Yocto: Add hello world application

In this section, you are going to learn

How to create a simple application in yocto ?

How to generate full yocto image for RPI-4B ?

How to flash full yocto image for RPI-4B ?

How to run minicom and work remotely with RPI-4B ?

Topics in this section,

How to clone and build base Yocto image ?

Step 1 : Add custom application

Step 1.1: Create a new meta layer inside yocto build directory

Step 1.2: Create layer.conf in conf directory

Step 1.3: Create source file for sample application

Step 1.4: Create recipe file for sample application

Step 1.5: Explanation of recipe file part by part

Step 2 : Add configurations in .conf files

Step 2.1: Add the meta custom layer in bblayers.conf

Step 2.2: Add the configurations in local.conf

Step 3: Do Pre-Build checks

Step 3.1: Check configurations and layers is added in .conf files or not

Step 3.2: Check the location of toolchain

Step 4: Build application package

Step 5: Do post build checks

Step 5.1: Check the location of the application binary file

Step 5.2: Check the “objdump” output of the application binary file

Step 5.3: Check the “readelf” output of the application binary file

Step 5.4: Check the “nm” output of the application binary file

Step 5.5: Check the “file” command output of application binary file

Step 5.6: Check the “size” command output of application binary file

Step 5.7: Check the “strings” command output of application binary file

Step 6: Build full yocto - Incremental

Step 6.1: Do Pre-Build checks

Step 6.2: Build full yocto

Step 6.3: Do Post-Build checks

How to clone and build base yocto image for RPI-4B ?

Step 1: Install all the required packages

$ sudo apt-get install gawk wget git diffstat unzip texinfo gcc-multilib build-essential chrpath cpio python3 python3-pip python3-pexpect xz-utils debianutils iputils-ping

Step 2: Clone the yocto source directory

Create a yocto build directory

$ cd $HOME
$ mkdir yocto_build
$ cd yocto_build
$ mkdir sources
$ cd sources

Clone the poky directory

$ git clone -b langdale git://git.yoctoproject.org/poky.git

To clone openembedded core layer

$ git clone -b langdale git://git.openembedded.org/meta-openembedded

To clone meta-raspberrypi layer

$ git clone -b langdale git://git.yoctoproject.org/meta-raspberrypi

Step 3: Initialize the build environment

$ cd ..
$ . sources/poky/oe-init-build-env

Step 4: Edit the local.conf and bblayers.conf file

local.conf

Add the following configurations in conf/local.conf

MACHINE ??= "raspberrypi4"

bblayers.conf

Add the following yocto layers in conf/bblayers.conf

BBLAYERS ?= " \
  /home/test/yocto/sources/poky/meta \
  /home/test/yocto/sources/poky/meta-poky \
  /home/test/yocto/sources/poky/meta-yocto-bsp \
  /home/test/yocto/sources/meta-openembedded/meta-oe \
  /home/test/yocto/sources/meta-openembedded/meta-python \
  /home/test/yocto/sources/meta-raspberrypi \
  "

Step 5: Set locale settings as “en_US.UTF-8”

export LC_ALL="en_US.UTF-8"
export LC_CTYPE="en_US.UTF-8"
sudo dpkg-reconfigure locales

Select en_US.UTF-8 and apply the changes.

Step 5: Build the image
$ bitbake core-image-base
Step 6: Once build is completed the image will be in present in the following directory.
$ pwd /home/test/yocto/build/tmp/deploy/images/raspberrypi4/
Step 7: Extract the base image file using bzip2 command to generate the .wic file.
$ bzip2 -d -f core-image-base-raspberrypi4.wic.bz2
Step 8: Flash the image file core-image-base-raspberrypi4.wic to SD card using Elena Batcher tool.
Step 9: Run minicom to open the RPI-4B console.
$ sudo minicom -D /dev/ttyUSB0

Step 1 : Add custom application

In this section you will learn how to add custom application

Step 1.1: Create a new meta layer inside the yocto build directory
Make sure the current directory is “yocto”
$ pwd
/home/test/yocto
Create the new meta-layer directory for the custom application
$ mkdir meta-custom

