): returns the PID of the calling process (unique for parent and child).
- In the problem the parent’s PID is given as 100 and the child’s PID is 101.

2) Step-by-step reasoning (complete)
- The single fork() call assigns its return value to pid.
  - In the child process, pid == 0.
  - In the parent process, pid == child PID == 101.
- In the child branch the code executes pid1 = getpid(); so pid1 becomes the child’s PID (101).
- In the parent branch the code executes pid1 = getpid(); so pid1 becomes the parent’s PID (100).

3) Intermediate values shown explicitly
- After fork():
  - Child: pid = 0
  - Parent: pid = 101
- After getpid() in child: pid1 = 101
- After getpid() in parent: pid1 = 100

4) Why the correct answer is correct
- Line A prints the child’s copy of pid, which is 0 -> A = 0.
- Line B prints pid1 in the child, which is getpid() in child -> 101 -> B = 101.
- Line C prints pid in the parent, which is fork()’s return to the parent -> child PID 101 -> C = 101.
- Line D prints pid1 in the parent, which is parent’s own PID via getpid() -> 100 -> D = 100.

5) Why the other options are incorrect (brief)
- Options that put A = 100 or A = 100/other assume fork() returns parent PID to child — incorrect; fork() returns 0 in the child.
- Options swapping B/C/D values confuse getpid() and fork() return values: getpid() gives process’s own PID (100 for parent, 101 for child), and fork() returns child PID to parent (101) and 0 to child — any option inconsistent with these rules is wrong.

Conclusion: The correct values are A = 0, B = 101, C = 101, D = 100, matching the provided answer.

What are the PID values at lines A, B, C, D? Let parent be pid 100 and child be pid 101.单项选择题

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