Dynamic Linking

When one uses a C compiler under a Unix system to generate an executable from the C source code, the C compiler driver will usually invoke a C preprocessor, compiler, assembler and link editor in that order to translate the C language code into the executable file.

• The C compiler driver will first pass the C source code into a C preprocessor which outputs the pure C language code with the processed macros and directives,

• The C compiler translates the resultant C language code into machine-dependent assembly language code.

• The assembler translates the resultant assembly language code into the machine instructions of the target machine. The resultant machine instructions are stored in an object file in a specific binary format. In our case, the object files use the ELF binary format.

• In the last stage, the link editor links all the object files together with the start up codes and library functions which are referenced in the program. There are two kinds of libraries one can use:
  • A static library is a collection of object files which contain library routines and data. It is built in such a way that the link editor will incorporate a copy of only those object files that hold the functions and data referenced in the program into the executable at link time.

  • A shared library is a shared object file that contains functions and data. It is built in such a way that the link editor will only store in the executable the name of the shared library and information about the symbols referenced by the executable. At run time the dynamic linker, a.k.a. the program interpreter in ELF, will map the shared library into the virtual address space of the process image of the executable and resolve by name the symbols in the shared library used by the executable. That is process is also called dynamic linking.

There is nothing special which needs to be done by the programmer to take advantage of shared libraries with dynamic linking. Everything is transparent to programmers as well as to users.