Step 1.2: Create layer.conf in conf directory
Create a new directory called “conf” inside the meta-custom directory
$ mkdir meta-custom/conf
Create the layer.conf file and add the following configurations.
$ vi meta-custom/conf/layer.conf
layer.conf
LAYERVERSION = "1"

BBPATH .= ":${LAYERDIR}"

BBFILES += "${LAYERDIR}/recipes-*/*/*.bb \
            ${LAYERDIR}/recipes-*/*/*.bbappend"


BBFILE_COLLECTIONS += "meta-custom"
BBFILE_PATTER_meta-custom = "^${LAYERDIR}/"
BBFILE_PRIORITY_meta-custom = "6"

Step 1.3: Create source file for the sample application
Create a directory recipe-custom
$ mkdir meta-custom/recipe-custom
Create helloworld.c inside recipe-custom and add the necessary instructions.
$ vi meta-custom/recipe-custom/helloworld.c
helloworld.c
#include <stdio.h>

void app_fun()
{
        printf("Inside app_fun()\n");
}

int main(void)
{
        printf("Hello,World\n");
        app_fun();

        return 0;
}

Step 1.4: Create recipe file for the sample application

Create recipe file helloworld.bb in the same directory.

$ vim meta-custom/recipe-custom/helloworld.bb

helloworld.bb

SUMMARY = "Hello World Application"
LICENSE = "CLOSED"

SRC_URI = "file://helloworld.c"

do_compile() {
        ${CC} ${CFLAGS} ${LDFLAGS} -o helloworld helloworld.c
}

do_install() {
        install -d ${D}${bindir}
        install -m 0755 helloworld ${D}${bindir}
}

Step 1.5: Explanation of recipe file part by part

SUMMARY = "Hello World Application"
LICENSE = "CLOSED"

Summary gives a basic definition about the application.
License is used to specifie the license related information for the recipe.

SRC_URI = "file://helloworld.c"

SRC_URI is used to indicate the uri for the recipe source files.
“file://” denote that the source code is present in the same directory where the recipe file is.

do_compile() {
        ${CC} ${CFLAGS} ${LDFLAGS} -o helloworld helloworld.c
}

do_compile() is used to define the rules to compile the source code.
CC, CFLAGS and LDFLAGS are set by Yocto to appropriate cross-compilation tools and flags.
Here the source code helloworld.c is compiled and the binary file helloworld is generated.

do_install() {
        install -d ${D}${bindir}
        install -m 0755 helloworld ${D}${bindir}
}

do_install() defines the rules on how to install the generated binary from do_compile() to the final target image.
install -d ${D}${bindir} : creates the destination directory in the target image where the binary will be installed.
install -m 0755 helloworld ${D}${bindir} : installs the helloworld binary in the destination directory with the following permissions (read,write and execute for owner and read,execute permission for group and others)

Step 2 : Add configurations in .conf files

In this section you will learn how to add application layer and adding the application binary to final target image.

Step 2.1: Add the meta-custom layer to the bblayers.conf

$ vim build/bblayers.conf

bblayers.conf

BBLAYERS ?= " \
  /home/test/yocto/sources/poky/meta \
  /home/test/yocto/sources/poky/meta-poky \
  /home/test/yocto/sources/poky/meta-yocto-bsp \
  /home/test/yocto/sources/meta-openembedded/meta-oe \
  /home/test/yocto/sources/meta-openembedded/meta-python \
  /home/test/yocto/sources/meta-raspberrypi \
  /home/test/yocto/sources/meta-custom \
  "

Step 2.2: Add the configurations in local.conf
$ vim build/local.conf
local.conf
IMAGE_INSTALL_append = "helloworld"

Step 3: Do Pre-Build checks

In this section you will learn how to do Pre-build checks

Step 3.1: Check configurations and layers added in .conf files or not
In .conf file application image and layer should be added.

