标准管道(popen)

NAME
       popen, pclose - pipe stream to or from a process

SYNOPSIS
       #include <stdio.h>

       FILE *popen(const char *command, const char *type);

       int pclose(FILE *stream);

DESCRIPTION
       The popen() function opens a process by creating a pipe,  forking,  and
       invoking  the shell.  Since a pipe is by definition unidirectional, the
       type argument may specify  only  reading  or  writing,  not  both;  the
       resulting stream is correspondingly read-only or write-only.

       The  command argument is a pointer to a null-terminated string contain-
       ing a shell command line.  This command is passed to /bin/sh using  the
       -c  flag;  interpretation, if any, is performed by the shell. 

RETURN VALUE
       The popen() function returns NULL if the fork(2) or pipe(2) calls fail,
       or if it cannot allocate memory.

       The pclose() function returns -1 if wait4(2) returns an error, or  some
       other error is detected.

写

popen.c，如下：

/*************************************************************************
    > File Name: popen.c
    > Author: KrisChou
    > Mail:[e-mail]
    > Created Time: Fri 22 Aug 2014 11:07:26 AM CST
 ************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[])
{
    char buf[1024];
    FILE *fp;

    while(memset(buf,0,1024),fgets(buf,1024,stdin) != NULL)
    {
        fp = popen(argv[1],"w");
        fputs(buf,fp);
        pclose(fp);
    }

    return 0;
}

被调用函数reverse.c，如下：

/*************************************************************************
    > File Name: reverse.c
    > Author: KrisChou
    > Mail:[e-mail]
    > Created Time: Sat 23 Aug 2014 11:21:27 AM CST
 ************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[])
{
    char word[256]; //从标准输入读取字符串
    char buf[256][256],index = 0; //保存从标准输入读取的字符串
    while(memset(word,0,256), scanf("%s",word) != EOF )
    {
        strcpy(buf[index++],word);
    }
    index--;
    while(index >=0 )
    {
        printf("%s ",buf[index]);
        index--;
    }
    printf("\n");
    return 0;
}

运行程序:

[purple@localhost popen]$ gcc popen.c -o main
[purple@localhost popen]$ gcc reverse.c -o reverse
[purple@localhost popen]$ ./main ./reverse
how are you
you are how
baby u r beautiful
beautiful r u baby
[purple@localhost popen]$

按 ctrl+D 退出popen.c中的循环，从而退出程序。

读

popen.c，如下：

/*************************************************************************
    > File Name: popen.c
    > Author: KrisChou
    > Mail:[e-mail]
    > Created Time: Sun 24 Aug 2014 08:53:14 AM CST
 ************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

int main(int argc, char *argv[])
{
    FILE *fp;        /* 标准管道描述符 */
    char buf[1024];  /* 存放所要调用程序的参数 */
    char cmd[1024];  /* 存放所要调用的程序的命令行 */
    while(memset(buf,0,1024),fgets(buf,1024,stdin))
    {
        sprintf(cmd,"%s %s",argv[1],buf);
        fp = popen(cmd,"r");
        memset(buf,0,1024);
        fgets(buf,1024,fp);
        puts(buf);
        pclose(fp);
    }
    return 0;
}

被调用函数reverse.c，如下：

/*************************************************************************
    > File Name: reverse.c
    > Author: KrisChou
    > Mail:[e-mail]
    > Created Time: Sun 24 Aug 2014 09:03:37 AM CST
 ************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[])
{
    int index;
    for(index = argc - 1; index > 0 ; index--)
    {
        printf("%s ",argv[index]);
    }
    return 0;
}

运行程序:

[purple@localhost popen_write]$ ./main ./reverse
how are you
you are how
hello world
world hello

按 ctrl+D 退出程序。

小结

1. 在主调程序中，若popen以读模式打开，说明主调函数需要从管道获取程序运行结果，因而重定向了被调函数的标准输出。此时，popen时，主调函数需要将运行被调函数的完整参数写入放进命令行。被调函数运行后会将运行结果送入管道，主调程序自行从管道取出运行结果，打印在屏幕上。

2. 在主调程序中，若popen以写模式打开，说明主调函数需要将被调函数的运行所需的参数送人管道，因而重定向了被调函数的标注输入。此时，popen时，主调函数只需将被调函数的path写入命令行即可。被调函数会从管道中取走自己的参数，运行结果由被调函数打印在屏幕上。