Questions on Compiling and Linking

1. What is meant by compiling and linking?

Ans: Creating an executable is a multistage process divided into two components.

Compiling: This refers to the processing of source code files (.c or .cpp) and the creation of an object file. This step doesn't create executable file. Instead, the compiler produces the machine language instructions that corresponding to the source file that was compiled. For example, you we compile three source files then we will have three object files created as output, each with name .o .

Linking: This refers to the creation of single executable file from multiple object files. In this step, it is common that the linker will complain about undefined functions. During compilation, if the compiler couldn't find the definition for a particular function, it would just assume that the function was defined in another file. Compiler doest look one file at a time. On other hand, the linker may look multiple files and try to find references for the functions that weren't mentioned.

Compiler errors are usually syntactic in nature -- a missing semicolon, an extra parenthesis. Linking errors usually have to do with missing or multiple declarations.

2. What are the steps of compilation?

Ans: Following are the steps that compiler does:

i) Lexical Analysis

ii) Syntactic Analysis

iii) Semantic Analysis

iv) Pre-Optimization of internal representation

v) Code Generation

vi) Post Optimization

3. What is static linkage and dynamic linkage?

Ans: If every program uses the standard C library, it means the each program would normally have a unique copy of that particular library present within it. This leads wasted of resources, degrade the efficiency and performance. Since C library is common, it is better to have each program reference the common one instance of the library, instead if having each program contain a copy of the library.

This implementation during the linking process where some objects are linked during the link time(static linkage) whereas some done during the run time(dynamci linkage).

Static Linkage: Static linkage means the the program and the particular library are combined together by the linker at link time. Programs that are linked statically are linked against archives of objects(libraries) that typically have the extension of .a.

For gcc, the -static option can be used during the compilation/linking of the program.

gcc -static filename.c -o filename

The drawback of this technique is that the executable is quite big in size.

Dynamic Linkage: Dynamic linkage means that the program and the particular library are not combined together at link time. Instead, the linker places information to the executable that tells the loader which shared object module the code is in and which run time linker should be used to find and bind the references. This means that the binding between the program and the shared object is done at run time that is before the program starts, the appropriate shared objects are found and bound. Programs that are linked dynamically that typically have the extension of .so.

4. What are the types of linkages?

Ans: The linkage determines whether identifiers that have identical names refers to the same object, function. The linkage of an identifier depends on how it was declared. There are three types of linkages:

i) Internal Linkage: Identifiers can only be seen within a translation unit(file). Ex: static

ii) External Linkage: Identifiers can be seen in other translation units(files). Ex: extern

iii) No Linkage: Identifiers can only be seen in the scope in which they are defined.