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:[email protected] > 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:[email protected] > 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:[email protected] > 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:[email protected] > 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。