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Code Snippets

Small snippets of code that I refer to in my daily work.

Table of Contents

C++

These assume that you are using C++14 and boost.

Activate std::string literals

... like auto s = "this is a std::string"s;

using namespace std::literals;

Activate std::chrono literals

... like auto microseconds = 1000us;

using namespace std::chrono_literals;

Regular Expressions

std::regex requires at least C++11. CTRE requires at least C++17.

Check if a string matches a regex

std::regex
#include <regex>

if (std::regex_match(string,std::regex("[a-zA-Z]*"))) {
    cout << "Matches\n";
}
CTRE
#include <ctre.hpp>
using namespace ctre::literals;

if ("[a-zA-Z]*"_ctre.match(string)) {
    cout << "Matches\n";
}

Replace a string with regex

std::regex
#include <regex>

const auto result = regex_replace(str,std::regex("<"),"&lt;");

Extract matching substrings with regex

std::regex
#include <regex>

const auto str = "Jul-18-2017";
std::smatch match;
if (std::regex_search(str,match,std::regex("([a-zA-Z]*)-([0-9]*)-([0-9]*)"))) {
	std::cout << "Month: " << match[1] << ", day: " << match[2] << ", year: " << match[3] << "\n";
} else {
	// No match
}
CTRE
#include <ctre.hpp>
using namespace ctre::literals;

const auto str = "Jul-18-2017";
if (const auto match="([a-zA-Z]*)-([0-9]*)-([0-9]*)"_ctre.match(str)) {
	std::cout << "Month: " <<  match.get<1>() << ", day: " <<  match.get<2>() << ", year: " <<  match.get<3>() << "\n";
} else {
	// No match
}

Split a string into lines

#include <boost/tokenizer.hpp>
auto const tokens = boost::tokenizer<boost::char_separator<char>>(mystring,
    boost::char_separator<char>("\n"));
for (std::string const &t : tokens) {
    std::cout << t << std::endl;
}

Trim white space from beginning and end of a string

#include <boost/algorithm/string.hpp>
std::string const untrimmed = ...;
auto const trimmed = boost::trim_copy(untrimmed);

Load a file into memory

#include <fstream>
auto ifs = std::ifstream(file);
auto const data = std::string(
    std::istreambuf_iterator<char>(ifs),
    std::istreambuf_iterator<char>()
);

Copy all from one stream into another

auto in = std::istream(...);
auto out = std::ostream(...);
out << in.rdbuf();

Make a long random string

#include <fstream>
std::string longstr;
{
    auto ifs = std::ifstream("/dev/urandom",std::ios::binary);
    auto isi = std::istream_iterator<char>(ifs);
    std::copy_n(isi,
        10'000'000,
        std::insert_iterator<std::string>(longstr,longstr.begin()));
}

Create all subdirectories required for a file

#include <boost/filesystem.hpp>
boost::filesystem::create_directories(boost::filesystem::path(file).parent_path());

Iterate over the files in a directory

#include <boost/filesystem.hpp>
#include <boost/range/iterator_range.hpp>

std::string dir = ...;
for(auto const &f: boost::make_iterator_range(boost::filesystem::directory_iterator(dir))) {
    auto const file = f.path().string();
    std::cout << "Found file: " << file << std::endl;
}

Get current local time as a struct tm

#include <boost/date_time/posix_time/posix_time.hpp>
auto const tm = boost::posix_time::to_tm(boost::posix_time::second_clock::local_time());

Load a URL with cpp-netlib

auto request = boost::network::http::client::request("http://...");
request << boost::network::header("Connection","close");
auto const result = body(boost::network::http::client().get(request));

Load a URL with licurl

Supports both GET and POST. Not complete code, just for illustration.

#include <curl/curl.h>

size_t WriteFct(void* contents,size_t size,size_t nmemb,void* userp) {
    std::string *const buf = static_cast<std::string*>(userp);
    *buf += std::string(static_cast<char*>(contents),size*nmemb);
    return size * nmemb;
}

void LoadWithCurl()
{
    const auto curl = curl_easy_init();
    if (!curl) {
        throw std::runtime_error("Error initializing the curl context");
    }

    // Close the curl session when done
    BOOST_SCOPE_EXIT_ALL(curl) {
        curl_easy_cleanup(curl);
    };

