C - プロセス間通信 - お話は楽しい(子プロセス、待機、waitpid関数、パイプの作成)

Head First C
原著

開発環境

• OS: macOS High Sierra - Apple
• IDE(統合開発環境): Xcode - Apple
• プログラミング言語: C

Head First C ―頭とからだで覚えるCの基本 (David Griffiths (著)、Dawn Griffiths (著)、中田 秀基 (監修)、木下 哲也 (翻訳)、オライリージャパン)の10章(プロセス間通信 - お話は楽しい)、エクササイズ(p. 447)を取り組んでみる。

エクササイズ(p. 447)

//
//  main.c
//  sample1
//
//  Created by kamimura on 2018/02/26.
//  Copyright © 2018 kamimura. All rights reserved.
//

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/wait.h>

void error(char *msg) {
    fprintf(stderr, "%s: %s\n", msg, strerror(errno));
    exit(1);
}

void open_url(char *url) {
    pid_t pid = fork();
    if (pid == -1) {
        error("プロセスをフォークできません");
    }
    if (!pid) {
        if (execlp("open", "open", url, NULL) == -1) {
            char s[255];
            sprintf(s, "'%s'を開けません", url);
            error(s);
        }
    }
    int pid_status;
    if (waitpid(pid, &pid_status, 0) == -1) {
        error("子プロセスの待機エラー");
    }
}

int main(int argc, const char * argv[]) {
    char *phrase = (char*)argv[1];
    char *vars[] = {
        "RSS_FEED=http://rss.cnn.com/rss/edition.rss",
        NULL
    };
    int fd[2];
    if (pipe(fd) == -1) {
        error("パイプを作成できません");
    }
    pid_t pid = fork();
    if (pid == -1) {
        error("プロセスをフォークできません");
    }
    if (!pid) {
        close(fd[0]);
        if (dup2(fd[1], 1) == -1) {
            error("標準出力をリダイレクトできません");
        }
        if (execle("/usr/bin/python",
                   "/usr/bin/python",
                   "rssgossip.py", "-u", phrase, NULL,
                   vars) == -1) {
            error("スクリプトを実行できません");
        }
    }
    close(fd[1]);
    if (dup2(fd[0], 0) == -1) {
        error("標準入力をリダイレクト出来ません");
    }
    char line[255];
    while(fgets(line, 255, stdin)) {
        printf("%s", line);
        if (line[0] == '\t') {
            open_url(line + 1);
        }
    }
    int pid_status;
    if (waitpid(pid, &pid_status, 0) == -1) {
        error("子プロセスの待機エラー");
    }
    return 0;
}

入出力結果(Terminal)

$ ./news 
Traceback (most recent call last):
  File "rssgossip.py", line 50, in <module>
    searcher = re.compile(args[0], re.IGNORECASE)
IndexError: list index out of range
$ ./news Japan
$ ./news US
President or Emperor? Xi pushes China back to one-man rule 
 https://www.cnn.com/2018/02/26/asia/china-xi-jinping-president-intl/index.html
North Korea 'willing to hold talks' but US plays coy
 https://www.cnn.com/2018/02/25/asia/north-korea-us-talks-intl/index.html
Axios: Trump pushing to have personal pilot run FAA
 https://www.cnn.com/2018/02/25/politics/trump-pilot-faa-nomination/index.html
No one knows who this unconscious Syrian toddler is
 https://www.cnn.com/2018/02/23/middleeast/syria-eastern-ghouta-lost-child-intl/index.html
Fear of Russian aggression driving US military sales to Europe
 https://www.cnn.com/2018/02/24/politics/fear-russia-aggression-driving-us-military-sales/index.html
US Supreme Court to hear digital privacy case with global impact
 http://money.cnn.com/2018/02/25/technology/microsoft-us-supreme-court-data-sharing/index.html
USA Swimming officials under fire for alleged culture of abuse 
 https://www.cnn.com/2018/02/24/us/usa-swimming-abuse-allegations/index.html
Stolen work by famed painter Degas found in bus
 https://www.cnn.com/2018/02/23/world/stolen-degas-painting-found-bus-intl/index.html
$