Open-Wrt: Add shared library

What is Shared Library ?

How to create a shared library in Open-WRT ?

How to link shared library with object files for RPI-4B ?

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

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

Topics in this section,

PART A: CREATING SHARED LIBRARY

• Step 1: Add custom built library

  • Step 1.1: Add directory structure of “Shared Library” application package

  • Step 1.2: Create directory under package/libs directory

  • Step 1.3: Create directories under my_shared_lib directory

  • Step 1.4: Create my_shared_lib.c under src directory

  • Step 1.5: Create my_shared_lib.h under inc directory

  • Step 1.6: Create Makefile in “my_shared_lib” directory

  • Step 1.7: Explanation of Makefile part by part

• Step 2 : Add application package to menuconfig

  • Step 2.1: Create feeds.conf file

  • Step 2.2: Content of feeds.conf

  • Step 2.3: Below example for feeds path

• Step 3 : Update and install feeds

  • Step 3.1: Commands for update and install feeds

• Step 4 : Enable application package in menuconfig

  • Step 4.1: Select your application in menuconfig

  • Step 4.2: Save the configuration

  • Step 4.3: Exit the menuconfig

• Step 5: Do Pre-Build checks

  • Step 5.1: Check .config file is present or not

  • Step 5.2 : Check the location of toolchain

• Step 6: Build application package

• Step 7: Do post build checks

  • Step 7.1 Check the location of the shared library file

  • Step 7.2: Check the “objdump” output of the shared library file

  • Step 7.3: Check the “readelf” output of the shared library file

  • Step 7.4: Check the “nm” output of the shared library file

  • Step 7.5: Check the “file” command output of shared library (.so) file

  • Step 7.6: Check the “size” command output of shared library file

  • Step 7.7: Check the “strings” command output of shared library file

  • Step 7.8: Check the location of .ipk file

  • Step 7.9: How to extract .ipk file

  • Step 7.10: How to check content of .ipk file

• Step 8: Build full openwrt - Incremental

  • Step 8.1: Do Pre-Build checks

  • Step 8.2: Build full openwrt

  • Step 8.3: Do Post-Build checks

Topics in this section,

PART B: CREATING SHARED APPLICATION

• Step 1 : Add custom built library

  • Step 1.1: Add directory structure of “Shared Application” package

  • Step 1.2: Create directory under package/libs directory

  • Step 1.3: Create directories under x_shared_app directory

  • Step 1.4: Create “app.c” under src directory

  • Step 1.5: Create “my_shared_lib.h” under inc directory

  • Step 1.6: Create Makefile in “x_shared_app” directory

  • Step 1.7: Explanation of Makefile part by part

• Step 2 : Add application package to menuconfig

  • Step 2.1: Create feeds.conf file

  • Step 2.2: Content of feeds.conf

  • Step 2.3: Below example for feeds path

• Step 3 : Update and install feeds

  • Step 3.1: Commands for update and install feeds

• Step 4 : Enable application package in menuconfig

  • Step 4.1: Select your application in menuconfig

  • Step 4.2: Save the configuration

  • Step 4.3: Exit the menuconfig

• Step 5: Do Pre-Build checks

  • Step 5.1: Check .config file is present or not

  • Step 5.2 : Check the location of toolchain

• Step 6: Build application package

• Step 7: Do post build checks

  • Step 7.1 Check the location of the shared application binary file

  • Step 7.2: Check the “objdump” output of the shared application file

  • Step 7.3: Check the “readelf” output of the shared application file

  • Step 7.4: Check the “nm” output of the shared application file

  • Step 7.5: Check the “file” command output of shared application file

  • Step 7.6: Check the “size” command output of shared application file

  • Step 7.7: Check the “strings” command output of shared application file

  • Step 7.8: Check the location of .ipk file

  • Step 7.9: How to extract .ipk file

  • Step 7.10: How to check content of .ipk file

• Step 8: Build full openwrt - Incremental

  • Step 8.1: Do Pre-Build checks

  • Step 8.2: Build full openwrt

  • Step 8.3: Do Post-Build checks

Note

PART A: Creating Shared Library

Note

Create 2 folders in alphabet order so that library side makefile should execute first and it will create .so shared library file

Step 1 : Add custom built library

In this section you will learn how to add custom built library in “Openwrt”

Step 1.1: Add directory structure of “Shared Library” application package

• Make sure the current directory “openwrt”

  $ pwd
  
    /home/test/openwrt
  

Step 1.2: Create directory under package/libs directory

• Make sure the current directory “openwrt”

  $ cd package/libs/
  
  $ mkdir -p shared_lib/my_shared_lib
  

Step 1.3: Create directories under “my_shared_lib” directory

• Make sure the current directory “my_shared_lib”

  $ cd my_shared_lib
  

Step 1.4: Create my_shared_lib.c under src directory

• Make sure the current directory “my_shared_lib/src”

• In my_shared_lib.c have “Function Definition”

• Create my_shared_lib.c file using vim command

vim my_shared_lib.c

• Content of src/my_shared_lib.c

#include "my_shared_lib.h"

void my_shared_function(void)
{
       printf("Hello from my_shared_function!\n");
}

Step 1.5: Create my_shared_lib.h under inc directory

• Make sure the current directory “my_shared_lib/inc”

• In my_shared_lib.h have “Function Prototype”

• Create my_shared_lib.h file using vim command

vim my_shared_lib.h

• Content of inc/my_shared_lib.h

#include <stdio.h>

void my_shared_function(void);

What is Shared Library ?

See Answer

• Collection of compiled object code that is linked to the source code only at the runtime.

What are the types of library ?

See Answer

• Two types of library:

  • Static Library

  • Shared Library

What is the use of library ?

See Answer

• It is a collection of pre-compiled functions that can be used by a program.

