linux 进程 ID分析 fork getpid getppid 函数

hu16happy

代码如下
1 e  S( u/ l, l, o#include <stdio.h>
  2 #include <stdlib.h>
6 _: E& H/ ^* B" U2 M0 h, f, S  3 #include <string.h>
  4 #include <unistd.h>
  5 #include <pthread.h>
  6 #include <sys/stat.h>
4 |2 k) d/ {6 B$ Q5 ^5 o$ f  7
( _* K# b: {' O  P+ X7 ]  8 int main(int argc,char *argv[])
  9 {
10     printf("before fork -1-\n");
! h7 l) j$ f) g1 {3 a 11     printf("before fork -2-\n");
# {& `8 R+ F: h9 o: f- ] 12     printf("before fork -3-\n");
13     printf("before fork -4-\n");
! l+ F# y7 m7 K; |4 p  D 14     printf("before fork -5-\n");
4 v, Y% X) ~2 H& a! T 15
16     pid_t pid = fork();
% O. q8 N1 `/ j( n; j8 B5 g 17     if(pid == -1){
18         perror("fork error");
- w* k( ]. ]9 n" _7 N 19         exit(1);
: J: y5 ~7 k% `% i 20     }else if(pid == 0){
21         printf("child was created\n");
7 s, D/ P; {% e* G. j 22         printf("child pid : %d;parent pid : %d\n",getpid(),getppid());
23     }else if(pid > 0){
24         printf("I am parent process\n");
25         printf("my child pid : %d from fork\n",pid);
" @# W) W- h- E1 i7 ^8 n% s 26         printf("current process pid : %d;parent parent pid : %d\n",getpid(),getppid());
27     }
28
( J' t" P+ Q1 R" L 29     printf("===============end of file\n");
' W4 f! P- f4 j- M4 v: z 30
( x2 n' L) \; ?$ v- V6 B! G( n 31     return 0;
32 }

2 s+ J, h' m  ~+ J/ V+ h+ j编译结果如下
hdh@ubuntu:~/work_linux/linux_c/4_test$ ./obj/fork
0 Q  }1 U: c) G3 g; ybefore fork -1-
before fork -2-
before fork -3-
before fork -4-
  e( k9 i/ N; T6 e# A- {. Lbefore fork -5-
7 U6 p  e: ^2 m/ X" T8 p0 wI am parent process
my child pid : 65472 from fork
5 ]: f  o# N: E5 Q! m; Y/ Y5 Y# D& dcurrent process pid : 65471;parent parent pid : 63901
) m" B. H0 u6 b% [4 b% i===============end of file
hdh@ubuntu:~/work_linux/linux_c/4_test$ child was created
5 l9 L+ M- D/ r4 ?: Rchild pid : 65472;parent pid : 1617
% ^; L9 k/ x3 ^3 D  \7 Q; t===============end of file
9 B  Q0 T; {4 X1 N& \- r
" d+ V4 Z4 J2 B8 O: B
问题点：
/ ?9 D) q7 m9 Y+ ^* Y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   
9 p: L6 s9 D2 S" X# V子进程打印出parent  PID  1617   而父进程说自己PID时65471
8 p" J: M0 K" A  w% i

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