fork函數

pid_t fork(void);
fork函數調用成功，  返回兩次
在fork函數執行完畢后
如果創建新進程成功，則出現兩個進程
一個是子進程，一個是父進程
在子進程中，fork函數返回0
在父進程中，fork返回新創建子進程的進程ID
我們可以通過fork返回的值來判斷當前進程是子進程還是父進程。

?返回值為0，代表當前進程是子進程
?返回值是非負數，代表當前進程為父進程
?調用失敗，返回-1
?返回值是pid_t 類型的，這里的pid_t類似一個類型，就像int型一樣，int型定義的變量都是整型的，pid_t定義的類型都是進程號類型。這個語句的意思是定義了一個pid_t類型的變量pid，fork()函數返回一個進程號，這個進程號賦給了pid。

代碼示例

#include<stdio.h>
#include <sys/types.h>
#include <unistd.h>
int main()
{pid_t pid;pid_t fpid;printf("father id :%d\n",getpid());fpid=fork();if(fpid>0){printf("this is father print,pid=%d\n",getpid());}else if(pid==0){printf("this is child print,pid=%d\n",getpid());}return 0;
}
運行結果：father id :17615this is father print,pid=17615this is child print,pid=17616

判斷進程的運行順序

#include<stdio.h>
#include <sys/types.h>
#include <unistd.h>
int main()
{pid_t pid;pid_t fpid;pid_t returnpid;printf("father id :%d\n",getpid());returnpid=fork();fpid=getpid();//獲取當前進程的pid，父子進程都會執行這句話getppid（）——獲取父進程的pidif(fpid==pid){printf("this is father print,pid=%d,returnpid=%d\n",getpid(),returnpid);}else{printf("this is child print,pid=%d,returnpid=%d\n",getpid(),returnpid);}return 0;
}
運行結果：father id :17638this is child print,pid=17638,returnpid=17639this is child print,pid=17639,returnpid=0

子進程改變變量父進程中的變量不變

#include<stdio.h>
#include <sys/types.h>
#include <unistd.h>
int main()
{pid_t pid;pid_t fpid;int a=10;printf("father id :%d\n",getpid());fpid=fork();if(fpid>0){printf("this is father print,pid=%d\n",getpid());}else if(pid==0){printf("this is child print,pid=%d\n",getpid());a=a+10;}printf("a=%d\n",a);return 0;
}
運行結果：	father id :17660this is father print,pid=17660a=10this is child print,pid=17661a=20

創建子進程的目的

模擬與客戶端交互

#include<stdio.h>
#include <sys/types.h>
#include <unistd.h>
int main()
{pid_t fpid;int data;while(1){printf("please input a data\n");scanf("%d",&data);if(data==1){fpid=fork();if(fpid>0){}else if(fpid==0){while(1){printf("do net request,pid=%d\n",getpid());sleep(3);}}}else{printf("do nothing\n");}}return 0;
}

vfork函數

pid_t vfork(void);
用法和fork函數一樣

代碼示例

#include<stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
int main()
{pid_t pid;pid=vfork();int cnt;if(pid>0){while(1){printf("this is father print,pid=%d\n",getpid());sleep(1);printf("cnt=%d\n",cnt);//和父進程共享內存，cnt的值被改變}}else if(pid==0){while(1){printf("this is child print,pid=%d\n",getpid());sleep(1);cnt++;if(cnt==3){exit(-1);}}}return 0;
}