• Used to reduce the redundancy of code

Step 1.6: Create Makefile in “my_shared_lib” directory

• Make sure the current directory “my_shared_lib”

• Create Makefile file using vim command

  vim Makefile
  

• Content of Makefile

  include $(TOPDIR)/rules.mk
  
  PKG_NAME:=my_shared_lib
  PKG_VERSION:=1.0
  PKG_RELEASE:=1
  
  SOURCE_DIR:=/home/test/openwrt_src/openwrt/package/libs/shared_lib/my_shared_lib
  
  include $(INCLUDE_DIR)/package.mk
  
  define Package/my_shared_lib
          SECTION:=examples
          CATEGORY:=Examples
          TITLE:=My Shared Library
  endef
  
  define Package/my_shared_lib/description
          A simple shared library example.
  endef
  
  define Build/Prepare
          mkdir -p $(PKG_BUILD_DIR)
          cp -r $(SOURCE_DIR)/src $(PKG_BUILD_DIR)
          cp -r $(SOURCE_DIR)/inc $(PKG_BUILD_DIR)
          cp -r $(SOURCE_DIR)/obj $(PKG_BUILD_DIR)
          cp $(SOURCE_DIR)/Makefile $(PKG_BUILD_DIR)
          $(Build/Patch)
  endef
  
  define Build/Compile
          $(TARGET_CC) $(TARGET_CFLAGS) -I$(PKG_BUILD_DIR)/inc -fPIC -o $(PKG_BUILD_DIR)/obj/my_shared_lib.o -c $(PKG_BUILD_DIR)/src/my_shared_lib.c
          $(TARGET_CC) $(TARGET_CFLAGS) -shared -fPIC -g -o $(PKG_BUILD_DIR)/obj/libmy_shared_lib.so $(PKG_BUILD_DIR)/obj/my_shared_lib.o
  
  endef
  
  
  define Package/my_shared_lib/install
          $(INSTALL_DIR) $(1)/usr/lib
          $(INSTALL_BIN) $(PKG_BUILD_DIR)/obj/libmy_shared_lib.so $(1)/usr/lib/
  endef
  
  $(eval $(call BuildPackage,my_shared_lib))
  

Step 1.7: Explanation of Makefile part by part

include $(TOPDIR)/rules.mk

• Explanation of include $(TOPDIR)/rules.mk

  include $(TOPDIR)/rules.mk
  

• The $(TOPDIR) is expected to be the top-level directory

• /rules.mk - The path to the file included

What is $(TOPDIR) ?

See Answer

• $(TOPDIR) path is /home/test/openwrt

Package Name, Version and Release number Block

• Explanation of Package Name, Version and Release number Block

PKG_NAME:=my_shared_lib
PKG_VERSION:=1.0
PKG_RELEASE:=1

• The name and version of your package are used to define the variable to point to the build directory of your package (Ex: my_shared_lib-1.0)

Path (absolute or relative) to a directory Block

• Explanation of SOURCE_DIR

SOURCE_DIR:=$(TOPDIR)/package/libs/shared_lib/my_shared_lib

What is SOURCE_DIR ?

See Answer

• SOURCE_DIR is relative or absolute path of directory

• In this case path of SOURCE_DIR $HOME/openwrt/package/libs/shared_lib/my_shared_lib

• Give path of the source directory

• SOURCE_DIR is used in Build/Prepare

include $(INCLUDE_DIR)/package.mk

• Explanation of include $(INCLUDE_DIR)/package.mk

include $(INCLUDE_DIR)/package.mk

• $(INCLUDE_DIR) - Directory containing include files

• /package.mk: The path to the file included

What is $(INCLUDE_DIR) ?

See Answer

• $(INCLUDE_DIR) - It will search for included files

• In this case path of $(INCLUDE_DIR) $HOME/openwrt/include

Package Information Block

• Explanation of Package Information

define Package/my_shared_lib
   SECTION:=examples
   CATEGORY:=Examples
   TITLE:=My Shared Library
endef

• This block defines the package information as section, category, and title

• SECTION: examples: This line tells the section to which the package belongs. In this case, the package will come under the “examples” section.

• CATEGORY: Examples: This line tells the category to which the package belongs. In this case, the category is set to “Examples

• TITLE: My Shared Library This line tells, title or name of the package. In this case, the title is set to “My Shared Library”.

• endef: This line will be the end of the define block.

What all information included in Package Information Block ?

See Answer

• It includes section, category, and title

Package Description Block

• Explanation of Package Description

define Package/my_shared_lib/description
        A simple shared library example
endef

• This function defines of package.

What is Package Description Block ?

See Answer

• Instructs on how and where our package will appear in the overall configuration menu

Build/Prepare Block

• Explanation of Build/Prepare

define Build/Prepare
        mkdir -p $(PKG_BUILD_DIR)
        cp -r $(SOURCE_DIR)/src $(PKG_BUILD_DIR)
        cp -r $(SOURCE_DIR)/inc $(PKG_BUILD_DIR)
        cp -r $(SOURCE_DIR)/obj $(PKG_BUILD_DIR)
        cp $(SOURCE_DIR)/Makefile $(PKG_BUILD_DIR)
        $(Build/Patch)
endef

• mkdir -p $(PKG_BUILD_DIR): This line creates the build directory

• cp -r $(SOURCE_DIR)/src $(PKG_BUILD_DIR): This line copies the contents from SOURCE_DIR (source) to PKG_BUILD_DIR (destination)

• cp -r $(SOURCE_DIR)/inc $(PKG_BUILD_DIR): This line copies the contents from SOURCE_DIR (source) to PKG_BUILD_DIR (destination)

• cp -r $(SOURCE_DIR)/obj $(PKG_BUILD_DIR): This line copies the contents from SOURCE_DIR (source) to PKG_BUILD_DIR (destination)

• cp -r $(SOURCE_DIR)Makefile $(PKG_BUILD_DIR): This line copies the contents from SOURCE_DIR (source) to PKG_BUILD_DIR (destination)

• -r : -r is recursive copy.

• $(Build/Patch): This line includes a call to $(Build/Patch), which is a variable or macro typically defined elsewhere in the Makefile. It is used to apply any necessary patches to the source code.

What is -r ?

See Answer

• -r is recursive copy.

Which path $(PKG_BUILD_DIR) ?

See Answer

• In this case path of $(PKG_BUILD_DIR) $HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0

What is SOURCE_DIR ?

See Answer

• In this case path of SOURCE_DIR $HOME/openwrt/package/mypackages/examples/my_shared_lib

Build/Compile Block

• Explanation of Build/Compile

define Build/Compile
        $(TARGET_CC) $(TARGET_CFLAGS) -I$(PKG_BUILD_DIR)/inc -fPIC -o $(PKG_BUILD_DIR)/obj/my_shared_lib.o -c $(PKG_BUILD_DIR)/src/my_shared_lib.c
        $(TARGET_CC) $(TARGET_CFLAGS) -shared -fPIC -g -o $(PKG_BUILD_DIR)/obj/libmy_shared_lib.so $(PKG_BUILD_DIR)/obj/my_shared_lib.o

endef

• This block defines the compilation process.

• $(TARGET_CC): It is said to arm cross compiler and it is defined globally

• $(TARGET_CFLAGS): It is used for additional flags

• -I$(PKG_BUILD_DIR)/inc : It will include prototype path

• -fPIC: fPIC is to generate position-independent code

• -o $(PKG_BUILD_DIR)/obj/my_shared_lib.o : Takes .c input file location -c $(PKG_BUILD_DIR)/src/my_shared_lib.c and generates .o output file in location $(PKG_BUILD_DIR)/obj

• -shared: -shared instructs the compiler that we are building a shared library

• -g: -g is to leave debugging information in the output file

• $(PKG_BUILD_DIR)/obj/libmy_shared_lib.so : Takes .o input file location $(PKG_BUILD_DIR)/obj/my_shared_lib.o and generates .so output file in location $(PKG_BUILD_DIR)/obj

What is $(TARGET_CC) ?

