journalctl usage..
journalctl 用法
journalctl 查看所有日志，基本上就是狂刷

journalctl --no-pager 等于　cat syslog.txt的效果。

journalctl -k

　查看内核日志（不显示应用日志）

加上--no-pager参数就直接输出到标准输出了。

journalctl -k --no-pager　

Jul 29 17:38:47 systemd[1]: systemd-journald.service: Unit entered failed state.
Jul 29 17:38:47 systemd[1]: systemd-journald.service: Failed with result 'signal'.
Jul 29 17:38:47 systemd[1]: systemd-journald.service: Service has no hold-off time, scheduling restart.
Jul 29 17:38:47 systemd[1]: Stopped Flush Journal to Persistent Storage.
Jul 29 17:38:47 systemd[1]: Stopping Flush Journal to Persistent Storage...
Jul 29 17:38:47 systemd[1]: Stopped Journal Service.
Jul 29 17:38:47 systemd[1]: Starting Journal Service...
Jul 29 17:38:47 systemd[1]: Started Journal Service.

查看系统本次启动的日志，基本上就是刷屏了。

journalctl -b
journalctl -b --no-pager　

查看指定程序的日志

journalctl /usr/sbin/sshd --no-pager

Jul 30 10:00:24 sshd[14609]: error: AuthorizedKeysCommand path is not absolute
Jul 30 10:00:24 sshd[14609]: Authentication refused: bad ownership or modes for directory /home/root
Jul 30 10:00:24 sshd[14609]: error: AuthorizedKeysCommand path is not absolute
Jul 30 10:00:24 sshd[14609]: Authentication refused: bad ownership or modes for directory /home/root
Jul 30 10:00:26 sshd[14609]: Accepted password for root from 192.168.5.64 port 52218 ssh2
Jul 30 10:00:26 sshd[14614]: lastlog_openseek: Couldn't stat /var/log/lastlog: No such file or directory
Jul 30 10:00:26 sshd[14614]: lastlog_openseek: Couldn't stat /var/log/lastlog: No such file or directory
Jul 30 10:56:45 sshd[16413]: error: AuthorizedKeysCommand path is not absolute
Jul 30 10:56:45 sshd[16413]: Authentication refused: bad ownership or modes for directory /home/root
Jul 30 10:56:45 sshd[16413]: error: AuthorizedKeysCommand path is not absolute
Jul 30 10:56:45 sshd[16413]: Authentication refused: bad ownership or modes for directory /home/root
Jul 30 10:56:47 sshd[16413]: Accepted password for root from 192.168.5.64 port 55484 ssh2
Jul 30 10:56:47 sshd[16418]: lastlog_openseek: Couldn't stat /var/log/lastlog: No such file or directory
Jul 30 10:56:47 sshd[16418]: lastlog_openseek: Couldn't stat /var/log/lastlog: No such file or directory
Jul 30 11:01:29 sshd[16585]: error: AuthorizedKeysCommand path is not absolute
Jul 30 11:01:29 sshd[16585]: error: AuthorizedKeysCommand path is not absolute
Jul 30 11:01:29 sshd[16585]: Accepted publickey for root from 192.168.5.64 port 55624 ssh2: RSA SHA256:Qji+sVx09n43QcM300pHJiiPWi4KzqulAMkLAaf1vXM
Jul 30 11:01:29 sshd[16587]: lastlog_openseek: Couldn't stat /var/log/lastlog: No such file or directory
Jul 30 11:01:29 sshd[16587]: lastlog_openseek: Couldn't stat /var/log/lastlog: No such file or directory
Jul 30 11:22:15 sshd[17223]: Accepted password for root from 192.168.5.44 port 64448 ssh2
Jul 30 11:22:15 sshd[17228]: lastlog_openseek: Couldn't stat /var/log/lastlog: No such file or directory
Jul 30 11:22:15 sshd[17228]: lastlog_openseek: Couldn't stat /var/log/lastlog: No such file or directory
Jul 30 11:33:52 sshd[17582]: Accepted password for root from 192.168.5.44 port 65228 ssh2
Jul 30 11:36:11 sshd[17696]: Accepted password for root from 192.168.5.44 port 65341 ssh2

journalctl /usr/bin/cv2x-daemon --no-pager

基本上就够用了。

按可执行文件的路径过滤
如果在参数中指定某个可执行文件(二进制文件或脚本文件)，则 journalctl 会显示与该可执行文件相关的全部条目。比如可以显示 /usr/lib/systemd/systemd 程序产生的日志：

