dmesg源码位置：
external/toybox/toys/lsb/dmesg.c
system/core/toolbox/dmesg.c（应该不是这个）
在external/toybox/Android.mk的ALL_TOOLS变量中添加了dmesg.c，编译出的/system/bin/dmesg其实是一个指向
/system/bin/toybox的符号链接，
toybox是android m上集成的项目，目的在于用一个单一bin文件统一linux下的常用命令
Toybox combines many common Linux command line utilities together into a single BSD-licensed executable. It's simple, small, fast, and reasonably standards-compliant (POSIX-2008 and LSB 4.1).
Toybox's main goal is to make Android self-hosting by improving Android's command line utilities so it can build an installable Android Open Source Project image entirely from source under a stock Android system. After a talk at the 2013 Embedded Linux Conference explaining this plan (outline, video), Google merged toybox into AOSP and began shipping toybox in Android Mashmallow.
adb shell后执行的ls cat等命令也其实都指向toybox
linux内核自3.5以后提供了/dev/kmsg，用于向用户空间提供访问内核log的借口，
https://www.kernel.org/doc/Documentation/ABI/testing/dev-kmsg
What: /dev/kmsgDate: Mai 2012KernelVersion: 3.5Contact: Kay Sievers <kay@vrfy.org>Description: The /dev/kmsg character device node provides userspace accessto the kernel's printk buffer.
external/toybox/toys/lsb/dmesg.c:
/* dmesg.c - display/control kernel ring buffer.
*
* Copyright 2006, 2007 Rob Landley <rob@landley.net>
*
*
http://refspecs.linuxfoundation.org/LSB_4.1.0/LSB-Core-generic/LSB-Core-generic/dmesg.html
// We care that FLAG_c is 1, so keep c at the end.
USE_DMESG(NEWTOY(dmesg, "w(follow)Ctrs#<1n#c[!tr]", TOYFLAG_BIN))
config DMESG
bool "dmesg"
default y
help
usage: dmesg [-Cc] [-r|-t] [-n LEVEL] [-s SIZE] [-w]
Print or control the kernel ring buffer.
-C Clear ring buffer without printing
-c Clear ring buffer after printing
-n Set kernel logging LEVEL (1-9)
-r Raw output (with <level markers>)
-s Show the last SIZE many bytes
-t Don't print kernel's timestamps
-w Keep waiting for more output (aka --follow)
*/
#define FOR_dmesg
#include "toys.h"
#include <sys/klog.h>
GLOBALS(
long
level;
long
size;
int
color;
)
static
int
xklogctl(
int
type,
char
*buf,
int
len)
{
int
rc = klogctl(type, buf, len);
if
(rc<0) perror_exit(
"klogctl"
);
return
rc;
}
// Use klogctl for reading if we're on a pre-3.5 kernel.
legacy_mode中调用klogct去读kmsg
static
void
legacy_mode() {
char
*data, *to, *from;
int
size;
// Figure out how much data we need, and fetch it.
if
(!(size = TT.size)) size = xklogctl(10, 0, 0);
data = to = from = xmalloc(size+1);
data[size = xklogctl(3 + (toys.optflags & FLAG_c), data, size)] = 0;
// Filter out level markers and optionally time markers
if
(!(toys.optflags & FLAG_r))
while
((from - data) < size) {
if
(from == data || from[-1] ==
'\n'
) {
char
*to;
if
(*from ==
'<'
&& (to =
strchr
(from,
'>'
))) from = ++to;
if
((toys.optflags&FLAG_t) && *from ==
'['
&& (to =
strchr
(from,
']'
)))
from = to+1+(to[1]==
' '
);
}
*(to++) = *(from++);
}
else
to = data+size;
// Write result. The odds of somebody requesting a buffer of size 3 and
// getting "<1>" are remote, but don't segfault if they do.
if
(to != data) {
//将/proc/kmsg中的内容写到标准输出中，这句完成将kmsg输出的任务
xwrite(1, data, to-data);
if
(to[-1] !=
'\n'
) xputc(
'\n'
);
}
if
(CFG_TOYBOX_FREE)
free
(data);
}
static
void
color(
int
c) {
if
(TT.color)
printf
(
"\033[%dm"
, c);
}
void
dmesg_main(
void
)
{
// For -n just tell kernel which messages to keep.
if
(toys.optflags & FLAG_n) {
xklogctl(8, 0, TT.level);
return
;
}
// For -C just tell kernel to throw everything out.
if
(toys.optflags & FLAG_C) {
xklogctl(5, 0, 0);
return
;
}
TT.color = isatty(1);
//
http://kernel.org/doc/Documentation/ABI/testing/dev-kmsg
// Each read returns one message. By default, we block when there are no
// more messages (--follow); O_NONBLOCK is needed for for usual behavior.
