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echo.c

BSD版 echo.c

/*-
 * Copyright (c) 1989, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#if 0
#ifndef lint
static char const copyright[] =
"@(#) Copyright (c) 1989, 1993\n\
	The Regents of the University of California.  All rights reserved.\n";
#endif /* not lint */

#ifndef lint
static char sccsid[] = "@(#)echo.c	8.1 (Berkeley) 5/31/93";
#endif /* not lint */
#endif
#include <sys/cdefs.h>
__FBSDID("$FreeBSD: src/bin/echo/echo.c,v 1.18.22.1 2010/02/10 00:26:20 kensmith Exp $");

#include <sys/types.h>
#include <sys/uio.h>

#include <assert.h>
#include <errno.h>
#include <limits.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

/*
 * Report an error and exit.
 * Use it instead of err(3) to avoid linking-in stdio.
 */
static void
errexit(const char *prog, const char *reason)
{
	char *errstr = strerror(errno);
	write(STDERR_FILENO, prog, strlen(prog));
	write(STDERR_FILENO, ": ", 2);
	write(STDERR_FILENO, reason, strlen(reason));
	write(STDERR_FILENO, ": ", 2);
	write(STDERR_FILENO, errstr, strlen(errstr));
	write(STDERR_FILENO, "\n", 1);
	exit(1);
}
	
int
main(int argc, char *argv[])
{
	int nflag;	/* if not set, output a trailing newline. */
	int veclen;	/* number of writev arguments. */
	struct iovec *iov, *vp; /* Elements to write, current element. */
	char space[] = " ";
	char newline[] = "\n";
	char *progname = argv[0];

	/* This utility may NOT do getopt(3) option parsing. */
        // -n オプションの解釈。
        // 解釈したら次の引数をフォーカスするために argv と argc を更新
	if (*++argv && !strcmp(*argv, "-n")) {
		++argv;
		--argc;
		nflag = 1;
	} else
		nflag = 0;

        // 引数と引数の間のスペースを含む要素数。
        // e.g. $ echo one two three ならば、
        //      argcは4なので、 (4 - 2) * 2 + 1 = 5 すなわち  one, " ", two, " ", three の5つが要素数。
        // 要するに 2*(argc - 1) - 1 なのだが、何故こんな書き方をしているのかは不明。
        // ちなみに argc - 1 なのはargv[0] の echo を排除するため。
	veclen = (argc >= 2) ? (argc - 2) * 2 + 1 : 0;

	if ((vp = iov = malloc((veclen + 1) * sizeof(struct iovec))) == NULL)
		errexit(progname, "malloc");

        // さりげなく -n の判定で argvがインクリメントされているので、ここでの argv[0]は表示する一つ目の引数
	while (argv[0] != NULL) {
		size_t len;
		
		len = strlen(argv[0]);

		/*
		 * If the next argument is NULL then this is this
		 * the last argument, therefore we need to check
		 * for a trailing \c.
		 */
                // UNIX系の echo は 引数末尾に \c がある場合、改行しないので、
                // それとの互換性維持のための処理
                // ちなみに Linux はこの処理がなさげ(少なくとも、\cで改行を挿入しないという動作ははしない)
		if (argv[1] == NULL) {
			/* is there room for a '\c' and is there one? */
            // 要するに末尾の引数の後ろから1文字目と2文字目を取って、\cか比較している。
			if (len >= 2 &&
			    argv[0][len - 2] == '\\' &&
			    argv[0][len - 1] == 'c') {
				/* chop it and set the no-newline flag. */
				len -= 2;
				nflag = 1;
			}
		}
		vp->iov_base = *argv;
		vp++->iov_len = len;
		// 関係ないけど、 *++argvという書き方は変態チックだと思った。
                if (*++argv) {
			vp->iov_base = space;
			vp++->iov_len = 1;
		}
	}
    // 改行挿入の処理
	if (!nflag) {
		veclen++;
		vp->iov_base = newline;
		vp++->iov_len = 1;
	}
	/* assert(veclen == (vp - iov)); */
        // IOV_MAXずつ引数を順次表示する。
	while (veclen) {
		int nwrite;
                // 3項演算子でわかりにくいが、要するに IOVMAXかveclen、小さい方を表示数に採用
		nwrite = (veclen > IOV_MAX) ? IOV_MAX : veclen;
		if (writev(STDOUT_FILENO, iov, nwrite) == -1)
			errexit(progname, "write");
                // 表示した分だけ先頭ポインタをスライドする
		iov += nwrite;
		veclen -= nwrite;
	}
	return 0;
}