See Answer

• It is said to arm cross compiler

• In this case path of $(TARGET_CC) $HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl

What is $(TARGET_CFLAGS) ?

See Answer

• It is used for additional flags

Package Install Block

• Explanation of Package Install

define Package/my_shared_lib/install
        $(INSTALL_DIR) $(1)/usr/lib
        $(INSTALL_BIN) $(PKG_BUILD_DIR)/obj/libmy_shared_lib.so $(1)/usr/lib/
endef

• $(INSTALL_DIR): It used for creating a directory and it is commonly used in build systems like OpenWrt.

• $(INSTALL_BIN): It used for copying an executable file.

• In this case path of $(PKG_BUILD_DIR) $HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0

• /usr/bin: It is the destination directory, .so file will be installed.

What is $(INSTALL_DIR) ?

See Answer

• It used for creating a directory

• In this case $(INSTALL_DIR) install -d -m0755

• Install command is used to copy files and set attributes.

• -d, –directory : It will act as directory names towards all arguments.

• m, –mode=MODE : Set permission mode (as in chmod).

What is $(INSTALL_BIN) ?

See Answer

• It used for copying an executable file.

• In this case $(INSTALL_BIN) install -m0755

• m, –mode=MODE : Set permission mode (as in chmod).

What is $(1)/usr/bin ?

See Answer

• /usr/bin: It is the destination directory where the binary executable will be installed.

• In this case path of $(1)/usr/bin $HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/.pkgdir/my_shared_lib/usr/lib

Package Build Block

• Explanation of Package Build

$(eval $(call BuildPackage,my_shared_lib))

• BuildPackage: It is function call with my_shared_lib as parameter.

Step 2 : Add application package to menuconfig

In this section you will learn how to add menuconfig

Step 2.1: Create feeds.conf file

• Make sure the current directory is “openwrt”

• Create new feeds.conf file using vim command in openwrt directory

$ vim feeds.conf

Step 2.2: Content of feeds.conf

• Inside feeds.conf

src-link <name> <path>

Step 2.3: Below example for feeds path

• Make sure the current directory is “openwrt”

• Create new feeds.conf file using vim command in openwrt directory

src-link libs /home/test/openwrt/package/libs

Step 3 : Update and install feeds

In this section you will learn how to install and update feeds

Step 3.1: Commands for update and install feeds

• Installs the most recent packages, replacing any earlier versions that were already on your system

$ ./scripts/feeds update -a

$ ./scripts/feeds install -a -f

Step 4 : Enable application package in menuconfig

In this section you will learn how to enable in menuconfig

Step 4.1: Select your application in menuconfig

• Select your category that reflects in menuconfig

• Go inside examples and select my_shared_lib

Step 4.2: Save the configuration

Step 4.3: Exit the menuconfig

Step 5: Do Pre-Build checks

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

Step 5.1: Check .config file is present or not

• In .config file package should be enabled in ‘y’

• Below line should be present in .config file

  CONFIG_PACKAGE_my_shared_lib=y
  

Step 5.2 : Check the location of toolchain

• Make sure the current directory “openwrt”

  $HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl
  

  $ ls -l
  
   drwxr-xr-x 2 test test 4096 Dec 23 02:58 bin
   drwxr-xr-x 3 test test 4096 Dec 23 03:39 include
   lrwxrwxrwx 1 test test    6 Dec 23 03:39 lib -> ../lib
   lrwxrwxrwx 1 test test    8 Dec 23 03:20 lib64 -> ../lib64
   lrwxrwxrwx 1 test test   10 Dec 23 03:39 sys-include -> ../include
  

Step 6: Build application package

• Make sure the current directory is “openwrt”

  $HOME/openwrt
  
  $ make package/my_shared_lib/compile
  

Step 7: Do post build checks

In this section you will learn about post build checks

Step 7.1 Check the location of the shared library file

• Make sure the current directory is ‘my_shared_lib-1.0’

  $HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0
  
  $ ls -l
  
  drwxr-xr-x 2 test test 4096 Jan  8 21:36 inc
  drwxr-xr-x 3 test test 4096 Jan  8 21:36 ipkg-aarch64_cortex-a72
  -rw-r--r-- 1 test test 1794 Jan  8 21:36 Makefile
  drwxr-xr-x 2 test test 4096 Jan  8 21:36 obj
  drwxr-xr-x 2 test test 4096 Jan  8 21:36 src
  

$ cd src

$ ls -l

-rw-r--r-- 1 test test 128 Jan  8 21:36 my_shared_lib.c

$ cd inc

$ ls -l

-rw-r--r-- 1 test test 47 Jan  8 21:36 my_shared_lib.h

$ cd obj

$ ls -l

-rwxr-xr-x 1 test test 70648 Jan  8 21:36 libmy_shared_lib.so
-rw-r--r-- 1 test test  1624 Jan  8 21:36 my_shared_lib.o

See that object file called my_shared_lib.o is present inside directory ?

See Answer

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj

$ ls -l

• -rw-r–r– 1 test test 1624 Jan 8 21:36 my_shared_lib.o

See that .so file called libmy_shared_lib.so is present inside directory ?

See Answer

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj

$ ls -l

• -rwxr-xr-x 1 test test 70648 Jan 8 21:36 libmy_shared_lib.so

Step 7.2: Check the “objdump” output of the shared library file

Step 7.2.1: Check “objdump” -t

• Let check objdump” output of the shared library file

• objdump -t : Displays the symbols of application binary file

• Check if “my_shared_function” is present or not in the “objdump”

• Make sure the current directory “obj”

$ pwd

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj

$ ls -l

-rwxr-xr-x 1 test test 70648 Jan  8 21:36 libmy_shared_lib.so
-rw-r--r-- 1 test test  1624 Jan  8 21:36 my_shared_lib.o

$HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/objdump libmy_shared_lib.so -t | grep -i my_shared_function

00000000000004e4 g     F .text     0000000000000014              my_shared_function

Step 7.2.2: Check “objdump” -S

• objdump -S : Display source code intermixed with disassembly

• Check if “my_shared_function” is present or not in the “objdump”

• Make sure the current directory “obj”

$ pwd

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj

$ ls -l

-rwxr-xr-x 1 test test 70648 Jan  8 21:36 libmy_shared_lib.so
-rw-r--r-- 1 test test  1624 Jan  8 21:36 my_shared_lib.o

$HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/objdump libmy_shared_lib.so -S | grep -i my_shared_function

00000000000004e4 <my_shared_function>:

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

See Answer

• -S : Display source code intermixed with disassembly

What “objdump” command does ?

