进程间通信（二）popen/pclose

popen其实就是获取一个外部程序的标准输出的的函数，整个调用过程如下所示： popen实现过程

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打开一个shell

运行程序

暂存至管道

返回管道描述符

他是一种进程间通信方式，通过创建的管道完成外部程序信息的传递，优点是能够利用到强大的shell，缺点是额外启动shell成本较高，单向读写。

在Linux系统提供的函数中execl也可以用来启动一个新的进程，可以调用一个外部程序到当前的进程空间里，但是不是一个新的进程。int system(const char * string);也可启动一个shell。

一、什么是管道

管道是连接两个进程的介质，进程A数据通过管道流入进程B。对于Linux用户而言，对下面的语句都不会陌生：

cmd1|cmd2

管道与缓冲区非常相似，cmd1的输出结果将会存放在管道中，管道再转发这个输出结果，对于cmd2而言，转发的内容为输入。

二、最简单的popen

popen简单来说就是打开一个终端并运行指定程序，将其输出放入管道，返回一个指针方便取出管道内容。

#include 
FILE * popen(const char * command, const char * open_mode);
int pclose(FILE * stream_to_close);

输入参数

const char * command 是一个shell命令或者程序;
const char * open_mode 取值"r"或"w”

返回值

FILE * fp 根据open_mode决定这个文件指针可以读还是写

三、例子 3.1 传递一小段数据

//test.cpp
#include 
#include 
#inlcude 
#include 
#include 
using namespace std;
int main()
{
    FILE * read_fp=popen("./app","r");
    if(!read_fp) cout

进程间通信（二）popen/pclose

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