Lesson 10

Dates: 4/10/2019
Application compilation and installation on Linux
Linux System Administration


Shared libraries


  • Shared libraries, unlike static ones, are not compiled into the application binaris. They are loaded into the memory and linked with the application at a run time. Other running codes can use the loaded shared libraries.



  • Building a shared library 
    gcc -c -fPIC lib1.c
gcc -c -fPIC lib2.c
gcc -c -fPIC lib3.c
gcc -shared lib1.o lib2.o lib3.o -o libextra.so
  • To compile the application with the library we would follow the step below: 
    gcc -o code.x code.c -L. -lextra
  • To see what the shared libraries the code needs, use ldd command 
    ldd code.x
  • If the library is not found, include it into LD_LIBRARY_PATH environment variable, for example: 
    export LD_LIBRARY_PATH=.
  • Alternatively the shared library path can be included into /etc/ld.so.conf. After /etc/ld.so.conf has been updated, we need to run command 
    /sbin/ldconfig -p

  • Exercise
    Get the list of shared libraries required by command, ls: 
    ldd /bin/ls

    Compile code.c with the Math shared library (default compilation): 
    gcc code.c -o code_shared.x -lm
    Get the list of shared libraries required by code_stat.x, and code_shared.x: 
    ldd code_stat.x
ldd code_shared.x
    Compare the sizes of the executable files: 
    ls -lh  code_*.x


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