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May 15, 2015

C program compilation process

The compilation and execution process of C can be divided in to multiple steps:
Preprocessing - Using a Preprocessor program to convert C source code in expanded source code. "#includes" and "#defines" statements will be processed and replaced actually source codes in this step.

Compilation - Using a Compiler program to convert C expanded source to assembly source code.

Assembly - Using a Assembler program to convert assembly source code to object code.

Linking - Using a Linker program to convert object code to executable code. Multiple units of object codes are linked to together in this step.

Loading - Using a Loader program to load the executable code into CPU for execution.

Here is a simple table showing input and output of each step in the compilation and execution process:

Input                                      Program                    Output

source code                     > Preprocessor                > expanded source code
expanded source code    > Compiler                      > assembly source code
assembly code                > Assembler                    > object code
object code                     > Linker                          > executable code
executable code               > Loader                        > execution

Here are examples of commonly used programs for different compilation and execution steps on a Linux system:

"cpp hello.c -o hello.i" - Preprocessor preprocessing hello.c and saving output to hello.i.
"cc1 hello.i -o hello.s" - Compiler compiling hello.i and saving output to hello.s.
"as hello.s -o hello.o" - Assembler assembling hello.s and saving output to hello.o.
"ld hello.o -o hello" - Linker linking hello.o and saving output to hello.
"load hello" - Loader loading hello and running hello.