Below line should be present in local.conf file
IMAGE_INSTALL_append = "helloworld"
Below line should be present in bblayers.conf file inside BBLAYERS parameter
/home/test/yocto/sources/meta-custom

Step 3.2 : Check the location of toolchain

Make sure the current directory “yocto”

$ cd build/tmp/hosttools/

See Output Below

$ ls -l
lrwxrwxrwx 1 test test 10 Jan  4 12:59 '['
lrwxrwxrwx 1 test test 11 Jan  4 12:59  ar
lrwxrwxrwx 1 test test 11 Jan  4 12:59  as
lrwxrwxrwx 1 test test 12 Jan  4 12:59  awk
lrwxrwxrwx 1 test test 17 Jan  4 12:59  basename
lrwxrwxrwx 1 test test 13 Jan  4 12:59  bash
lrwxrwxrwx 1 test test 14 Jan  4 12:59  bzip2
lrwxrwxrwx 1 test test 12 Jan  4 13:00  bzr
lrwxrwxrwx 1 test test 12 Jan  4 12:59  cat
lrwxrwxrwx 1 test test 14 Jan  4 12:59  chgrp
lrwxrwxrwx 1 test test 14 Jan  4 12:59  chmod
lrwxrwxrwx 1 test test 14 Jan  4 12:59  chown
lrwxrwxrwx 1 test test 16 Jan  4 12:59  chrpath
lrwxrwxrwx 1 test test 12 Jan  4 12:59  cmp
lrwxrwxrwx 1 test test 13 Jan  4 12:59  comm
lrwxrwxrwx 1 test test 11 Jan  4 12:59  cp
lrwxrwxrwx 1 test test 13 Jan  4 12:59  cpio
lrwxrwxrwx 1 test test 12 Jan  4 12:59  cpp
lrwxrwxrwx 1 test test 12 Jan  4 12:59  cut
lrwxrwxrwx 1 test test 13 Jan  4 12:59  date
lrwxrwxrwx 1 test test 11 Jan  4 12:59  dd
lrwxrwxrwx 1 test test 13 Jan  4 12:59  diff
lrwxrwxrwx 1 test test 17 Jan  4 12:59  diffstat
lrwxrwxrwx 1 test test 16 Jan  4 12:59  dirname
lrwxrwxrwx 1 test test 11 Jan  4 12:59  du
lrwxrwxrwx 1 test test 13 Jan  4 12:59  echo
lrwxrwxrwx 1 test test 14 Jan  4 12:59  egrep
lrwxrwxrwx 1 test test 12 Jan  4 12:59  env
lrwxrwxrwx 1 test test 15 Jan  4 12:59  expand
lrwxrwxrwx 1 test test 13 Jan  4 12:59  expr
lrwxrwxrwx 1 test test 14 Jan  4 12:59  false
lrwxrwxrwx 1 test test 14 Jan  4 12:59  fgrep
lrwxrwxrwx 1 test test 13 Jan  4 12:59  file
lrwxrwxrwx 1 test test 13 Jan  4 12:59  find
lrwxrwxrwx 1 test test 14 Jan  4 12:59  flock
lrwxrwxrwx 1 test test 12 Jan  4 12:59  g++
lrwxrwxrwx 1 test test 13 Jan  4 12:59  gawk
lrwxrwxrwx 1 test test 12 Jan  4 12:59  gcc
lrwxrwxrwx 1 test test 15 Jan  4 13:00  gcc-ar
lrwxrwxrwx 1 test test 16 Jan  4 12:59  getconf
lrwxrwxrwx 1 test test 15 Jan  4 12:59  getopt
lrwxrwxrwx 1 test test 12 Jan  4 12:59  git
lrwxrwxrwx 1 test test 12 Jan  4 13:00  gpg
lrwxrwxrwx 1 test test 18 Jan  4 13:00  gpg-agent
lrwxrwxrwx 1 test test 13 Jan  4 12:59  grep
lrwxrwxrwx 1 test test 15 Jan  4 12:59  gunzip
lrwxrwxrwx 1 test test 13 Jan  4 12:59  gzip
lrwxrwxrwx 1 test test 13 Jan  4 12:59  head
lrwxrwxrwx 1 test test 17 Jan  4 12:59  hostname
lrwxrwxrwx 1 test test 14 Jan  4 12:59  iconv
lrwxrwxrwx 1 test test 11 Jan  4 12:59  id
lrwxrwxrwx 1 test test 16 Jan  4 12:59  install
lrwxrwxrwx 1 test test 13 Jan  4 13:00  join
lrwxrwxrwx 1 test test 11 Jan  4 12:59  ld
lrwxrwxrwx 1 test test 15 Jan  4 13:00  ld.bfd
lrwxrwxrwx 1 test test 12 Jan  4 12:59  ldd
lrwxrwxrwx 1 test test 16 Jan  4 13:00  ld.gold
lrwxrwxrwx 1 test test 11 Jan  4 12:59  ln