$ sudo journalctl /usr/lib/systemd/systemd

也可以显示 /usr/bin/bash 程序产生的日志：

$ sudo journalctl /usr/bin/bash

用json输出略吊。。

journalctl /usr/sbin/sshd --no-pager -o json

{ "__CURSOR" : "s=1f967cdb38644934b6add6dca6147120;i=ab550;b=81a1ae5398824daea9b502bf3ddfc092;m=2488403b14;t=5c84eebbadaf5;x=3e575c4f570ffd13", "__REALTIME_TIMESTAMP" : "1627616171449077", "__MONOTONIC_TIMESTAMP" : "156904733460", "_BOOT_ID" : "81a1ae5398824daea9b502bf3ddfc092", "_TRANSPORT" : "syslog", "_UID" : "0", "_GID" : "0", "_CAP_EFFECTIVE" : "3fffffffff", "_MACHINE_ID" : "df9d1eef11ee45fd92b407191b390e56", "_HOSTNAME" : "imx6qsabresd", "PRIORITY" : "6", "SYSLOG_FACILITY" : "4", "SYSLOG_IDENTIFIER" : "sshd", "_COMM" : "sshd", "_EXE" : "/usr/sbin/sshd", "_CMDLINE" : "sshd: root [priv]", "_SYSTEMD_SLICE" : "system-sshd.slice", "SYSLOG_PID" : "17696", "MESSAGE" : "Accepted password for root from 192.168.5.44 port 65341 ssh2", "_PID" : "17696", "_SYSTEMD_CGROUP" : "/system.slice/system-sshd.slice/sshd@57-192.168.5.242:22-192.168.5.44:65341.service", "_SYSTEMD_UNIT" : "sshd@57-192.168.5.242:22-192.168.5.44:65341.service", "_SOURCE_REALTIME_TIMESTAMP" : "1627616171448199" }

journalctl /usr/sbin/sshd --no-pager -o json-pretty

{

"__CURSOR" : "s=1f967cdb38644934b6add6dca6147120;i=ab550;b=81a1ae5398824daea9b502bf3ddfc092;m=2488403b14;t=5c84eebbadaf5;x=3e575c4f570ffd13",
"__REALTIME_TIMESTAMP" : "1627616171449077",
"__MONOTONIC_TIMESTAMP" : "156904733460",
"_BOOT_ID" : "81a1ae5398824daea9b502bf3ddfc092",
"_TRANSPORT" : "syslog",
"_UID" : "0",
"_GID" : "0",
"_CAP_EFFECTIVE" : "3fffffffff",
"_MACHINE_ID" : "df9d1eef11ee45fd92b407191b390e56",
"_HOSTNAME" : "imx6qsabresd",
"PRIORITY" : "6",
"SYSLOG_FACILITY" : "4",
"SYSLOG_IDENTIFIER" : "sshd",
"_COMM" : "sshd",
"_EXE" : "/usr/sbin/sshd",
"_CMDLINE" : "sshd: root [priv]",
"_SYSTEMD_SLICE" : "system-sshd.slice",
"SYSLOG_PID" : "17696",
"MESSAGE" : "Accepted password for root from 192.168.5.44 port 65341 ssh2",
"_PID" : "17696",
"_SYSTEMD_CGROUP" : "/system.slice/system-sshd.slice/sshd@57-192.168.5.242:22-192.168.5.44:65341.service",
"_SYSTEMD_UNIT" : "sshd@57-192.168.5.242:22-192.168.5.44:65341.service",
"_SOURCE_REALTIME_TIMESTAMP" : "1627616171448199"

}

查看了一下帮助文件，支持的格式还不少。

 -o --output=STRING       Change journal output mode (short, short-iso,
                                    short-precise, short-monotonic, verbose,
                                    export, json, json-pretty, json-sse, cat)
                               

