linux 进程 ID分析 fork getpid getppid 函数

hu16happy

代码如下
. W. |) p( C7 Y# X) M9 P8 H2 ~#include <stdio.h>
! ^* c" v0 x' ]) n" x6 Y' O, N3 b7 u$ f/ S  2 #include <stdlib.h>
  3 #include <string.h>
# u% r1 c$ G7 N( a. ~  4 #include <unistd.h>
6 N% H, W/ i( u0 g" G  5 #include <pthread.h>
  6 #include <sys/stat.h>
/ A& Q+ H5 x# Y: T% d/ ~9 B* o  7
  8 int main(int argc,char *argv[])
  9 {
10     printf("before fork -1-\n");
11     printf("before fork -2-\n");
12     printf("before fork -3-\n");
% d: K* u7 V, A; { 13     printf("before fork -4-\n");
14     printf("before fork -5-\n");
15
* E9 I+ }# B% Y 16     pid_t pid = fork();
17     if(pid == -1){
18         perror("fork error");
19         exit(1);
20     }else if(pid == 0){
21         printf("child was created\n");
22         printf("child pid : %d;parent pid : %d\n",getpid(),getppid());
6 C2 w& P. F0 @. l- c* | 23     }else if(pid > 0){
24         printf("I am parent process\n");
25         printf("my child pid : %d from fork\n",pid);
26         printf("current process pid : %d;parent parent pid : %d\n",getpid(),getppid());
3 g! N$ h1 F0 x0 H, T# R/ L 27     }
28
29     printf("===============end of file\n");
8 u! N' ]* K3 E) g# x 30
31     return 0;
4 ~9 ?4 n' e+ w# C. C6 S 32 }

+ S; y+ u- B5 |: ?6 z4 w编译结果如下
hdh@ubuntu:~/work_linux/linux_c/4_test$ ./obj/fork
- d, x8 a/ x) ]: {: ebefore fork -1-
before fork -2-
( W& {' |, g7 U* b8 Qbefore fork -3-
before fork -4-
( T% q  F, g  I3 ^5 E5 mbefore fork -5-
I am parent process
" J: f. H/ }2 v# R) H2 g0 \, Imy child pid : 65472 from fork
$ p% @! [0 V: k0 s' `; ]current process pid : 65471;parent parent pid : 63901
! D+ }+ n2 E4 ^8 q# e===============end of file
hdh@ubuntu:~/work_linux/linux_c/4_test$ child was created
4 |" d9 F* j# m  D3 `, ^5 Rchild pid : 65472;parent pid : 1617
4 L% J6 r0 S! a===============end of file


问题点：
为什么子进程读到的父进程id与父进程读到自己ID不一致。能否解释一下   
子进程打印出parent  PID  1617   而父进程说自己PID时65471

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