源码阅读: echo 回显程序

1. 目的

阅读 netbsd 9.3 的 echo.c, 练习 C 语言源码阅读的技能。

2. 原始代码

https://github.com/NetBSD/src/blob/trunk/bin/echo/echo.c

/* $NetBSD: echo.c,v 1.23 2021/11/16 21:38:29 rillig Exp $	*/

/*
 * 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.
 * 3. 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.
 */

#include 
__COPYRIGHT(
"@(#) Copyright (c) 1989, 1993\
 The Regents of the University of California.  All rights reserved.");

#if 0
static char sccsid[] = "@(#)echo.c	8.1 (Berkeley) 5/31/93";
#else
__RCSID("$NetBSD: echo.c,v 1.23 2021/11/16 21:38:29 rillig Exp $");
#endif

#include 
#include 
#include 
#include 
#include 
#include 

/* ARGSUSED */
int
main(int argc, char *argv[])
{
	bool nflag;

	setprogname(argv[0]);
	(void)setlocale(LC_ALL, "");

	/* This utility may NOT do getopt(3) option parsing. */
	nflag = *++argv != NULL && strcmp(*argv, "-n") == 0;
	if (nflag)
		++argv;

	while (*argv != NULL) {
		(void)printf("%s", *argv);
		if (*++argv != NULL)
			(void)putchar(' ');
	}
	if (!nflag)
		(void)putchar('\n');
	(void)fflush(stdout);
	if (ferror(stdout) != 0)
		err(1, "write error");
	return 0;
}

3. 化简和跨平台支持

删除头部版权信息的文本。

err.h 在 MSVC 编译器下不存在，err() 改为 fprintf().

使用 clang-format 修改了编码风格：

BasedOnStyle: Microsoft
IndentWidth: 4

修改后的完整代码:

#include 
#include 
#include 
#include 

int main(int argc, char *argv[])
{
    bool nflag;

    // detect "-n" option
    nflag = *++argv != NULL && strcmp(*argv, "-n") == 0;
    if (nflag)
        ++argv;

    while (*argv != NULL)
    {
        printf("%s", *argv);
        if (*++argv != NULL)
        {
            putchar(' ');
        }
    }
    if (!nflag)
    {
        putchar('\n');
    }
    fflush(stdout);
    if (ferror(stdout) != 0)
    {
        fprintf(stderr, "write error");
    }
    return 0;
}

4. 修改后代码的代码分析

检查是否传入了 -n 参数：

其中 *++argv 表示， 先让 argv = argv + 1, 再取*argv.

原样输出每个输入的字符： 如果不是最后一个字符， 则跟随输出一个空格：

如果输入了 -n 选项， 那么主动输出一个换行符：

刷新io的缓冲区，立即输出到屏幕上。如果输出出现了错误，输出到标准错误输出上：

5. References

• Cannot open source file err.h in visual studio?