fork和exec一起使用

先预览一下工程的目录树：

实现的功能：master进程启动slave进程。

看看Makefile内容：

all: master.out slave.out

master.out: master.cpp
    g++ master.cpp -o master.out

slave.out: slave.cpp
    g++ slave.cpp -o slave.out

clean:
    rm -rf *.out

master.cpp内容：

/*************************************************************************
    * File: a.cpp
    * Brief: 
    * Created Time: Wed 23 Dec 2015 08:50:13 AM CST
 ************************************************************************/

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<iostream>
#include<signal.h>
using namespace std;

#define Sleep(ms) usleep(ms * 1000)

int main(int argc,char* argv[])
{
    printf("pid=%d, ",getpid());
    for(int i=0; i<argc;i++)
        printf("[%d]=%s, ",i,argv[i]);
    printf("
");
    
    struct sigaction sigchld_action;
    sigchld_action.sa_handler = SIG_DFL;
    sigchld_action.sa_flags = SA_NOCLDWAIT;
    sigaction(SIGCHLD, &sigchld_action, NULL);
    
    //new process
    int m_subProcessID=fork(); 
    if (m_subProcessID==0)
    {
        int ret=execl("./slave.out","a=abc","b=def","c=12345",NULL);
        if(0!=ret)
        {
            printf("execl fails.
");
            return -1;
        }
        printf("execl done wit ok
");
        return 0;
    }
    for(int i=0;i<10;i++)
    {
        Sleep(2000);
        printf("master say: i=%d
",i);
    }
    printf("master say: i am done
");
    return 0;
}

slave.cpp内容：

/*************************************************************************
    * File: a.cpp
    * Brief: 
    * Created Time: Wed 23 Dec 2015 08:50:13 AM CST
 ************************************************************************/

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<iostream>
#include<signal.h>
using namespace std;

#define Sleep(ms) usleep(ms * 1000)

int main(int argc,char* argv[])
{
    printf("pid=%d, ",getpid());
    for(int i=0; i<argc;i++)
        printf("[%d]=%s, ",i,argv[i]);
    printf("
");
    
    for(int i=0;i<40;i++)
    {
        Sleep(800);
        printf("slave say: -----------------------------------------------------ndex=%d
",i);
    }
    printf("slave say: i am done
");
    return 0;
}

make编译，看看运行效果：

[root@localhost testfork]# ./master.out 
pid=7749, [0]=./master.out, 
pid=7750, [0]=a=abc, [1]=b=def, [2]=c=12345, 
slave say: -----------------------------------------------------ndex=0
slave say: -----------------------------------------------------ndex=1
master say: i=0
slave say: -----------------------------------------------------ndex=2
slave say: -----------------------------------------------------ndex=3
master say: i=1
slave say: -----------------------------------------------------ndex=4
slave say: -----------------------------------------------------ndex=5
slave say: -----------------------------------------------------ndex=6
master say: i=2
slave say: -----------------------------------------------------ndex=7
slave say: -----------------------------------------------------ndex=8
master say: i=3
slave say: -----------------------------------------------------ndex=9
slave say: -----------------------------------------------------ndex=10
slave say: -----------------------------------------------------ndex=11
master say: i=4
slave say: -----------------------------------------------------ndex=12
slave say: -----------------------------------------------------ndex=13
master say: i=5
slave say: -----------------------------------------------------ndex=14
slave say: -----------------------------------------------------ndex=15
slave say: -----------------------------------------------------ndex=16
master say: i=6
slave say: -----------------------------------------------------ndex=17
slave say: -----------------------------------------------------ndex=18
master say: i=7
slave say: -----------------------------------------------------ndex=19
slave say: -----------------------------------------------------ndex=20
slave say: -----------------------------------------------------ndex=21
master say: i=8
slave say: -----------------------------------------------------ndex=22
slave say: -----------------------------------------------------ndex=23
master say: i=9
master say: i am done
[root@localhost testfork]# slave say: -----------------------------------------------------ndex=24
slave say: -----------------------------------------------------ndex=25
slave say: -----------------------------------------------------ndex=26
slave say: -----------------------------------------------------ndex=27
slave say: -----------------------------------------------------ndex=28
slave say: -----------------------------------------------------ndex=29
slave say: -----------------------------------------------------ndex=30
slave say: -----------------------------------------------------ndex=31
slave say: -----------------------------------------------------ndex=32
slave say: -----------------------------------------------------ndex=33
slave say: -----------------------------------------------------ndex=34
slave say: -----------------------------------------------------ndex=35
slave say: -----------------------------------------------------ndex=36
slave say: -----------------------------------------------------ndex=37
slave say: -----------------------------------------------------ndex=38
slave say: -----------------------------------------------------ndex=39
slave say: i am done

[root@localhost testfork]# 

可见，在第38行，master正常退出。在第55行，slave也正常退出。

注意到，我们常规启动进程的时候，进程打印的启动参数里面第一个是程序文件名称(也可能包含了路径)。而在这里，我们看到slave.out通过exec函数启动的时候，启动参数里的第一个参数变成了我们在exec函数里设置的参数"a=abc"。

在两个进程都运行的时候，我们查看一下进程信息：

ps -A |grep -sn --color=auto "master.out|slave.out"

master的进程ID为7749，slave的进程ID为7750，两个进程为父子进程。

这里说说fork，在父进程运行到fork时，会分离出一个子进程。两个进程的内存完全一样，子进程会共享父进程当前持有的内核对象，包括互斥量这些同步变量。

例如，父进程打开了一个文件FILE* fp，子进程fclose后，父进程调用fwrite不会报错，但实际上数据并没有写入文件。

资源：

     百度网盘软件源码 estfork.zip

完。