See Answer

• The “objdump” tool is to debug and understand the executable file

• Symbols “my_shared_function” are present in .so shared library file

• Hence we can confirm application is compiled successfully

Step 7.3: Check the “readelf” output of the shared library file

Step 7.3.1: Check “readelf” -s

• readelf -s: Display the symbol table

• Check if “my_shared_function” is present or not in the “readelf”

• Make sure the current directory “obj”

$ pwd

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj

$ ls -l

-rwxr-xr-x 1 test test 70648 Jan  8 21:36 libmy_shared_lib.so
-rw-r--r-- 1 test test  1624 Jan  8 21:36 my_shared_lib.o

$HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/readelf libmy_shared_lib.so -s | grep -i my_shared_function

7: 00000000000004e4    20 FUNC    GLOBAL DEFAULT    9 my_shared_function
60: 00000000000004e4    20 FUNC    GLOBAL DEFAULT    9 my_shared_function

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

• Symbols “my_shared_function” are present in .so shared library file

• Hence we can confirm application is compiled successfully

Step 7.4: Check the “nm” output of the shared library file

Step 7.4.1: Check “nm” -S

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

• Check if “my_shared_function” is present or not in the “nm”

• Make sure the current directory “obj”

$ pwd

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj

$ ls -l

-rwxr-xr-x 1 test test 70648 Jan  8 21:36 libmy_shared_lib.so
-rw-r--r-- 1 test test  1624 Jan  8 21:36 my_shared_lib.o

$HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/nm libmy_shared_lib.so -S | grep -i my_shared_function

00000000000004e4 0000000000000014 T my_shared_function

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.

• Symbols “my_shared_function” are present in .so shared library file

• Hence we can confirm application is compiled successfully

Step 7.4.2: 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 “my_shared_function” is present or not in the “nm”

• Make sure the current directory “obj”

$ pwd

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj

$ ls -l

-rwxr-xr-x 1 test test 70648 Jan  8 21:36 libmy_shared_lib.so
-rw-r--r-- 1 test test  1624 Jan  8 21:36 my_shared_lib.o

$HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/nm libmy_shared_lib.so -s | grep -i my_shared_function

00000000000004e4 T my_shared_function

• Symbols “my_shared_function” are present in .so shared library file

• Hence we can confirm application is compiled successfully

Step 7.5: Check the “file” command output of shared library (.so) file

• file: Determine file type

$ file libmy_shared_lib.so

libmy_shared_lib.so: ELF 64-bit LSB shared object, ARM aarch64, version 1 (SYSV), dynamically linked, with debug_info, not stripped

Why “file” command is used ?

See Answer

• file command is used to determine the file type.

Step 7.5.1: Check the “file” command output of shared library object file

$ file my_shared_lib.o

my_shared_lib.o: ELF 64-bit LSB relocatable, ARM aarch64, version 1 (SYSV), not stripped

Step 7.6: Check the “size” command output of shared library file

• size: List section sizes and total size of binary files

$ size libmy_shared_lib.a

  text    data     bss     dec     hex filename
  1053     536      56    1645     66d libmy_shared_lib.so

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 7.7: Check the “strings” command output of shared library file

• strings - print the sequences of printable characters in files

• As we can see string output are added in the program are confirmed.

$ strings libmy_shared_lib.so | grep -i Hello

  Hello from my_shared_function!

What “strings” command does ?

See Answer

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

Step 7.8: Check the location of .ipk file

• Make sure the current directory “openwrt”

$HOME/openwrt/staging_dir/packages/bcm27xx

$ ls -l | grep -i my_shared_lib

lrwxrwxrwx 1 test test 123 Jan  8 21:36 my_shared_lib_1.0-1_aarch64_cortex-a72.ipk -> /home/test/openwrt/bin/packages/aarch64_cortex-a72/libs/my_shared_lib_1.0-1_aarch64_cortex-a72.ipk

• ipk file: It is compressed archive format derived from the Debian package (. DEB) format

What is “IPK” file ?

See Answer

• IPK file is a compressed archive format derived from the Debian package (. DEB) format

Step 7.9: How to extract .ipk file

• Make sure the current directory “openwrt”

$HOME/openwrt/staging_dir/packages/bcm27xx

• Command to extract .ipk file is below

$ tar -xzf my_shared_lib_1.0-1_aarch64_cortex-a72.ipk

$ ls

  control.tar.gz
  data.tar.gz
  debian-binary

what is the command to extract IPK file ?

See Answer

• tar -xzf my_shared_lib_1.0-1_aarch64_cortex-a72.ipk

Step 7.10: How to check content of .ipk file

• First extract control.tar.gz and data.tar.gz

• Command to check content for control.tar.gz file is below

$ tar -xzvf control.tar.gz

./control
./postinst
./prerm

• Check ./control using “vim command”

$ vim ./control

• Content of ./control

Package: my_shared_lib
Version: 1.0-1
Depends: libc
Source: package/libs/shared_lib/my_shared_lib
SourceName: my_shared_lib
Section: examples
SourceDateEpoch: 1703276774
Architecture: aarch64_cortex-a72
Installed-Size: 1056
Description:  A simple shared library example.

• Check inside for other files ./postinst and ./prerm using “vim” command

• Command to check content for control.tar.gz file is below

$ tar -xzvf data.tar.gz

 ./usr/
 ./usr/lib/
 ./usr/lib/libmy_shared_lib.so

• Check ./usr/lib/libmy_shared_lib.so is a shared library file

Step 8: Build full openwrt - Incremental

In this section you will learn how to build full openwrt

Step 8.1: Do Pre-Build checks

• Make sure the current directory “openwrt”

$HOME/openwrt

Step 8.2: Build full openwrt

$ make -j1 V=s

• It compile all packages

• -j1: This option tells the build system work on one task at a time.

• V=s: This sets the verbosity level to ‘s’, which means “silent” or “verbose.” When set to ‘s’, it makes the build system display more detailed information about what it’s doing. You’ll see the commands it’s running and the files it’s working on. This verbose output is helpful for understanding and debugging the build process.

