程式間協作方式-shell out

標籤：popen   shellout   

程式間協作方式-shell out

shell out這個名詞雖然沒怎麼聽說，但是說不定在默默的使用，說白了就是通過一個程式通過命令的形式調用另一個程式，即system(2)系統調用。其中存在的互動（或者稱為協議）很簡單，一個執行完了，接著另一個再執行，不會有通訊過程，最多有調用程式向被調用程式傳遞參數。
這種模式的變形，就是C庫函數popen()和pclose()，我們可以把popen這種機制看做是對shell out 和 管道的一個上層封裝：建立管道，fork一個子進程，關閉不使用的端，運行shell命令，接受輸入，或者運行主程式，給shell命令提供輸出。情境是相似的。

In a common variant of this pattern, the specialist program may accept input on its standard input, and be called with the C library entry point popen(…, “w”) or as part of a shellscript. Or it may send output to its standard output, and be called with popen(…, “r”) or as part of a shellscript. (If it both reads from standard input and writes to standard output, it does so in a batch mode, completing all reads before doing any writes.) This kind of child process is not usually referred to as a shellout; there is no standard jargon for it, but it might well be called a ‘bolt-on’.
–《Unix編程藝術》

下面是APUE上面的一個執行個體，通過popen，向分頁程式輸送內容。

#include "util.h"#define PAGER "${PAGER:-more}" // environment var or defaultint main(int argc, char *argv[]){    char line[MAXLINE];    FILE *fpin, *fpout;    if(argc != 2)        err_quit("usage:a.out <pathname>");    if((fpin = fopen(argv[1], "r")) == NULL)        err_sys("cannot open %s", argv[1]);    if((fpout = popen(PAGER, "w")) == NULL)        err_sys("popen error");    // copy the file to pager    while(fgets(line, MAXLINE, fpin) != NULL){        if(fputs(line, fpout) == EOF)            err_sys("fputs error to pipe");    }    if(ferror(fpin))        err_sys("fgets error");    pclose(fpout);    exit(0);}

程式間協作方式-shell out