    // Configure the curl session
    std::string buf;
    char err[CURL_ERROR_SIZE] = "";
    curl_easy_setopt(curl,CURLOPT_URL,url.c_str());
    curl_easy_setopt(curl,CURLOPT_SSL_VERIFYPEER,0L);
    curl_easy_setopt(curl,CURLOPT_WRITEFUNCTION,WriteFct);
    curl_easy_setopt(curl,CURLOPT_WRITEDATA,static_cast<void*>(&buf));
    curl_easy_setopt(curl,CURLOPT_ERRORBUFFER,err);

    // Set the POST data (and activate POST) if requested by the caller
    if (postdata) {
        // Note: CURLOPT_POSTFIELDSIZE is restricted to 2 GB or less.
        curl_easy_setopt(curl,CURLOPT_POSTFIELDSIZE,postdata->size());
        curl_easy_setopt(curl,CURLOPT_POSTFIELDS,postdata->data());
    }

    // Perform the actual HTTP query
    const auto res1 = curl_easy_perform(curl);
    if (res1!=CURLE_OK) {
        throw std::runtime_error("Error accessing " + url + ": " + (err[0] ? err : curl_easy_strerror(res1)));
    }

    // Check the HTTP result status
    long status = -1;
    const auto res2 = curl_easy_getinfo(curl,CURLINFO_RESPONSE_CODE,&status);
    if (res2!=CURLE_OK) {
        throw std::runtime_error("Error getting HTTP result status: "s + curl_easy_strerror(res2));
    }

)

Convert a file descriptor into an I/O stream

#include <iostream>
#include <boost/iostreams/stream.hpp>
#include <boost/iostreams/device/file_descriptor.hpp>

// Output (file opened for writing)
int fd = ...;
boost::iostreams::file_descriptor_sink snk(fd,boost::iostreams::close_handle);
boost::iostreams::stream<boost::iostreams::file_descriptor_sink> os(snk);
os << "Hello World\n";

// Input (file opened for reading)
int fd = ...;
boost::iostreams::file_descriptor_source src(fd,boost::iostreams::close_handle);
boost::iostreams::stream<boost::iostreams::file_descriptor_source> is(src);
is >> myvariable;

Build Instructions

Build Boost from source

Download the Boost source to your home directory, then:

$ chmod a+wrx /usr/local/lib /usr/local/include
$ rm -f /usr/local/lib/*boost*
$ rm -rf /usr/local/include/boost
$ mkdir -p boost
$ cd boost
$ rm -fr build
$ mkdir build
$ rm -fr boost_*
$ tar xzf ~/boost_*.tar.gz
$ cd boost_*
$ ./bootstrap.sh --without-libraries=python
$ ./b2 --build-dir=~/boost/build $MINUSJ variant=release link=static threading=multi runtime-link=static install

Build gcc 9 from source

It's surprisingly easy.

$ cd
$ mkdir gcc
$ cd gcc
$ wget ftp://ftp.fu-berlin.de/unix/languages/gcc/releases/gcc-9.1.0/gcc-9.1.0.tar.gz
$ tar xzf gcc-9.1.0.tar.gz
$ cd gcc-9.1.0
$ contrib/download_prerequisites

$ cd ~/gcc
$ mkdir build
$ cd build
$ ../gcc-9.1.0/configure --prefix=$HOME/gcc/install --disable-multilib
$ make
$ make install

$ ~/gcc/install/bin/g++ --version
g++ (GCC) 9.1.0

$ sudo update-alternatives --install /usr/bin/gcc gcc $HOME/gcc/install/bin/gcc 60 --slave /usr/bin/g++ g++ $HOME/gcc/install/bin/g++
$ sudo update-alternatives --config gcc
$ gcc --version
gcc (GCC) 9.1.0
$ g++ --version
g++ (GCC) 9.1.0

To run executables built with gcc 9 (unless linked statically):

$ export LD_LIBRARY_PATH=$HOME/gcc/install/lib64:$LD_LIBRARY_PATH

Build cmake from source

$ cd /opt
$ wget https://github.com/Kitware/CMake/releases/download/v3.14.5/cmake-3.14.5.tar.gz
$ tar xzf cmake-3.14.5.tar.gz
$ cd cmake-3.14.5
$ mkdir build
$ cd $_
$ cmake ..
$ make -j8
$ sudo mkdir -p -m 777 /usr/local/doc /usr/local/share
$ make install
$ cmake --version
cmake version 3.14.5

If there's no cmake already installed, do the following instead of invoking cmake (in the source directory, don't create a build directory):

$ ./bootstrap -- -DCMAKE_BUILD_TYPE:STRING=Release

More info: https://cmake.org/install/

git

Create the “git godlog” command

git config --global alias.godlog "log --graph --oneline --decorate"

Manage Office files in git

This allows you to use "git diff" with Microsoft Office (Word, PowerPoint) and Libre Office (Writer, Calc, Impress) files stored in a git repository. The trick is to tell git how to extract plain text from these files, on which the regular diff can be used.