lrwxrwxrwx 1 test test 11 Jan  4 12:59  ls
lrwxrwxrwx 1 test test 13 Jan  4 13:00  lz4c
lrwxrwxrwx 1 test test 13 Jan  4 12:59  make
lrwxrwxrwx 1 test test 15 Jan  4 12:59  md5sum
lrwxrwxrwx 1 test test 14 Jan  4 12:59  mkdir
lrwxrwxrwx 1 test test 15 Jan  4 12:59  mkfifo
lrwxrwxrwx 1 test test 14 Jan  4 12:59  mknod
lrwxrwxrwx 1 test test 15 Jan  4 12:59  mktemp
lrwxrwxrwx 1 test test 11 Jan  4 12:59  mv
lrwxrwxrwx 1 test test 11 Jan  4 13:00  nc
lrwxrwxrwx 1 test test 11 Jan  4 13:00  nl
lrwxrwxrwx 1 test test 11 Jan  4 12:59  nm
lrwxrwxrwx 1 test test 16 Jan  4 12:59  objcopy
lrwxrwxrwx 1 test test 16 Jan  4 12:59  objdump
lrwxrwxrwx 1 test test 11 Jan  4 12:59  od
lrwxrwxrwx 1 test test 14 Jan  4 12:59  patch
lrwxrwxrwx 1 test test 13 Jan  4 12:59  perl
lrwxrwxrwx 1 test test 11 Jan  4 12:59  pr
lrwxrwxrwx 1 test test 15 Jan  4 12:59  printf
lrwxrwxrwx 1 test test 12 Jan  4 12:59  pwd
lrwxrwxrwx 1 test test 16 Jan  4 12:59  python3
lrwxrwxrwx 1 test test 14 Jan  4 12:59  pzstd
lrwxrwxrwx 1 test test 15 Jan  4 12:59  ranlib
lrwxrwxrwx 1 test test 16 Jan  4 12:59  readelf
lrwxrwxrwx 1 test test 17 Jan  4 12:59  readlink
lrwxrwxrwx 1 test test 17 Jan  4 12:59  realpath
lrwxrwxrwx 1 test test 11 Jan  4 12:59  rm
lrwxrwxrwx 1 test test 14 Jan  4 12:59  rmdir
lrwxrwxrwx 1 test test 15 Jan  4 12:59  rpcgen
lrwxrwxrwx 1 test test 12 Jan  4 13:00  scp
lrwxrwxrwx 1 test test 12 Jan  4 12:59  sed
lrwxrwxrwx 1 test test 12 Jan  4 12:59  seq
lrwxrwxrwx 1 test test 13 Jan  4 13:00  sftp
lrwxrwxrwx 1 test test 11 Jan  4 12:59  sh
lrwxrwxrwx 1 test test 16 Jan  4 12:59  sha1sum
lrwxrwxrwx 1 test test 18 Jan  4 12:59  sha224sum
lrwxrwxrwx 1 test test 18 Jan  4 12:59  sha256sum
lrwxrwxrwx 1 test test 18 Jan  4 12:59  sha384sum
lrwxrwxrwx 1 test test 18 Jan  4 12:59  sha512sum
lrwxrwxrwx 1 test test 13 Jan  4 13:00  size
lrwxrwxrwx 1 test test 14 Jan  4 12:59  sleep
lrwxrwxrwx 1 test test 14 Jan  4 13:00  socat
lrwxrwxrwx 1 test test 13 Jan  4 12:59  sort
lrwxrwxrwx 1 test test 14 Jan  4 12:59  split
lrwxrwxrwx 1 test test 12 Jan  4 13:00  ssh
lrwxrwxrwx 1 test test 13 Jan  4 12:59  stat
lrwxrwxrwx 1 test test 16 Jan  4 12:59  strings
lrwxrwxrwx 1 test test 14 Jan  4 12:59  strip
lrwxrwxrwx 1 test test 13 Jan  4 13:00  sudo
lrwxrwxrwx 1 test test 13 Jan  4 12:59  tail
lrwxrwxrwx 1 test test 12 Jan  4 12:59  tar
lrwxrwxrwx 1 test test 12 Jan  4 12:59  tee
lrwxrwxrwx 1 test test 13 Jan  4 12:59  test
lrwxrwxrwx 1 test test 14 Jan  4 12:59  touch
lrwxrwxrwx 1 test test 11 Jan  4 12:59  tr
lrwxrwxrwx 1 test test 13 Jan  4 12:59  true
lrwxrwxrwx 1 test test 14 Jan  4 12:59  uname
lrwxrwxrwx 1 test test 13 Jan  4 12:59  uniq
lrwxrwxrwx 1 test test 11 Jan  4 12:59  wc
lrwxrwxrwx 1 test test 13 Jan  4 12:59  wget
lrwxrwxrwx 1 test test 14 Jan  4 12:59  which
lrwxrwxrwx 1 test test 14 Jan  4 12:59  xargs
lrwxrwxrwx 1 test test 12 Jan  4 13:00  yes
lrwxrwxrwx 1 test test 13 Jan  4 13:00  zcat
lrwxrwxrwx 1 test test 13 Jan  4 12:59  zstd

