There are three basic
phases that are important in programming e.g. C- programming. The phases are
Pre-processing, compiling and Linking, only two phases are required: compiler
and linking.
prog.càPREPROCESSORà
temp.càCOMPILERà
prog.objàLINKERàprog.exe
A
preprocessor basically reads line by line from the input and replicates the
line to the output. If the line is a preprocessing statement, then it performs
the preprocessing directives. All preprocessing statements in C begin with the
# symbol and are at the beginning of a line. The operations of taking the input
file and generating the output file, pre-processed file, is called
pre-processing.
PRE-PROCESSOR à COMPILER à LINKER
(reads program line by line) (identify the uncoded (search for libraries files)
PRE-PROCESSOR à COMPILER à LINKER
(reads program line by line) (identify the uncoded (search for libraries files)
functions
like printf,scanf)
Example
#include<stdio.h> // pre-processor
#define Max 10 //
pre-processor
int main(void)
{
int i;
for(i=0;i<Max;i++)
printf(“Hello World!”);
return 0;
}
From
above example #include and #define defines header files. Pre-processor
reads line by line. Its starts from main() reading line by line, generates the
output file as i.e. temp.c is pre-processor output file.
Compiler
takes pre-processor output file as input for compiler and generated object file
i.e. prog.obj.
This object file contains machine code generated from the program
you wrote in your original C file. It does not as of yet contain code for
functions such as printf.
Since the code for printf is not in your code, the object file
contains symbol printf.
Usually a special file extension(obj) is given to an object file.
Think of an object file
as a partially complete program with missing blocks of code (ie the code for
printf). During the next phase called the Link Phase, it is the linker
program that is responsible to find the missing code "printf" and
link it(bind it) to the object file generated by the compiler. The compiler
also removes all comments from the input file.
The linker is a process
that accepts as input object files and libraries to produce the final
executable program. Libraries contain object code. This object code in turn
contains functions. In the link phase the object code from the compile phase is
bound to the missing function code such as printf. The function printf
is located in the standard input and output libraries. The final executable now
contains all the necessary code for execution.