Install the conversion tools:

$ sudo apt install unoconv catdoc

Optionally, only needed for .pptx files:

  • $ pip install python-pptx
  • Copy git-pptx-textconv.py from this repo to a location of your choice and make it executable

Add the following to ~/.gitconfig:

[diff "doc"]
    textconv=catdoc
[diff "odt"]
    textconv=odt2txt
[diff "odp"]
    textconv=odp2txt
[diff "ods"]
    textconv=ods2txt
[diff "ppt"]
    textconv=catppt
[diff "pptx"]
    textconv=/location/of/git-pptx-textconv.py

Add the following to ~/.config/git/attributes (or, alternatively, to the .gitattributes file in the project root):

*.doc diff=doc
*.odp diff=odp
*.ods diff=ods
*.odt diff=odt
*.ppt diff=ppt
*.pptx diff=pptx

If git diff displays the following error:

Error: Unable to connect or start own listener. Aborting.
fatal: unable to read files to diff

then type unoconv -l and retry.

More information:

Put git version number into PDF

... that is created from a Libre Office document which is managed in git as described in the previous section, using the tag-based version number generated by git describe. Tested with Libre Office 5 in Linux.

Preparation (once per document):

  1. Open the document in LibreOffice Writer
  2. Move cursor to where the version number is to be displayed
  3. Insert → Field → More fields ...
  4. Variables → Set variable, Name: version, Value: 0.0, Format: Text, Insert, Close
  5. To show the version number elsewhere: Insert → Field → More fields ... → Variables → Show variable, version, Insert, Close
  6. Close and save the document
  7. Add/commit the document to git

To convert the document into a PDF, replacing the "0.0" placeholder by the current git version number:

$ odt2pdf -o myname.pdf -F version=$(git describe --dirty --always) filename.odt

About tag-based git version numbers: https://git-scm.com/docs/git-describe

cmake

Activate C++14

... and be as portable as possible by switching off stuff like GNU language extensions.

set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_EXTENSIONS OFF)

Link with the pthreads library

cmake_minimum_required (VERSION 3.5)
project (myproject CXX)
find_package (Threads REQUIRED)
add_executable (myproject ...)
target_link_libraries (myproject Threads::Threads)

More: https://stackoverflow.com/questions/1620918/cmake-and-libpthread

Same output directory for all sub-projects

set (EXECUTABLE_OUTPUT_PATH "${CMAKE_SOURCE_DIR}/build")

Use compiler cache (ccache)

find_program (CCACHE_FOUND ccache)
if (CCACHE_FOUND)
    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
endif (CCACHE_FOUND)

Unix/Shell

Where "Shell" means bash, or something sufficiently compatible.

Shell Prompt And Git Status

Show the last command's exit status in the shell prompt, and show the prompt with a dark background to make it stand out better. When in a git repository, show the current git status: a cross if something needs to be committed, an up arrow if something needs to be pushed, a down arrow when something can be pulled, three stacked horizontal lines if there's a shash, a check mark if everything is clean.

Put the following into your .profile or .bashrc.

gitstatus() {
    if [ "$1" = "--color" ];then
        local RED=$(tput setaf 1)
        local BLU=$(tput setaf 6)
        local GRN=$(tput setaf 2)
        local OFF=$(tput sgr0)
    else
        local RED=""
        local BLU=""
        local GRN=""
        local OFF=""
    fi

    local S
    S=$(LANG=C git status -sb --porcelain 2>/dev/null) || return
    if [[ $S =~ [[:cntrl:]][^#] ]];then
        echo "${RED}×${OFF} "
    elif [[ $S =~ ^##.*\[behind ]];then
        echo "${BLU}↓${OFF} "
    elif [[ $S =~ ^##.*\[ahead ]];then
        echo "${BLU}↑${OFF} "
    elif git stash list | grep -q .;then
        echo "${BLU}≡${BLU} "
    else
        echo "${GRN}✓${OFF} "
    fi
}

gits() {
    local G
    find ${*:-.} -type d -name .git 2>/dev/null \
    | while read G;do
        local D=$(dirname "$G")
        (cd "$D" >/dev/null && git remote update &>/dev/null && echo "$(gitstatus --color)$D")
    done
}

PS1BEFORE=$(tput sgr0)$(tput rev)$(tput setaf 4)
PS1AFTER=$(tput sgr0)
PS1='\[$PS1BEFORE\]$? [\h:\w]\[$PS1AFTER\] $(gitstatus)\$ '

Screenshot showing bash prompt

This also defines the gits command which recursively searches the current directory (or directories you specify on the command line) for git repositories and displays their status (cross, up/down arrow, check mark as described above).