Step 4: Build application package

Make sure the current directory is ‘yocto’

$ cd build
$ bitbake helloworld

Step 5: Do post build checks

In this section you will learn about post build checks

Step 5.1 Check the location of the application binary file
Make sure the current directory is ‘yocto’
$ pwd
/home/test/yocto

$ cd build/tmp/work/raspberrypi4-poky-linux-gnueabi/helloworld/1.0-r0/image/usr/bin/
See that binary called helloworld is present inside directory ?
See Answer
$ cd build/tmp/work/raspberrypi4-poky-linux-gnueabi/helloworld/1.0-r0/image/usr/bin/
$ ls -l
-rwxrwxr-x 1 test test 15992 Jan  3 18:23 helloworld

Step 5.2: Check the “objdump” output of the application binary file
Step 5.2.1: Check “objdump” -t
objdump -t : Displays the symbols of application binary file

Check if “main” is present or not in the “objdump”
$HOME/yocto/build/tmp/hosttools/objdump -t helloworld | grep -i main

0000000000001163 g     F .text  0000000000000028              main
Check if “app_fun” is present or not in the “objdump”
$HOME/yocto/build/tmp/hosttools/objdump -t helloworld | grep -i app_fun

0000000000001149 g     F .text  000000000000001a              app_fun
Symbols “main” and “app_fun” are present in application binary file

Hence we can confirm application is compiled successfully

What is “-t” option is used in “objdump” command ?

See Answer

-t: Displays the symbols of application binary file

What “objdump” command does ?

See Answer

objdump: Display information from object files
Step 5.2.2: Check “objdump” -S
objdump -S : Display source code intermixed with disassembly
Check if “main” is present or not in the “objdump”
$HOME/yocto/build/tmp/hosttools/objdump -S helloworld | grep -i main

1078:   48 8d 3d e4 00 00 00    lea    1163 <main>
Check if “app_fun” is present or not in the “objdump”
$ $HOME/yocto/build/tmp/hosttools/objdump -S helloworld | grep -i app_fun

0000000000001149 <app_fun>:
117f:   e8 c5 ff ff ff          call   1149 <app_fun>
Symbols “main” and “app_fun” are present in application binary file

Hence we can confirm application is compiled successfully

What is “-S” option is used in “objdump” command ?

See Answer

-S : Display source code intermixed with disassembly

What is the purpose of “objdump” command ?