// dmesg命令在3.5内核之后读取的是
/dev/kmsg
int
fd = xopen(
"/dev/kmsg"
, O_RDONLY | ((toys.optflags&FLAG_w)?0:O_NONBLOCK));
while
(1) {
char
msg[8192];
// CONSOLE_EXT_LOG_MAX.
unsigned
long
long
time_us;
int
facpri, subsystem, pos;
char
*p, *text;
ssize_t len;
// kmsg fails with EPIPE if we try to read while the buffer moves under
// us; the next read will succeed and return the next available entry.
do
{
len = read(fd, msg,
sizeof
(msg));
}
while
(len == -1 &&
errno
== EPIPE);
// All reads from kmsg fail if you're on a pre-3.5 kernel.
if
(len == -1 &&
errno
== EINVAL) {
close(fd);
return
legacy_mode
();//3.5之前内核还是在
legacy_mode中调用klogct去读kmsg
}
if
(len <= 0)
break
;
//msg中保存的是从/dev/kmsg中读到的原始字符串，eg
//4,222,12150,-;ACPI: 4 ACPI AML tables successfully acquired and loaded
msg[len] = 0;
// 用sscanf解析msg
// char *msg = "4,222,12150,-;ACPI: 4 ACPI AML tables successfully acquired and loaded";
// int retval = sscanf(msg, "%u,%*u,%llu,%*[^;];%n", &facpri, &time_us, &pos);
// %u, 对应优先级"4,"，因此输出facpri--4
// %*u, 跳过行数"222,"
// %llu,对应时间"12150," 下来剩下的就到了"-;ACPI: 4 ACPI AML tables successfully acquired and loaded";
//%*[^;]; 这个意义是跳过接下来从不是字符;开始的到字符; 也就是会跳过"-;"，剩下就到了"ACPI: 4 ACP。。。。"，这些//就是真正的log
//对于%n,gnu c 实现了 C 标准的 format specify 的 %n，它的含义是返回从该次 XXscanf 调用开始到此读了多少个字节,//目前已经读取到了"ACPI...",
//从4到A前面的;一共14个字符，A是第15个，因此%n将使pos获取14
//整个sscanf匹配到的输入项只有facpri和time_us，pos不算在内，因此返回值才与2做比较
//获取到了pos后，text = msg + pos的意义就很明确了，从原始的msg中提取出";"后面真正的文本,作为text
//这里text就是ACPI: 4 ACPI AML tables successfully acquired and loaded
http://kernel.org/doc/Documentation/ABI/testing/dev-kmsg
这个文档上也对/dev/kmsg的标准格式有所说明：
if
(
sscanf (msg,
"%u,%*u,%llu,%*[^;];%n"
, &facpri, &time_us, &pos) != 2)
continue ;
// Drop extras after end of message text.
text = msg + pos;
if
((p =
strchr (text,
'\n'
))) *p = 0;
// subsystem
是指kkernel log中带有 子系统名:详细信息的log，比如
//
//usb usb2: New USB device found, idVendor=1d6b, idProduct=0002
// Is there a subsystem? (The ": " is just a convention.)
p =
strstr (text,
": "
);
subsystem = p ? (p - text) : 0;
// "Raw" is a lie for /dev/kmsg. In practice, it just means we show the
// syslog facility/priority at the start of each line.
// 打印优先级
if
(toys.optflags&FLAG_r)
printf (
"<%d>"
, facpri);
if
(!(toys.optflags&FLAG_t)) {
color(32); //调用printf("\033[%dm", 32); 用绿色打印
// 如果不带-t参数就打印时间戳
printf (
"[%5lld.%06lld] "
, time_us/1000000, time_us%1000000);
color(0);
}
// Errors (or worse) are shown in red, subsystems are shown in yellow.
// 1 时间戳之后如果有subsystem先打印subsystem
if
(subsystem) {
color(33); // 调用printf("\033[%dm", 32); 用黄色打印
printf (
"%.*s"
, subsystem, text);
text += subsystem;
color(0);
}
// 2 下来按优先级，4级及以上调用xputs输出，1-3级用printf红色输出
if
(!((facpri&7) <= 3)) xputs(text);
else
{
color(31);
printf (
"%s"
, text);
color(0);
xputc(
'\n'
);
}
}
close(fd);
}
