[英]Understanding how fork() system call works
I have this C code sequence: 我有这个C代码序列:
printf("\nThe PID of this (main) process is: %d\n", getpid());
if(fork() != -1) { // #1
printf("\n\nParent 1 PID: %d\n", getpid());
for(int i = 0; i < 10; i++) {
printf("\n%d", i);
}
if(fork() != -1) { // #2
//sleep(1);
printf("\n\nParent 2 PID: %d\n", getpid());
for(char i = 'a'; i != 'f'; i++) {
printf("\n%c", i);
}
}
else { // #3
sleep(3);
printf("\n\nChild 2 PID: %d\n", getpid());
for(char i = 'F'; i != 'J'; i++) {
printf("\n%c", i);
}
}
}
else { // #4
sleep(4);
printf("\n\nChild 1 PID: %d\n", getpid());
for(int i = 10; i < 20; i++) {
printf("\n%d", i);
}
}
I expect that I will have 4 processes: two parents and two childs. 我希望我有四个过程:两个父母和两个孩子。 At line #1 I call
fork()
for first time, and everything from line #1 to line #4 will be executed in first parent process. 在第1行,我第一次调用
fork()
,从第1行到第4行的所有内容都将在第一个父进程中执行。 In the parent process (1) I call fork()
one more time, so from line #2 to line #3 I will have the parent 2 process, and from line #3 to #4 child 2 process. 在父进程(1)中,我再调用一次
fork()
,因此从第2行到第3行,我将拥有父2进程,从第3行到#4是子2进程。
What I expect to be printed: 我希望打印的内容:
Parent 1 PID: ....
0
1
2
3
4
5
6
7
8
9
Parent 2 PID: ....
a
b
c
d
e
Child 2 PID: ....
F
G
H
I
Child 1 PID: ....
10
11
12
13
14
15
16
17
18
19
What I actually got: 我实际得到的是:
Parent 1 PID: 3877
0
1
2
3
4
5
6
7
8
Parent 1 PID: 3878
0
1
2
3
4
5
6
7
8
9
Parent 2 PID: 3877
a
b
c
d
e9
Parent 2 PID: 3878
9
a
b
c
d
Parent 2 PID: 3879
a
b
c
d
e9
eParent 2 PID: 3880
a
b
c
d
e
What I do wrong? 我做错了什么?
This line isn't doing what you think: 这条线没有按照您的想法做:
if(fork() != -1) { // #1
That will succeed for both the parent and the child (as long as fork
is possible, which is almost always the case). 父母和孩子都会成功(只要有可能使用
fork
,几乎总是这样)。 You mean to test against 0 here. 您的意思是在这里测试0。 The parent will get 0, the child will get >0.
父母将获得0,孩子将获得> 0。 -1 is an error.
-1是错误。
In your case, what you've marked as the "child" legs should never be executed unless there are errors. 就您而言,除非有错误,否则切勿执行已标记为“子”腿的操作。 I don't think that's what you meant.
我认为那不是你的意思。 What you're seeing are the initial 2 (parent and child) forks plus 4 (parent+child * 2) second forks.
您看到的是最初的2个(父代和子代)叉子加上4个(父代和子代* 2)第二个叉子。 That's 6 forks, which is what the output indicates.
这是6个fork,这就是输出所指示的内容。
from man fork
: 从
man fork
:
RETURN VALUE
On success, the PID of the child process is returned in the parent, and 0 is returned in the child. On failure, -1 is returned in the parent, no child process is created, and errno
is set appropriately.
this means, that you should expect 0 in the child process and the childs pid in the parent process, so your code should look something like this: 这意味着,您应该在子进程中期望0,在父进程中期望childs pid,因此您的代码应如下所示:
switch(pid = fork()) {
case -1: //error handling here
case 0: // child process code here
default: // parent process code here.
}
merry christmas :) 圣诞节快乐 :)
For understanding detail functioning of how Fork System call works ? 为了了解Fork System调用如何工作的详细功能? you can go to this link : http://linuxtrainers.wordpress.com/2014/12/31/how-fork-system-call-works-what-is-shared-between-parent-and-child-process/
您可以转到以下链接: http : //linuxtrainers.wordpress.com/2014/12/31/how-fork-system-call-works-what-is-shared-between-parent-and-child-process/
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