Step 8.3: Do Post-Build checks

Step 8.3.1 : Check the location of the full image

• Make sure the current directory “openwrt”

• To check package is successfully compiled, below is the path

$HOME/openwrt/bin/targets/bcm27xx/bcm2711

• To check the image file is present or not

$ ls -l

-rw-r--r-- 1 test test     1655 Jan  8 21:35 config.buildinfo
-rw-r--r-- 1 test test       71 Jan  8 21:35 feeds.buildinfo
-rw-r--r-- 1 test test 17116537 Jan  8 21:37 openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
-rw-r--r-- 1 test test 17117384 Jan  8 21:37 openwrt-bcm27xx-bcm2711-rpi-4-ext4-sysupgrade.img.gz
-rw-r--r-- 1 test test     4210 Jan  8 21:37 openwrt-bcm27xx-bcm2711-rpi-4.manifest
-rw-r--r-- 1 test test 15351092 Jan  8 21:37 openwrt-bcm27xx-bcm2711-rpi-4-squashfs-factory.img.gz
-rw-r--r-- 1 test test 15351939 Jan  8 21:37 openwrt-bcm27xx-bcm2711-rpi-4-squashfs-sysupgrade.img.gz
drwxr-xr-x 2 test test     4096 Jan  8 21:37 packages
-rw-r--r-- 1 test test     2141 Jan  8 21:37 profiles.json
-rw-r--r-- 1 test test      912 Jan  8 21:37 sha256sums
-rw-r--r-- 1 test test       18 Jan  8 21:35 version.buildinfo

• As we can see “image” is present then it is confirmed that fully image is successfull

• Image file is openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz

Step 8.3.2 : How to extract full image file

• To extract image file command and pathis below

• Make sure the current directory “openwrt”

$HOME/openwrt/bin/targets/bcm27xx/bcm2711

$ ls -l

-rw-r--r-- 1 test test     1655 Jan  8 21:35 config.buildinfo
-rw-r--r-- 1 test test       71 Jan  8 21:35 feeds.buildinfo
-rw-r--r-- 1 test test 17116537 Jan  8 21:37 openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
-rw-r--r-- 1 test test 17117384 Jan  8 21:37 openwrt-bcm27xx-bcm2711-rpi-4-ext4-sysupgrade.img.gz
-rw-r--r-- 1 test test     4210 Jan  8 21:37 openwrt-bcm27xx-bcm2711-rpi-4.manifest
-rw-r--r-- 1 test test 15351092 Jan  8 21:37 openwrt-bcm27xx-bcm2711-rpi-4-squashfs-factory.img.gz
-rw-r--r-- 1 test test 15351939 Jan  8 21:37 openwrt-bcm27xx-bcm2711-rpi-4-squashfs-sysupgrade.img.gz
drwxr-xr-x 2 test test     4096 Jan  8 21:37 packages
-rw-r--r-- 1 test test     2141 Jan  8 21:37 profiles.json
-rw-r--r-- 1 test test      912 Jan  8 21:37 sha256sums
-rw-r--r-- 1 test test       18 Jan  8 21:35 version.buildinfo

• Command to extract full image file

$ gunzip -d openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz

Step 8.3.3: Check the content of image file

• Content of full image file

Step 8.3.4 : Check the size of the full image

• The du (disk usage) that allows users to analyze and report on disk usage within directories and files.

• Displays sizes in human-readable format, using units such as KB, MB, GB, etc.

$ du -h openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz

17M     openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz

• As we can see size of image file is around 17M

Note

PART B: Creating Shared Application

Step 1 : Add custom built library

In this section you will learn how to add custom built library in “Openwrt”

Step 1.1: Add directory structure of “Shared Application” package

• Make sure the current directory “openwrt”

  $ pwd
  
    /home/test/openwrt
  

Step 1.2: Create directory under package/libs directory

• Make sure the current directory “openwrt”

  $ cd package/libs/shared_lib
  
  $ mkdir x_shared_app
  

Step 1.3: Create directories under “x_shared_app” directory

• Make sure the current directory “x_shared_app”

  $ cd x_shared_app
  
  $ mkdir src obj inc
  

Step 1.4: Create “app.c” under src directory

• Make sure the current directory “x_shared_app/src”

• In app.c have “Function Definition”

• Create app.c file using vim command

vim app.c

• Content of src/app.c

#include <stdio.h>
#include "my_shared_lib.h"

extern void my_shared_function(void);

int main(void)
{
       my_shared_function();

       return 0;
}

Step 1.5: Create “my_shared_lib.h” under inc directory

• Make sure the current directory “x_shared_app/inc”

• In my_shared_lib.h have “Function Prototype”

• Create my_shared_lib.h file using vim command

vim my_shared_lib.h

• Content of inc/my_shared_lib.h

#include <stdio.h>

void my_shared_function(void);

Step 1.6: Create Makefile in “x_shared_app” directory

• Make sure the current directory “x_shared_app”

• Create Makefile file using vim command

  vim Makefile
  

• Content of Makefile

  include $(TOPDIR)/rules.mk

  PKG_NAME:=x_shared_app PKG_VERSION:=1.0 PKG_RELEASE:=1

  SOURCE_DIR:=$(TOPDIR)/package/libs/shared_lib/x_shared_app

  include $(INCLUDE_DIR)/package.mk

  define Package/x_shared_app SECTION:=examples CATEGORY:=Examples TITLE:=My Shared Library DEPENDS:=+my_shared_lib endef

  define Package/x_shared_app/description A simple shared library example. endef

  define Build/Prepare mkdir -p $(PKG_BUILD_DIR) cp -r $(SOURCE_DIR)/src $(PKG_BUILD_DIR) cp -r $(SOURCE_DIR)/inc $(PKG_BUILD_DIR) cp -r $(SOURCE_DIR)/obj $(PKG_BUILD_DIR) cp $(SOURCE_DIR)/Makefile $(PKG_BUILD_DIR) $(Build/Patch) endef

  define Build/Compile $(TARGET_CC) $(TARGET_CFLAGS) -I$(PKG_BUILD_DIR)/inc -L/home/test/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj -o $(PKG_BUILD_DIR)/obj/app $(PKG_BUILD_DIR)/src/app.c -l my_shared_lib endef

  define Package/x_shared_app/install $(INSTALL_DIR) $(1)/usr/bin $(INSTALL_BIN) $(PKG_BUILD_DIR)/obj/app $(1)/usr/bin/ endef

  $(eval $(call BuildPackage,x_shared_app))

Step 1.7: Explanation of Makefile part by part

include $(TOPDIR)/rules.mk

• Explanation of include $(TOPDIR)/rules.mk

  include $(TOPDIR)/rules.mk
  

• The $(TOPDIR) is expected to be the top-level directory

• /rules.mk - The path to the file included

What is $(TOPDIR) ?

