c - fork 过程的输出

Question

大家好我想问一个关于 fork 的问题

这是代码

#include  <stdio.h>
#include  <string.h>
#include  <sys/types.h>

#define   MAX_COUNT  5
#define   BUF_SIZE   100

void  main(void)
{
     pid_t  pid;
     int    i;
     char   buf[BUF_SIZE];

     fork();
     pid = getpid();
     for (i = 1; i <= MAX_COUNT; i++) {
          sprintf(buf, "This line is from pid %d, value = %d\n", pid, i);
          write(1, buf, strlen(buf));
     } 
     printf("child id :%d\n" , pid);

     printf("parent id :%d\n" , getppid());
}

当我运行它时这里是输出

This line is from pid 31130, value = 1
This line is from pid 31130, value = 2
This line is from pid 31130, value = 3
This line is from pid 31130, value = 4
This line is from pid 31130, value = 5
child id :31130
This line is from pid 31131, value = 1
parent id :31052
This line is from pid 31131, value = 2
This line is from pid 31131, value = 3
This line is from pid 31131, value = 4
This line is from pid 31131, value = 5
child id :31131
parent id:31130

这让我很困惑，为什么

1. 打印两次parent id和child id的行

2. 为什么我调用时两次的 child ID 值都不同 只有一次

3. 为什么第二次父id值等于第一个子id值

实际上什么是父 ID 和智利 ID 非常感谢

Answer 1

fork 函数被调用一次，返回两次。所以 child 和 parent 都执行 printf。

printf("child id :%d\n" , pid); /* Both child and parent print this. */
printf("parent id :%d\n" , getppid()); /* Both child and parent print this. */

您说“ child ”和“ parent ”这一事实并没有真正改变任何东西，所以这就是您得到误导性结果的原因。

一个常见的模式是测试 fork 返回的值。它在子节点中返回 0，在父节点中返回子节点的 PID。

pid_t pid = fork();
if (pid)
    printf("I am the parent, pid: %d. The child has pid: %d\n", getpid(), pid);
else
    printf("I am the child, pid: %d\n", getpid());

/* Common code. */
for...