Compilation pipeline

Phase	Command	Input	Output
Preprocessing	gcc -E	source code .c	compilation unit .i
Compilation	gcc -S	compilation unit .i	assembly code .s
Assemble	gcc -c	assembly code .s	relocatable object file .o
Linking	gcc	relocatable object file .o	executable

// hello_world.c
#include <stdio.h>

int main(int argc, char const *argv[]){
    printf("Hello world");
    return 0;
}

gcc -E hello_world.c -o hello_world.i
gcc -S hello_world.i -o hello_world.s
gcc -c hello_world.s // generates hello_world.o
gcc hello_world.o // generates a.out

To execute each of the phases in the pipeline, gcc invokes different programs:

the preprocessor cpp
the assembler as
the linker ld