关了手机，定好机械闹钟，一心就把上午读的几个代码写写心得。
wc 这个命令用得很熟悉了，看《flex and bison》中编写一个wc是很简单的，但打开看，wc.c命令也有几百行，596行。
从main开始说起，我读代码是从头到尾看，我总想全部翻一次，再慢慢从尾到头分析。main中，根据传入文件的多少，如果传入多个文件，再打印一个合计信息。
我验证了一下，果然如此。
for (; optind < argc; ++optind)
wc_file (argv[optind]);
if (nfiles > 1)
write_counts (total_lines, total_words, total_chars, total_bytes,
max_line_length, _("total"));
其实我难的方法就是再加一句打印语句，读程序时，若找不清头绪，我就根据自己的大致推测，加打印语句。
所以干活的主要是wc_file这个函数。
wc_file调用wc来解决问题
wc (fd, file);
再看wc
程序用几个变量来累计计算的结果
lines = words = chars = bytes = linelength = 0;
主要的结构是一个大的if语句。
if (count_bytes && !count_chars && !print_lines && !count_complicated)
{
}
else if (!count_chars && !count_complicated)
{
}
其中还有宏的一段，不管了，其实开头是有注释的，
/* When counting only bytes, save some line- and word-counting
overhead. If FD is a `regular' Unix file, using lseek is enough
to get its `size' in bytes. Otherwise, read blocks of BUFFER_SIZE
bytes at a time until EOF. Note that the `size' (number of bytes)
that wc reports is smaller than stats.st_size when the file is not
positioned at its beginning. That's why the lseek calls below are
necessary. For example the command
`(dd ibs=99k skip=1 count=0; ./wc -c) < /etc/group'
should make wc report `0' bytes. */
write_counts (total_lines, total_words, total_chars, total_bytes,
max_line_length, _("total"));
其实我难的方法就是再加一句打印语句，读程序时，若找不清头绪，我就根据自己的大致推测，加打印语句。
所以干活的主要是wc_file这个函数。
wc_file调用wc来解决问题
wc (fd, file);
再看wc
程序用几个变量来累计计算的结果
lines = words = chars = bytes = linelength = 0;
主要的结构是一个大的if语句。
if (count_bytes && !count_chars && !print_lines && !count_complicated)
{
}
else if (!count_chars && !count_complicated)
{
}
其中还有宏的一段，不管了，其实开头是有注释的，
/* When counting only bytes, save some line- and word-counting
overhead. If FD is a `regular' Unix file, using lseek is enough
to get its `size' in bytes. Otherwise, read blocks of BUFFER_SIZE
bytes at a time until EOF. Note that the `size' (number of bytes)
that wc reports is smaller than stats.st_size when the file is not
positioned at its beginning. That's why the lseek calls below are
necessary. For example the command
`(dd ibs=99k skip=1 count=0; ./wc -c) < /etc/group'
should make wc report `0' bytes. */
/* Use a separate loop when counting only lines or lines and bytes --
but not chars or words. */
这段注释我看，大致就是计算行数的处理在此。于是就加打印语句，看它是否走到这儿，执行时，果然走到这里。
else if (!count_chars && !count_complicated)
{
/* Use a separate loop when counting only lines or lines and bytes --
but not chars or words. */
while ((bytes_read = safe_read (fd, buf, BUFFER_SIZE)) > 0)
{
register char *p = buf;
but not chars or words. */
这段注释我看，大致就是计算行数的处理在此。于是就加打印语句，看它是否走到这儿，执行时，果然走到这里。
else if (!count_chars && !count_complicated)
{
/* Use a separate loop when counting only lines or lines and bytes --
but not chars or words. */
while ((bytes_read = safe_read (fd, buf, BUFFER_SIZE)) > 0)
{
register char *p = buf;
while ((p = memchr (p, '\n', (buf + bytes_read) - p)))
{
++p;
++lines;
}
bytes += bytes_read;
}
if (bytes_read < 0)
{
error (0, errno, "%s", file);
exit_status = 1;
}
从代码看，就是看有几个\n，就认为有几行。用的memchr库函数没用过，我在想，争对一些特殊情况，这段代码是正确的吗？后来就想，如果仔细调的话，不划算，先留点尾巴。以后典再看吧。
{
++p;
++lines;
}
bytes += bytes_read;
}
if (bytes_read < 0)
{
error (0, errno, "%s", file);
exit_status = 1;
}
从代码看，就是看有几个\n，就认为有几行。用的memchr库函数没用过，我在想，争对一些特殊情况，这段代码是正确的吗？后来就想，如果仔细调的话，不划算，先留点尾巴。以后典再看吧。