不知道json-sse是什么东西，试试。

journalctl /usr/sbin/sshd --no-pager -o json-sse

data: { "__CURSOR" : "s=1f967cdb38644934b6add6dca6147120;i=ab550;b=81a1ae5398824daea9b502bf3ddfc092;m=2488403b14;t=5c84eebbadaf5;x=3e575c4f570ffd13", "__REALTIME_TIMESTAMP" : "1627616171449077", "__MONOTONIC_TIMESTAMP" : "156904733460", "_BOOT_ID" : "81a1ae5398824daea9b502bf3ddfc092", "_TRANSPORT" : "syslog", "_UID" : "0", "_GID" : "0", "_CAP_EFFECTIVE" : "3fffffffff", "_MACHINE_ID" : "df9d1eef11ee45fd92b407191b390e56", "_HOSTNAME" : "imx6qsabresd", "PRIORITY" : "6", "SYSLOG_FACILITY" : "4", "SYSLOG_IDENTIFIER" : "sshd", "_COMM" : "sshd", "_EXE" : "/usr/sbin/sshd", "_CMDLINE" : "sshd: root [priv]", "_SYSTEMD_SLICE" : "system-sshd.slice", "SYSLOG_PID" : "17696", "MESSAGE" : "Accepted password for root from 192.168.5.44 port 65341 ssh2", "_PID" : "17696", "_SYSTEMD_CGROUP" : "/system.slice/system-sshd.slice/sshd@57-192.168.5.242:22-192.168.5.44:65341.service", "_SYSTEMD_UNIT" : "sshd@57-192.168.5.242:22-192.168.5.44:65341.service", "_SOURCE_REALTIME_TIMESTAMP" : "1627616171448199"}

linux socket 设置缓冲区大小时内核会翻倍
示例代码：

#include <sys/socket.h>
#include <stdio.h>

int main(int argc, char **argv)
{
 int sockfd, sendbuff;
 socklen_t optlen;

 sockfd = socket(AF_INET, SOCK_DGRAM, 0);
 if(sockfd == -1)
     printf("Error");

 int res = 0;

 // Get buffer size
 optlen = sizeof(sendbuff);
 res = getsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, &sendbuff, &optlen);

 if(res == -1)
     printf("Error getsockopt one");
 else
     printf("send buffer size = %d\n", sendbuff);

 // Set buffer size
 sendbuff = 98304;

 printf("sets the send buffer to %d\n", sendbuff);
 res = setsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, &sendbuff, sizeof(sendbuff));

 if(res == -1)
     printf("Error setsockopt");


 // Get buffer size
 optlen = sizeof(sendbuff);
 res = getsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, &sendbuff, &optlen);

 if(res == -1)
     printf("Error getsockopt two");
 else
     printf("send buffer size = %d\n", sendbuff);

 return 0;
}

运行结果：

send buffer size = 129024
sets the send buffer to 98304
new send buffer size = 196608

官方手册说明

编译结果

./protoc --version

libprotoc 3.17.3

git clone https://github.com/protocolbuffers/protobuf.git
cd protobuf
git submodule update --init --recursive
mkdir x64
cd x64
../configure --prefix=$(pwd)/install
make
make install

运行编译好的程序

./install/bin/protoc --version

libprotoc 3.17.3
查看所有库文件

tree install/lib/

install/lib/
├── libprotobuf.a
├── libprotobuf.la
├── libprotobuf-lite.a
├── libprotobuf-lite.la
├── libprotobuf-lite.so -> libprotobuf-lite.so.28.0.3
├── libprotobuf-lite.so.28 -> libprotobuf-lite.so.28.0.3
├── libprotobuf-lite.so.28.0.3
├── libprotobuf.so -> libprotobuf.so.28.0.3
├── libprotobuf.so.28 -> libprotobuf.so.28.0.3
├── libprotobuf.so.28.0.3
├── libprotoc.a
├── libprotoc.la
├── libprotoc.so -> libprotoc.so.28.0.3
├── libprotoc.so.28 -> libprotoc.so.28.0.3
├── libprotoc.so.28.0.3
└── pkgconfig

├── protobuf-lite.pc
└── protobuf.pc

编译arm版本protobuf库

cd ../
mkdir arm
cd arm/
CC=arm-linux-gnueabihf-gcc CXX=arm-linux-gnueabihf-g++ ../configure --prefix=$(pwd)/install --host=arm-linux
make -j8
make install
file ./install/bin/protoc 

./install/bin/protoc: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, for GNU/Linux 2.6.32, BuildID[sha1]=cea786b94a0b64b866983c8b4541df34e9cc7c1d, with debug_info, not stripped

protobuf/arm$ tree ./install/lib/

./install/lib/
├── libprotobuf.a
├── libprotobuf.la
├── libprotobuf-lite.a
├── libprotobuf-lite.la
├── libprotobuf-lite.so -> libprotobuf-lite.so.28.0.3
├── libprotobuf-lite.so.28 -> libprotobuf-lite.so.28.0.3
├── libprotobuf-lite.so.28.0.3
├── libprotobuf.so -> libprotobuf.so.28.0.3
├── libprotobuf.so.28 -> libprotobuf.so.28.0.3
├── libprotobuf.so.28.0.3
├── libprotoc.a
├── libprotoc.la
├── libprotoc.so -> libprotoc.so.28.0.3
├── libprotoc.so.28 -> libprotoc.so.28.0.3
├── libprotoc.so.28.0.3
└── pkgconfig