See Answer

• $(TOPDIR) path is /home/test/openwrt

Package Name, Version and Release number Block

• Explanation of Package Name, Version and Release number Block

PKG_NAME:=x_shared_app
PKG_VERSION:=1.0
PKG_RELEASE:=1

• The name and version of your package are used to define the variable to point to the build directory of your package (Ex: x_shared_app-1.0)

Path (absolute or relative) to a directory Block

• Explanation of SOURCE_DIR

SOURCE_DIR:=$(TOPDIR)/package/libs/shared_lib/x_shared_app

What is SOURCE_DIR ?

See Answer

• SOURCE_DIR is relative or absolute path of directory

• In this case path of SOURCE_DIR $HOME/openwrt/package/libs/shared_lib/x_shared_app

• Give path of the source directory

• SOURCE_DIR is used in Build/Prepare

• Explanation of include $(INCLUDE_DIR)/package.mk

include $(INCLUDE_DIR)/package.mk

• $(INCLUDE_DIR) - Directory containing include files

• /package.mk: The path to the file included

What is $(INCLUDE_DIR) ?

See Answer

• $(INCLUDE_DIR) - It will search for included files

• In this case path of $(INCLUDE_DIR) $HOME/openwrt/include

Package Information Block

• Explanation of Package Information

define Package/x_shared_app
   SECTION:=examples
   CATEGORY:=Examples
   TITLE:=My Shared Library
   DEPENDS:=+my_shared_lib
endef

• This block defines the package information as section, category, and title

• SECTION: examples: This line tells the section to which the package belongs. In this case, the package will come under the “examples” section.

• CATEGORY: Examples: This line tells the category to which the package belongs. In this case, the category is set to “Examples

• TITLE: My Shared Library This line tells, title or name of the package. In this case, the title is set to “My Shared Library”.

• DEPENDS: =+my_shared_lib: It is the library dependencies of the package.

• endef: This line will be the end of the define block.

What all information included in Package Information Block ?

See Answer

• It includes section, category, and title

Package Description Block

• Explanation of Package Description

define Package/x_shared_app/description
        A simple shared library example
endef

• This function defines of package.

What is Package Description Block ?

See Answer

• Instructs on how and where our package will appear in the overall configuration menu

Build/Prepare Block

• Explanation of Build/Prepare

define Build/Prepare
        mkdir -p $(PKG_BUILD_DIR)
        cp -r $(SOURCE_DIR)/src $(PKG_BUILD_DIR)
        cp -r $(SOURCE_DIR)/inc $(PKG_BUILD_DIR)
        cp -r $(SOURCE_DIR)/obj $(PKG_BUILD_DIR)
        cp $(SOURCE_DIR)/Makefile $(PKG_BUILD_DIR)
        $(Build/Patch)
endef

• mkdir -p $(PKG_BUILD_DIR): This line creates the build directory

• cp -r $(SOURCE_DIR)/src $(PKG_BUILD_DIR): This line copies the contents from SOURCE_DIR (source) to PKG_BUILD_DIR (destination)

• cp -r $(SOURCE_DIR)/inc $(PKG_BUILD_DIR): This line copies the contents from SOURCE_DIR (source) to PKG_BUILD_DIR (destination)

• cp -r $(SOURCE_DIR)/obj $(PKG_BUILD_DIR): This line copies the contents from SOURCE_DIR (source) to PKG_BUILD_DIR (destination)

• cp -r $(SOURCE_DIR)Makefile $(PKG_BUILD_DIR): This line copies the contents from SOURCE_DIR (source) to PKG_BUILD_DIR (destination)

• -r : -r is recursive copy.

• $(Build/Patch): This line includes a call to $(Build/Patch), which is a variable or macro typically defined elsewhere in the Makefile. It is used to apply any necessary patches to the source code.

What is -r ?

See Answer

• -r is recursive copy.

Which path $(PKG_BUILD_DIR) ?

See Answer

• In this case path of $(PKG_BUILD_DIR) $HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/x_shared_app-1.0

What is SOURCE_DIR ?

See Answer

• In this case path of SOURCE_DIR $HOME/openwrt/package/mypackages/examples/x_shared_app

Build/Compile Block

• Explanation of Build/Compile

define Build/Compile
        $(TARGET_CC) $(TARGET_CFLAGS) -I$(PKG_BUILD_DIR)/inc -L/home/test/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj -o $(PKG_BUILD_DIR)/obj/app $(PKG_BUILD_DIR)/src/app.c -l my_shared_lib
endef

• This block defines the compilation process.

• $(TARGET_CC): It is said to arm cross compiler and it is defined globally

• -I$(PKG_BUILD_DIR)/inc : It will include prototype path

• $(PKG_BUILD_DIR)/src/app.c : .c file path

• -L/home/test/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj : Giving the path of .so file which already generated at “Shared Library” side

• l my_shared_lib : It will link during the compilation process (should give 1 space after –l)

• How to give path for -L

• It is most important to check before giving path in “Shared Application”

• Before copying path to –L first we need to check objdump xxxxx.so (in this case libmy_shared_lib.so) with –t or –dSs it will reflect with the symbol table if not symbol table reflect then this path should not be used in –L.

• Check table of shared library file

• Make sure the current directory “openwrt”

  $HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/my_shared_lib-1.0/obj
  
  $ ls -l
  
  -rwxr-xr-x 1 test test 70648 Jan  8 21:36 libmy_shared_lib.so
  -rw-r--r-- 1 test test  1624 Jan  8 21:36 my_shared_lib.o
  

• Check shared library for table

  $HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/objdump libmy_shared_lib.so -dSs | grep -i my_shared_function
  
  00000000000004e4 <my_shared_function>:
  

• We got the table so should give this path(.so file) to -L path in “Shared Application”

What is -L ?

See Answer

• -L : It check in directory for library files

What is -l ?

See Answer

• -l : It will link with the library files

Package Install Block

• Explanation of Package Install

define Package/x_shared_app/install
        $(INSTALL_DIR) $(1)/usr/bin
        $(INSTALL_BIN) $(PKG_BUILD_DIR)/obj/app $(1)/usr/bin/
endef

• $(INSTALL_DIR): It used for creating a directory and it is commonly used in build systems like OpenWrt.

• $(INSTALL_BIN): It used for copying an executable file.

• In this case path of $(PKG_BUILD_DIR) $HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/x_shared_app-1.0

• /usr/bin: It is the destination directory, binary file will be installed.

What is $(INSTALL_DIR) ?

See Answer

• It used for creating a directory

• In this case $(INSTALL_DIR) install -d -m0755

• Install command is used to copy files and set attributes.

• -d, –directory : It will act as directory names towards all arguments.

• m, –mode=MODE : Set permission mode (as in chmod).

What is $(INSTALL_BIN) ?

See Answer

• It used for copying an executable file.

• In this case $(INSTALL_BIN) install -m0755

• m, –mode=MODE : Set permission mode (as in chmod).

What is $(1)/usr/bin ?

