Compiling and linking

The C compilation system consists of a compiler, assembler, and link editor. The cc command invokes each of these components automatically unless you use command line options to specify otherwise. An executable C program is created with the following four steps:

1. The preprocessor component of the compiler reads lines in your source files that direct it to replace a name with a token string (#define), perhaps conditionally (#if, for example). It also accepts directives in your source files to include the contents of a named file in your program (#include). Included header files for the most part consist of #define directives and declarations of external symbols, definitions and declarations that you want to make available to more than one source file. See ``Libraries and header files'' for details.

2. The compiler proper, cc(CP), translates the C language code in your source files, which now contain the preprocessed contents of any included header files, into assembly language code.

3. The assembler, as(CP), translates the assembly language code into the machine instructions of the computer your program is to run on. These instructions are stored in object files that correspond to each of your source files. Each object file contains a binary representation of the C language code in the corresponding source file. Object files are made up of sections, of which there are usually at least two. The text section consists mainly of program instructions. Text sections normally have read and execute, but not write, permissions. Data sections normally have read, write, and execute permissions.

4. See ``Common Object File Format (COFF)'', and ``ELF object files'' for details of the supported object file formats.

5. The link editor, ld(CP) links these object files with each other and with any library functions that you have called in your program. The link editing model you have chosen determines when the library functions are linked:

• An archive, or statically linked library, is a collection of object files each of which contains the code for a function or a group of related functions in the library. When you use a library function in your program, a copy of the object file that contains the function is incorporated in your executable at link time.
  

• A static shared library is a file containing object code that several processes may use simultaneously while executing. When a program is link edited with a static shared library, the library code that defines the program's external references is not copied into the program's object file. Instead, a special section called .lib that identifies the library code is created in the object file. When the resulting a.out file is executed, it uses the information in this section to bring the required shared library code into the address space of the process. It should be noted that only one copy of the static shared library code is in memory and is shared by all processes using that library.

• A shared object, or dynamically linked library as in our case here, is a single object file that contains the code for every function in the library. When you call a library function in your program and specify a dynamic linking option (-dy) on the cc command line, the entire contents of the dynamically linked library are mapped into the virtual address space of your process at run time. As its name implies, a dynamically linked library contains code that can be used simultaneously by different programs at run time.

See ``Link editing'' for more information on how statically and dynamically linked libraries are implemented, and details on how to combine the static and dynamic linking approaches in different ways according to your needs.

``Organization of C Compilation System'' shows the organization of the C compilation system. Details concerning the optimizer are omitted here because it is optional. See ``Commonly used cc command line options'' for more information concerning invoking the optimizer.

Organization of C Compilation System