├── protobuf-lite.pc
└── protobuf.pc

使用git 直接git clone github的库，不会报googletest 相关的警告，自动下载好了。如果自己在github.com protobuf release哪儿下载的放在，编译时候会报一个没有googletest的警告。好烦。。

configure: WARNING: no configuration information is in third_party/googletest

用户名ubuntu
最终命令

sudo usermod -aG www-data ubuntu

添加用户到组命令：sudo usermod -aG 组名 用户名

usermod -h
Usage: usermod [options] LOGIN

Options:
  -b, --badnames                allow bad names
  -c, --comment COMMENT         new value of the GECOS field
  -d, --home HOME_DIR           new home directory for the user account
  -e, --expiredate EXPIRE_DATE  set account expiration date to EXPIRE_DATE
  -f, --inactive INACTIVE       set password inactive after expiration
                                to INACTIVE
  -g, --gid GROUP               force use GROUP as new primary group
  -G, --groups GROUPS           new list of supplementary GROUPS
  -a, --append                  append the user to the supplemental GROUPS
                                mentioned by the -G option without removing
                                the user from other groups
  -h, --help                    display this help message and exit
  -l, --login NEW_LOGIN         new value of the login name
  -L, --lock                    lock the user account
  -m, --move-home               move contents of the home directory to the
                                new location (use only with -d)
  -o, --non-unique              allow using duplicate (non-unique) UID
  -p, --password PASSWORD       use encrypted password for the new password
  -R, --root CHROOT_DIR         directory to chroot into
  -P, --prefix PREFIX_DIR       prefix directory where are located the /etc/* files
  -s, --shell SHELL             new login shell for the user account
  -u, --uid UID                 new UID for the user account
  -U, --unlock                  unlock the user account
  -v, --add-subuids FIRST-LAST  add range of subordinate uids
  -V, --del-subuids FIRST-LAST  remove range of subordinate uids
  -w, --add-subgids FIRST-LAST  add range of subordinate gids
  -W, --del-subgids FIRST-LAST  remove range of subordinate gids
  -Z, --selinux-user SEUSER     new SELinux user mapping for the user account

c/c++中使用mktime时，有时候会返回-1错误，主要的原因是输入了非法值，下面是一些常用的示例

#include <bits/stdc++.h>
#include <ctime>
#include <cstdio>
using namespace std;
//g++ mktime.cpp -std=c++11 -o mktime
int main(int argc, char* argv[])
{
    cout<<"mktime example"<<endl;
    struct tm stm;
    stm.tm_sec = 0;
    stm.tm_min = 0;
    stm.tm_hour = 0;
    stm.tm_mday = 1;
    stm.tm_mon = 0;
    stm.tm_year = 70;
    time_t ret;
    setenv("TZ","Etc/GMT",1);
    tzset();
    ret = mktime(&stm);
    printf("Etc/GMT ret = %ld\n", ret);
    setenv("TZ","Europe/Berlin",1);
    tzset();
    ret = mktime(&stm);
    printf("UTC+01:00 ret = %ld\n", ret);
    setenv("TZ","Atlantic/Azores",1);
    tzset();
    ret = mktime(&stm);
    printf("UTC-01:00 ret = %ld\n", ret);
    setenv("TZ","Asia/Shanghai",1);
    tzset();
    ret = mktime(&stm);
    printf("Asia/Shanghai ret = %ld\n", ret);
}

编译，运行，结果

g++ mktime.cpp -std=c++11 -o mktime
./mktime
mktime example
Etc/GMT ret = 0
UTC+01:00 ret = -3600
UTC-01:00 ret = 3600
Asia/Shanghai ret = -28800

Referenced from:https://jp.cybozu.help/general/zh/admin/list_systemadmin/list_localization/timezone.html

需要注意的是：
time():返回自00:00:00 UTC, January 1, 1970到現在所經過的秒數,注意,是UTC，中文默认时间是CST，也是UTC+08:00