See Answer

• /usr/bin: It is the destination directory where the binary executable will be installed.

• In this case path of $(1)/usr/bin $HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/x_shared_app-1.0/.pkgdir/x_shared_app/usr/bin

Package Build Block

• Explanation of Package Build

$(eval $(call BuildPackage,x_shared_app))

• BuildPackage: It is function call with x_shared_app as parameter.

Step 2 : Add application package to menuconfig

In this section you will learn how to add menuconfig

Step 2.1: Create feeds.conf file

• Make sure the current directory is “openwrt”

• Create new feeds.conf file using vim command in openwrt directory

$ vim feeds.conf

Step 2.2: Content of feeds.conf

• Inside feeds.conf

src-link <name> <path>

Step 2.3: Below example for feeds path

• Make sure the current directory is “openwrt”

• Create new feeds.conf file using vim command in openwrt directory

src-link libs /home/test/openwrt/package/libs

Step 3 : Update and install feeds

In this section you will learn how to install and update feeds

Step 3.1: Commands for update and install feeds

• Installs the most recent packages, replacing any earlier versions that were already on your system

$ ./scripts/feeds update -a

$ ./scripts/feeds install -a -f

Step 4 : Enable application package in menuconfig

In this section you will learn how to enable in menuconfig

Step 4.1: Select your application in menuconfig

• Select your category that reflects in menuconfig

• Go inside examples and select x_shared_app

Step 4.2: Save the configuration

Step 4.3: Exit the menuconfig

Step 5: Do Pre-Build checks

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

Step 5.1: Check .config file is present or not

• In .config file package should be enabled in ‘y’

• Below line should be present in .config file

  CONFIG_PACKAGE_x_shared_app=y
  

Step 5.2 : Check the location of toolchain

• Make sure the current directory “openwrt”

  $HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl
  

  $ ls -l
  
   drwxr-xr-x 2 test test 4096 Dec 23 02:58 bin
   drwxr-xr-x 3 test test 4096 Dec 23 03:39 include
   lrwxrwxrwx 1 test test    6 Dec 23 03:39 lib -> ../lib
   lrwxrwxrwx 1 test test    8 Dec 23 03:20 lib64 -> ../lib64
   lrwxrwxrwx 1 test test   10 Dec 23 03:39 sys-include -> ../include
  

Step 6: Build application package

• Make sure the current directory is “openwrt”

  $HOME/openwrt
  
  $ make package/x_shared_app/compile
  

Step 7: Do post build checks

In this section you will learn about post build checks

Step 7.1 Check the location of the shared application binary file

• Make sure the current directory is ‘x_shared_app-1.0’

  $HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/x_shared_app-1.0
  
  $ ls -l
  
  drwxr-xr-x 2 test test 4096 Jan  8 23:03 inc
  drwxr-xr-x 3 test test 4096 Jan  8 23:03 ipkg-aarch64_cortex-a72
  -rw-r--r-- 1 test test 1280 Jan  8 23:03 Makefile
  drwxr-xr-x 2 test test 4096 Jan  8 23:03 obj
  drwxr-xr-x 2 test test 4096 Jan  8 23:03 src
  

$ cd src

$ ls -l

-rw-r--r-- 1 test test 141 Jan  8 23:03 app.c

$ cd inc

$ ls -l

-rw-r--r-- 1 test test 46 Jan  8 23:03 my_shared_lib.h

$ cd obj

$ ls -l

-rwxr-xr-x 1 test test 72736 Jan  8 23:03 app

Step 7.2: Check the “objdump” output of the shared application file

Step 7.2.1: Check “objdump” -t

• Let check objdump” output of the shared application file

• objdump -t : Displays the symbols of application binary file

• Check if “my_shared_function” is present or not in the “objdump”

• Make sure the current directory “obj”

$ pwd

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/x_shared_app-1.0/obj

$ ls -l

-rwxr-xr-x 1 test test 72736 Jan  8 23:03 app

$HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/objdump app -t | grep my_shared_function

0000000000000000       F *UND*     0000000000000000              my_shared_function

Step 7.2.2: Check “objdump” -S

• objdump -S : Display source code intermixed with disassembly

• Check if “my_shared_function” is present or not in the “objdump”

• Make sure the current directory “obj”

$ pwd

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/x_shared_app-1.0/obj

$ ls -l

-rwxr-xr-x 1 test test 72736 Jan  8 23:03 app

$HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/objdump app -S | grep -i my_shared_function

00000000004004f0 <my_shared_functionplt>:
400528: 97fffff2        bl      4004f0 <my_shared_functionplt>

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

See Answer

• -S : Display source code intermixed with disassembly

What “objdump” command does ?

See Answer

• The “objdump” tool is to debug and understand the executable file

• Symbols “my_shared_function” are present in app binary file

• Hence we can confirm application is compiled successfully

Step 7.3: Check the “readelf” output of the shared application file

Step 7.3.1: Check “readelf” -s

• readelf -s: Display the symbol table

• Check if “my_shared_function” is present or not in the “readelf”

• Make sure the current directory “obj”

$ pwd

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/x_shared_app-1.0/obj

$ ls -l

-rwxr-xr-x 1 test test 72736 Jan  8 23:03 app

$HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/readelf app -s | grep -i my_shared_function

2: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND my_shared_function
68: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND my_shared_function

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

• Symbols “my_shared_function” are present in app binary file

• Hence we can confirm application is compiled successfully

Step 7.4: Check the “nm” output of the shared application file

Step 7.4.1: Check “nm” -S

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

• Check if “my_shared_function” is present or not in the “nm”

• Make sure the current directory “obj”

$ pwd

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/x_shared_app-1.0/obj

$ ls -l

-rwxr-xr-x 1 test test 72736 Jan  8 23:03 app

$HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/nm app -S | grep -i my_shared_function

U my_shared_function

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.

• Symbols “my_shared_function” are present in binary file

• Hence we can confirm application is compiled successfully

Step 7.4.2: 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 “my_shared_function” is present or not in the “nm”

• Make sure the current directory “obj”

$ pwd

$HOME/openwrt/build_dir/target-aarch64_cortex-a72_musl/x_shared_app-1.0/obj

$ ls -l

-rwxr-xr-x 1 test test 72736 Jan  8 23:03 app

$HOME/openwrt/staging_dir/toolchain-aarch64_cortex-a72_gcc-12.3.0_musl/aarch64-openwrt-linux-musl/bin/nm app -s | grep -i my_shared_function

U my_shared_function

• Symbols “my_shared_function” are present in binary application file

• Hence we can confirm application is compiled successfully

Step 7.5: Check the “file” command output of shared application file

• file: Determine file type

$ file app

app: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-musl-aarch64.so.1, with debug_info, not stripped

Why “file” command is used ?