$ gits
↑ ./afl-demo
✓ ./cloud-backup
✓ ./cmake-hello-world
✓ ./ecs
↑ ./helloworld
✓ ./itunes-in-nextcloud
× ./snippets
↓ ./ThreadPool
✓ ./version

Host Name And Work Directory In Terminal Window Title

PS1="\[\e]0;\h:\w\a\]$PS1"

Unlimited History

export HISTFILE="$HOME/.bash-history"
export HISTFILESIZE=
export HISTSIZE=

Changing the history file name from the default .bash_history to something else (here, by using a dash instead of an underscore, but could be anything) prevents non-login shells (that use the default history size) from deleting our history.

Convert To/From time_t

Requires gawk (the Gnu implementation of awk, e. g. as Linux has it).

$ gawk 'BEGIN {print systime()}'
1497429720

$ gawk 'BEGIN {print mktime("2017 06 14 10 42 00")}'
1497429720

$ gawk 'BEGIN {print strftime("%F %T %Z",1497429720)}'
2017-06-14 10:42:00 CEST

$ gawk 'BEGIN {print strftime("%F %T %Z",1497429720,1)}'
2017-06-14 08:42:00 GMT

Asymmetric Encryption

Make key pair:

$ openssl req -newkey rsa:2048 -new -nodes -x509 -days 36500 -out cert.pem -keyout key.pem

cert.pem is the certificate file (public key), used for encryption. key.pem is the private key file, used for decryption.

Encrypt:

$ openssl smime -encrypt -aes256 -binary -outform DER cert.pem <cleartext.dat  >ciphertext.dat

Decrypt:

$ openssl smime -decrypt -inform DER -inkey key.pem <ciphertext.dat >cleartext.dat

Show contents of a pem file:

$ openssl x509 -in filename.pem -text

Extract Pages From PDF File

Replace 11 and 22 with the range of pages you want to extract.

With pdftk:

$ pdftk your-input-file.pdf cat 11-22 output your-output-file.pdf

pdftk can extract arbitrary collections of pages (e. g., use cat 1 3-5 8 to extract pages 1, 3, 4, 5, and 8).

With Ghostscript:

$ gs -sDEVICE=pdfwrite -dNOPAUSE -dBATCH -dSAFER \
	-dFirstPage=11 \
	-dLastPage=22 \
	-sOutputFile=your-output-file.pdf \
	your-input-file.pdf

vi/vim

Convert line endings from DOS to Unix

:set ff=unix

Linux

Disable pager in systemctl

which is annoying as hell and completely different from the way Unix programs classically work.

In shell profile:

export SYSTEMD_PAGER=cat

Then:

$ sudo visudo

Add the following line in the "Defaults" section:

Defaults        env_keep += "SYSTEMD_PAGER"

More info: https://unix.stackexchange.com/questions/343168/can-i-prevent-service-foo-status-from-paging-its-output-through-less

Natural Scrolling

Make your mouse wheel work like on a Mac: Roll up to scroll up, roll down to scroll down.

Put the following into /usr/share/X11/xorg.conf.d/20-natural-scrolling.conf, then reboot:

Section "InputClass"
        Identifier "Natural Scrolling"
        MatchIsPointer "on"
        MatchDevicePath "/dev/input/event*"
        Option "VertScrollDelta" "-1"
        Option "HorizScrollDelta" "-1"
        Option "DialDelta" "-1"
EndSection

If this doesn't work, try:

$ xinput set-prop 9 275 1

9 and 275 are just examples, find the actual values as follows.

