Runtime loading of dynamic libraries in Linux

-
In Linux, libraries which are linked during linking stage of the program, gets loaded during the startup of the program, unless a copy is already present in memory.

Another approach provided by Linux operating system is to load/unload the libraries at run time whenever it is needed.

API's are provided by Linux for

  • Loading the Library -- dlopen()
  • Looking up for symbols -- dlsym()
  • Handling Errors -- dlerror()
  • Unloading the Library -- dlclose()
Header File : #include <dlfcn.h>

Note: While building the program, you need to link 'ldl' library: -ldl


Flow:
1. Process begins with a call to dlopen API, providing the path of the shared library and the mode. Main two values for this mode are RTLD_NOW, RTLD_LAZY. This informs the dynamic linker when to perform relocations. RTLD_NOW will make all the necessary relocations at the dlopen call time, whereas RTLD_LAZY will perform relocations only when they're needed. dlopen function returns a handle which is used in all the subsequent API's
2. You then call dlsym() to get the addresses of the symbols (functions)
3. dlerror() function will returns a human readable error message if an API fails
4. When all the functions are accessed of the library, you can unload the shared library with a call to dlclose()

Example Code:

 Let's create a shared library which we want to load at runtime using dlopen, dlsym calls

arithmetic.c:

arithmetic.h:


Then write application code for loading this library at run time

main.c:


Output:

Comments

  1. Mitchel Marsh23 December 2020 at 19:03

    nice

    ReplyDelete
  2. Matthew BRad24 December 2020 at 00:22

    Such an informative blog.
    I also faced a DLL error in my Linux window computer then DLLWin guides me it is happening because sometimes due to malware and guides me to which DLL files I need to keep update or install on my Linux computer for a smooth run of my system. If you require any kind of DLL file either it is the newest or popular file then DLLWin is the place Download DLL Files

    ReplyDelete

Post a Comment

Popular posts from this blog

bb.utils.contains yocto

-
Image
bb.utils.contains  is most commonly used function in Yocto. grep in poky directory returned with 696 count. $ grep -nr 'bb.utils.contains' poky/ | wc -l 696 It's definition is present in 'poky/bitbake/lib/bb/utils.py' file This function returns third argument if the second argument is subset of the first argument else returns fourth argument Let's take an example to understand this. You can see from the above screenshot, when the second argument is a subset of first argument, "present" was returned, else "notpresent" was returned We can also use bb.utils.contains inside if loop. This we can use inside a recipe. if ${@bb.utils.contains('SOMEFLAG','NEWVALUE3','true','false',d)}; then     FOO = 'HELLO' fi
Read more

Difference between RDEPENDS and DEPENDS in Yocto

-
We normally observe these two (RDEPENDS and DEPENDS) whenever we open any recipe. What should we choose when we write our own recipe. DEPENDS -> Build Time Dependency RDEPENDS -> Run Time Dependency DEPENDS: When a recipe 'A' is DEPENDS on recipe 'B'. In this case, Bitbake first builds recipe 'B' and then recipe 'A' E.g. You need 'dbus' to be built before 'wpa_supplicant'. RDEPENDS: When a recipe 'A' is RDEPENDS on recipe 'B'. In this case, Bitbake deploys 'B' on the target system when it deploys 'A'.  E.g. 'perf' RDEPENDS on 'bash' In other words, DEPENDS are those set of packages that should be available while building package, whereas RDEPENDS are set of packages that should be available during execution of the program.
Read more

make config vs oldconfig vs defconfig vs menuconfig vs savedefconfig

-
Image
Building the Linux Kernel is a two-step process: Configuring the Kernel options Building the Kernel with those options. There are many methods available for configuring the kernel. make config: Text-based Configuration.  Options are prompted one after another. All options need to be answered Access to former options is not possible make menuconfig: Menu-driven user interface Allows to navigate forwards and backward directly between features Allows to load and save files with filenames different from ".config" Provides search feature It uses ncurses library for GUI. If the ncurses library is not installed, make menuconfig option fails. To install ncurses library on Ubuntu: sudo apt-get install libncurses5-dev make defconfig: Creates a ".config" file with default options from the ARCH supplied defconfig Configurations are generally stored in the directory: arch/$(ARCH)/configs $ ls arch/x86/configs/ i386_defconfi
Read more