See Answer

The main purpose of the objdump tool is to debug and understand the executable file

Step 5.3: Check the “readelf” output of the application binary file
Step 5.3.1: Check “readelf” -s
readelf -s: Display the symbol table

Check if “main” is present or not in the “readelf”
$HOME/yocto/build/tmp/hosttools/readelf -s helloworld | grep -i main

32: 0000000000001163    40 FUNC    GLOBAL DEFAULT   16 main
Check if “app_fun” is present or not in the “readelf”
$ $HOME/yocto/build/tmp/hosttools/readelf -s helloworld | grep -i app_fun

18: 0000000000001149    26 FUNC    GLOBAL DEFAULT   16 app_fun
Symbols “main” and “app_fun” are present in application binary file

Hence we can confirm application is compiled successfully

What is “-s” option is used in “readelf” command ?

See Answer

-s: Display the symbol table

What “readelf” command does ?

See Answer

readelf: Display information about ELF files

What is the purpose of “readelf” command ?

See Answer

The main purpose of the readelf tool is to display the headers of an ELF (Executable and Linkable Format) files

Step 5.4: Check the “nm” output of the application binary file
Step 5.4.1: Check “nm” -S
nm -S: Print both value and size of defined symbols for the “bsd” output style

Check if “main” is present or not in the “nm”
$HOME/yocto/build/tmp/hosttools/nm -S helloworld | grep -i main

0000000000001163 0000000000000028 T main
Check if “app_fun” is present or not in the “nm”
$HOME/yocto/build/tmp/hosttools/nm -S helloworld | grep -i app_fun

0000000000001149 000000000000001a T app_fun
Symbols “main” and “app_fun” are present in application binary file

Hence we can confirm application is compiled successfully

What is “-S” option is used in “nm” command ?

See Answer

-S: Print both value and size of defined symbols for the “bsd” output style

What “nm” command does ?

See Answer

nm is used to dump the symbol table and their attributes from a binary executable file.

What is the purpose of “nm” command ?

See Answer

The main purpose of the nm tool is to display information about symbols in the specified File, which can be an object file, an executable file, or an object-file library.
Step 5.4.2: Check “nm” -s
nm -s: When listing symbols from archive members, include the index: a mapping of which modules contain definitions for which names.

Check if “main” is present or not in the “nm”
$HOME/yocto/build/tmp/hosttools/nm -s helloworld | grep -i main

0000000000001163 T main
Check if “app_fun” is present or not in the “nm”
$HOME/yocto/build/tmp/hosttools/nm -s helloworld | grep -i app_fun

0000000000001149 T app_fun
Symbols “main” and “app_fun” are present in application binary file

Hence we can confirm application is compiled successfully

What is “-s” option is used in “nm” command ?

See Answer

-s: When listing symbols from archive members, include the index: a mapping of which modules contain definitions for which names.

Step 5.5: Check the “file” command output of application binary file
file: Determine file tye
$ file helloworld

helloworld: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-musl-aarch64.so.1, for GNU/Linux3.7.0, not stripped
Why “file” command is used ?

See Answer

file command is used to determine the file type.

Step 5.6: Check the “size” command output of application binary file
size: List section sizes and total size of binary files
$ size helloworld

text       data     bss     dec     hex filename
1465        600       8    2073     819 helloworld
What “size” command does ?

See Answer

size command will display the output that will give you information on the size command in 5 values like data, text, dec, bss, and hex

Step 5.7: Check the “strings” command output of application binary file
strings - print the sequences of printable characters in files

As we can see string output are added in the program are confirmed.
$ strings helloworld | grep -i Hello
Hello,World
What “strings” command does ?

See Answer

strings command is used to return the string characters into files.

Step 6: Build full yocto - Incremental

In this section you will learn how to build full yocto

Step 6.1: Do Pre-Build checks
Make sure the current directory “build”
$ cd $HOME/yocto/build

Step 6.2: Build full yocto
$ bitbake core-image-base
It downloads, compiles and configures all the necessary packages required for the build.

Step 6.3: Do Post-Build checks
Make sure the current directory “build”

To check package is successfully compiled, below is the path
$ cd tmp/deploy/images/raspberrypi4/
$ ls
-rw-r--r-- 1 test test 16136058 Jan  3 16:52 core-image-base-raspberrypi4.wic.bz2
As we can see image is present in the tmp/deploy/images/raspberrypi4 directory

Image file is core-image-base-raspberrypi4.wic.bz