First, the 9 is the id of your mouse device, to be found like this:

$ xinput list
⎡ Virtual core pointer                    	id=2	[master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer              	id=4	[slave  pointer  (2)]
⎜   ↳ Logitech USB-PS/2 Optical Mouse         	id=9	[slave  pointer  (2)]
⎣ Virtual core keyboard                   	id=3	[master keyboard (2)]
    ↳ Virtual core XTEST keyboard             	id=5	[slave  keyboard (3)]
    ↳ Power Button                            	id=6	[slave  keyboard (3)]
    ↳ Video Bus                               	id=7	[slave  keyboard (3)]
    ↳ Power Button                            	id=8	[slave  keyboard (3)]
    ↳ Eee PC WMI hotkeys                      	id=10	[slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard            	id=11	[slave  keyboard (3)]

Once you got the 9, find the 275 like this:

$ xinput list-props 9 | grep -i natural
	libinput Natural Scrolling Enabled (275):	0
	libinput Natural Scrolling Enabled Default (276):	0

Sources:

Show System Messages During Boot/Shutdown

Make your machine start up and shut down like a real computer.

In /etc/default/grub, remove quiet splash from GRUB_CMDLINE_LINUX_DEFAULT, so it's

GRUB_CMDLINE_LINUX_DEFAULT=""

Uncomment GRUB_GFXMODE and set it to one one more resolutions supported by your monitor, followed by auto, for example

GRUB_GFXMODE="1920x1080,640x480,auto"

Then:

$ sudo update-grub

Reboot and enjoy.

Sources:

Mount Nextcloud

Access the files in your Nextcloud without syncing them to your harddisk, using Nextcloud's WebDAV interface. Doesn't require disk space to store your Nextcloud files locally. Doesn't use the Nextcloud client software.

Tested with Ubuntu 16.04. Will probably work with Owncloud or any other WebDAV-based file service.

The following examples assume that

  • mycloud.example.com is your Nextcloud server
  • myname is your Nextcloud user name
  • mypasswordis your Nextcloud password

Preparation:

$ sudo apt install ca-certificates
$ sudo apt install davfs2
$ sudo mkdir /mnt/myname
$ sudo usermod -aG davfs2 $USER

Add the following line to /etc/fstab:

https://mycloud.example.com/remote.php/webdav /mnt/myname davfs user,noauto 0 0

If you want read-only access (you can read your cloud files but not change them):

https://mycloud.example.com/remote.php/webdav /mnt/myname davfs user,noauto,ro,dir_mode=555,file_mode=444 0 0

Add the following to /etc/davfs2/secrets:

/mnt/myname myname mypassword

Note: Every user on your Linux machine can mount your Nextcloud files, which may or may not be desired.

Finally, to mount your Nextcloud files:

$ sudo mount /mnt/myname

More information: https://wiki.ubuntuusers.de/WebDAV/

Using it to back up your Nextcloud: https://gitlab.com/wolframroesler/back-up-nextcloud

Screen Recording

$ sudo add-apt-repository ppa:maarten-baert/simplescreenrecorder
$ sudo apt-get update
$ sudo apt-get install simplescreenrecorder

More: http://www.maartenbaert.be/simplescreenrecorder/

macOS

Eject CD/DVD

$ drutil tray eject

Don't Create .DS_Store Files

$ defaults write com.apple.desktopservices DSDontWriteNetworkStores true

Log off or reboot to activate.

Install Package Manager

To install software like a boss.

$ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
$ brew analytics off

More information: http://brew.sh

You may like:

$ brew install htop
$ brew install tree

Install boost

To use the boost C++ libraries. Requires the brew package manager desribed above.

$ brew install boost
$ ln -s /usr/local/Cellar/boost/1.67.0_1/ /usr/local/boost

And update the symlink whenever a new boost version comes out. Why don't they install in /usr/local/boost directly like everyone else, or at least create the symlink automatically?

If you need to build different projects with different boost versions you can leave the symlink away and use an environment variable instead:

$ export BOOST_ROOT=/usr/local/Cellar/boost/1.67.0_1/

MySQL/MariaDB

SQL Profiling

set profiling=1;
select ...;
show profile;

Show Explain Plan

explain select ...;

Hang the database

Useful in test cases, e. g. when testing where clauses constructed from user input. Causes a lot of load in the database that makes the application hang, hogs one more more CPUs, prevents new clients from connecting, and might require a DB bounce.

SELECT something
FROM somewhere
WHERE BENCHMARK(100000000000,ENCODE('a','b'))='c';

Wolfram Rösler • wolfram@roesler-ac.de • https://gitlab.com/wolframroesler • https://twitter.com/wolframroesler • https://www.linkedin.com/in/wolframroesler/