See Answer

• file command is used to determine the file type.

Step 7.6: Check the “size” command output of shared application file

• size: List section sizes and total size of binary files

$ size app

  text    data     bss     dec     hex filename
  1144     664      56    1864     748 app

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 7.7: Check the “strings” command output of shared application file

• strings - print the sequences of printable characters in files

• As we can see string output are added in the program are confirmed.

$ strings app | grep -i my_shared_function

  my_shared_function
  my_shared_function

What “strings” command does ?

See Answer

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

Step 7.8: Check the location of .ipk file

• Make sure the current directory “openwrt”

$HOME/openwrt/staging_dir/packages/bcm27xx

$ ls -l | grep -i "x_shared_app"

 lrwxrwxrwx 1 test test 122 Jan  8 22:45 x_shared_app_1.0-1_aarch64_cortex-a72.ipk -> /home/test/openwrt/bin/packages/aarch64_cortex-a72/libs/x_shared_app_1.0-1_aarch64_cortex-a72.ipk

• ipk file: It is compressed archive format derived from the Debian package (. DEB) format

What is “IPK” file ?

See Answer

• IPK file is a compressed archive format derived from the Debian package (. DEB) format

Step 7.9: How to extract .ipk file

• Make sure the current directory “openwrt”

$HOME/openwrt/staging_dir/packages/bcm27xx

• Command to extract .ipk file is below

$ tar -xzf x_shared_app_1.0-1_aarch64_cortex-a72.ipk

$ ls

  control.tar.gz
  data.tar.gz
  debian-binary

what is the command to extract IPK file ?

See Answer

• tar -xzf helloworld_1.0-1_aarch64_cortex-a72.ipk

Step 7.10: How to check content of .ipk file

• First extract control.tar.gz and data.tar.gz

• Command to check content for control.tar.gz file is below

$ tar -xzvf control.tar.gz

./control
./postinst
./prerm

• Check ./control using “vim command”

$ vim ./control

• Content of ./control

Package: x_shared_app
Version: 1.0-1
Depends: libc, my_shared_lib
Source: package/libs/shared_lib/x_shared_app
SourceName: x_shared_app
Section: examples
SourceDateEpoch: 1703276774
Architecture: aarch64_cortex-a72
Installed-Size: 1201
Description:  A simple shared library example.

• Check inside for other files ./postinst and ./prerm using “vim” command

• Command to check content for control.tar.gz file is below

$ tar -xzvf data.tar.gz

 ./usr/
 ./usr/bin/
 ./usr/bin/app

• Check ./usr/bin/app is a shared application binary file

Step 8: Build full openwrt - Incremental

In this section you will learn how to build full openwrt

Step 8.1: Do Pre-Build checks

• Make sure the current directory “openwrt”

$HOME/openwrt

Step 8.2: Build full openwrt

$ make -j1 V=s

• It compile all packages

• -j1: This option tells the build system work on one task at a time.

• V=s: This sets the verbosity level to ‘s’, which means “silent” or “verbose.” When set to ‘s’, it makes the build system display more detailed information about what it’s doing. You’ll see the commands it’s running and the files it’s working on. This verbose output is helpful for understanding and debugging the build process.

Step 8.3: Do Post-Build checks

Step 8.3.1 : Check the location of the full image

• Make sure the current directory “openwrt”

• To check package is successfully compiled, below is the path

$HOME/openwrt/bin/targets/bcm27xx/bcm2711

• To check the image file is present or not

$ ls -l

-rw-r--r-- 1 test test     1685 Jan  8 22:44 config.buildinfo
-rw-r--r-- 1 test test       71 Jan  8 22:44 feeds.buildinfo
-rw-r--r-- 1 test test 17118327 Jan  8 22:45 openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
-rw-r--r-- 1 test test 17119174 Jan  8 22:45 openwrt-bcm27xx-bcm2711-rpi-4-ext4-sysupgrade.img.gz
-rw-r--r-- 1 test test     4231 Jan  8 22:45 openwrt-bcm27xx-bcm2711-rpi-4.manifest
-rw-r--r-- 1 test test 15351852 Jan  8 22:45 openwrt-bcm27xx-bcm2711-rpi-4-squashfs-factory.img.gz
-rw-r--r-- 1 test test 15352699 Jan  8 22:45 openwrt-bcm27xx-bcm2711-rpi-4-squashfs-sysupgrade.img.gz
drwxr-xr-x 2 test test     4096 Jan  8 22:45 packages
-rw-r--r-- 1 test test     2141 Jan  8 22:45 profiles.json
-rw-r--r-- 1 test test      912 Jan  8 22:46 sha256sums
-rw-r--r-- 1 test test       18 Jan  8 22:44 version.buildinfo

• As we can see “image” is present then it is confirmed that fully image is successfull

• Image file is openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz

Step 8.3.2 : How to extract full image file

• To extract image file command and pathis below

• Make sure the current directory “openwrt”

$HOME/openwrt/bin/targets/bcm27xx/bcm2711

$ ls -l

-rw-r--r-- 1 test test     1685 Jan  8 22:44 config.buildinfo
-rw-r--r-- 1 test test       71 Jan  8 22:44 feeds.buildinfo
-rw-r--r-- 1 test test 17118327 Jan  8 22:45 openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
-rw-r--r-- 1 test test 17119174 Jan  8 22:45 openwrt-bcm27xx-bcm2711-rpi-4-ext4-sysupgrade.img.gz
-rw-r--r-- 1 test test     4231 Jan  8 22:45 openwrt-bcm27xx-bcm2711-rpi-4.manifest
-rw-r--r-- 1 test test 15351852 Jan  8 22:45 openwrt-bcm27xx-bcm2711-rpi-4-squashfs-factory.img.gz
-rw-r--r-- 1 test test 15352699 Jan  8 22:45 openwrt-bcm27xx-bcm2711-rpi-4-squashfs-sysupgrade.img.gz
drwxr-xr-x 2 test test     4096 Jan  8 22:45 packages
-rw-r--r-- 1 test test     2141 Jan  8 22:45 profiles.json
-rw-r--r-- 1 test test      912 Jan  8 22:46 sha256sums
-rw-r--r-- 1 test test       18 Jan  8 22:44 version.buildinfo

• Command to extract full image file

$ gunzip -d openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz

Step 8.3.3: Check the content of image file

• Content of full image file

Step 8.3.4 : Check the size of the full image

• The du (disk usage) that allows users to analyze and report on disk usage within directories and files.

• Displays sizes in human-readable format, using units such as KB, MB, GB, etc.

  $ du -h openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
  
  17M     openwrt-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
  

• As we can see size of image file is around 17M