read CAT source file

我学习cat命令的源码及注释。

/*    $OpenBSD: cat.c,v 1.20 2009/10/27 23:59:19 deraadt Exp $    */
/*    $NetBSD: cat.c,v 1.11 1995/09/07 06:12:54 jtc Exp $    */

/*
* Copyright (c) 1989, 1993
*    The Regents of the University of California.  All rights reserved.
*
* This code is derived from software contributed to Berkeley by
* Kevin Fall.
*
* 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 <sys/param.h>
#include <sys/stat.h>

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

extern char *__progname;

/* 使用extern表示变量来自另外一个源文件 */

int bflag, eflag, nflag, sflag, tflag, vflag;
int rval;
char *filename;

void cook_args(char *argv[]);
void cook_buf(FILE *);
void raw_args(char *argv[]);
void raw_cat(int);

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

setlocale(LC_ALL, "");
/* setlocale为库中某些特定的例程（routines）设定自然语言的显示格式。

每种这样的显示格式称为一个locale
* 用法：setlocale(int category, const char *locale);
其中第一个参数LC_ALL类（category）定义全部通用的显示格式；

其他的还有LC_COLLATE,LC_CTYPE,LC_MONETRAY等。
第二个参数为空“”，表示程序调用时所处的环境（native environment）；

其他的还有“C”，“POSIX"。它们是默认的三种locale
如果参数中category和locale是一个没有意义的组合，

setlocale（）返回一个空指针（NULL）

*/

while ((ch = getopt(argc, argv, "benstuv")) != -1)

/* getopt用于获取cat命令的参数，有效参数为benstuv中的一个或多个 */
switch (ch) {
case 'b':

/* 不计算空白行，且输出行号 */
bflag = nflag = 1;

/* -b implies -n */
break;

/* 跳出当前的switch循环 */
case 'e':

/* 在行尾输出$字符，且显示非打印字符（=v）*/
eflag = vflag = 1;

/* -e implies -v */
break;
case 'n':

/* 输出行号 */
nflag = 1;
break;
case 's':

/* 合并多个相邻的空白行 */
sflag = 1;
break;
case 't':

/* 将tab字符显示为^I,且显示非打印字符 */
tflag = vflag = 1;

/* -t implies -v */
break;
case 'u':

/* 保证输出不被缓冲 */
setbuf(stdout, NULL);
break;
case 'v':

/* 显示非打印字符 */
vflag = 1;
break;
default:
(void)fprintf(stderr,
"usage: %s [-benstuv] [file ...]\n", __progname);
exit(1);
/* NOTREACHED */
}
argv += optind;
/* optind 用于多次调用getopt；第一次从getopt返回后，

optind含有指向下一个argv参数的索引
* optind初始化为1
* The variables "opterr" and "optind" are both initialized to 1.  The optind
* variable may be set to another value larger than 0 before a set of calls
* to getopt() in order to skip over more or less argv entries.  An optind
* value of 0 is reserved for compatibility with GNU getopt().
*/

if (bflag || eflag || nflag || sflag || tflag || vflag)
cook_args(argv);

/* 如果有参数就执行cook_args函数*/
else
raw_args(argv);

/* 如果没有参数就执行raw_args函数*/

if (fclose(stdout))
err(1, "stdout");
exit(rval);
/* NOTREACHED */
}

void
cook_args(char **argv)
{
FILE *fp;

fp = stdin;
filename = "stdin";

/* starting a do...while loop ... */
do {
if (*argv) {
if (!strcmp(*argv, "-"))  /

* strcmp，如果两个字符相同则返回0 */
fp = stdin;
else if ((fp = fopen(*argv, "r")) == NULL) {
warn("%s", *argv);

/* warn 用于在stderr上显示格式化的错误信息 */
rval = 1;
++argv;
continue;

/* 结束本次循环,即跳过循环体下面尚未执行的语句,接着进行下一次是否执行循环的判断,
* (就是执行到continue时,立即结束本次循环,重新去判断循环条件是否为真) */
}
filename = *argv++;
}
cook_buf(fp);
if (fp == stdin)
clearerr(fp);

/* clearerr的作用是清楚fp的EOF和错误标志*/
else
(void)fclose(fp);
} while (*argv);
}

/* end of a do...while loop ... */

void
cook_buf(FILE *fp)
{
int ch, gobble, line, prev;

line = gobble = 0;
for (prev = '\n'; (ch = getc(fp)) != EOF; prev = ch) {

/* prev = '\n'; while (ch = getc(fp) != EOF) { ....;prev = ch;} */
if (prev == '\n') {              /* 第一个IF */
if (sflag) {

/* 第二个IF */

/* 如果设置sflag=1，表示多个相邻的空白行要合并成一个*/
if (ch == '\n') {
if (gobble)
continue;
gobble = 1;
} else

/* else和离它最近的一个没有else的IF关键词结合，这里应该是if （ch == '\n'),
如果有大括号进行分段则以大括号为准 */
gobble = 0;
} /* 第二个IF 结束*/

if (nflag &amp;&amp; (!bflag || ch != '\n')) {

/* nflag 控制是否显示行号, bflag也是要求显示行号，但不计算空白行 */

(void)fprintf(stdout, "%6d\t", ++line);

/* fprintf中“%6d\t"这种格式是要将打印的变量至少占6个字符的宽度，
不够6个字符的情况下前面补上空格 */
if (ferror(stdout))

/* ferror函数测试文件流的错误指示标志（error indicator），

如果标志已经设置则返回非0值 */
break;
}
} /* 第一个IF 结束*/

if (ch == '\n') {
if (eflag &amp;&amp; putchar('$') == EOF)
break;
} else if (ch == '\t') {
if (tflag) {
if (putchar('^') == EOF || putchar('I') == EOF)
break;
continue;
}
} else if (vflag) {
if (!isascii(ch)) {
if (putchar('M') == EOF || putchar('-') == EOF)
break;
ch = toascii(ch);
}
if (iscntrl(ch)) {
if (putchar('^') == EOF ||
putchar(ch == '\177' ? '?' :
ch | 0100) == EOF)
break;
continue;
}
}
if (putchar(ch) == EOF)
break;
}  /* FOR循环结束 */

if (ferror(fp)) {
warn("%s", filename);
rval = 1;
clearerr(fp);
}
if (ferror(stdout))
err(1, "stdout");
}

void
raw_args(char **argv)
{
int fd;

fd = fileno(stdin);
filename = "stdin";
do {
if (*argv) {
if (!strcmp(*argv, "-"))
fd = fileno(stdin);
else if ((fd = open(*argv, O_RDONLY, 0)) < 0) {
warn("%s", *argv);
rval = 1;
++argv;
continue;
}
filename = *argv++;
}
raw_cat(fd);
if (fd != fileno(stdin))
(void)close(fd);
} while (*argv);
}

void
raw_cat(int rfd)
{
int wfd;
ssize_t nr, nw, off;
static size_t bsize;
static char *buf = NULL;
struct stat sbuf;

wfd = fileno(stdout);
if (buf == NULL) {
if (fstat(wfd, &amp;sbuf))
err(1, "stdout");
bsize = MAX(sbuf.st_blksize, BUFSIZ);
if ((buf = malloc(bsize)) == NULL)
err(1, "malloc");
}
while ((nr = read(rfd, buf, bsize)) != -1 &amp;&amp; nr != 0)
for (off = 0; nr; nr -= nw, off += nw)
if ((nw = write(wfd, buf + off, (size_t)nr)) == 0 ||
nw == -1)
err(1, "stdout");
if (nr < 0) {
warn("%s", filename);
rval = 1;
}
}