Steps of C++ codes compilation

g++ vs gcc

• g++ can compile any .c or .cpp files but they will be treated as C++ files only
• gcc can compile any .c or .cpp files but they will be treated as C and C++ respectively
• Using g++ to link the object files, files automatically links in the std C++ libraries. gcc does not do this.

Compilation

Suppose we have simplest code in main.cpp file, then

g++ main.cpp # Produces the executable file (with default name i.e a.out)
./a.out  # Runs a.out

g++ main.cpp -o main # Specifies a filename to output rather than default name
./main  # Runs main

Not a Single Step Process!

• Word Compilation is using wrongly.
• Compilation is just Step 2 of the complete pipeline below.

Barebone Source file

#define LUCKY_NUMBER 7

int main()
{
    int num = LUCKY_NUMBER;
    return 0;
}

Step 1: Preprocessing

g++ -E main.cpp

Outputs:

# 1 "main.cpp"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "main.cpp"


int main()
{
    int num = 7;
    return 0;
}

Step 2: Compilation

g++ -S main.cpp

Outputs: main.s

	.file	"main.cpp"
	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	endbr64
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register 6
	movl	$7, -4(%rbp)
	movl	$0, %eax
	popq	%rbp
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.ident	"GCC: (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0"
	.section	.note.GNU-stack,"",@progbits
	.section	.note.gnu.property,"a"
	.align 8
	.long	 1f - 0f
	.long	 4f - 1f
	.long	 5
0:
	.string	 "GNU"
1:
	.align 8
	.long	 0xc0000002
	.long	 3f - 2f
2:
	.long	 0x3
3:
	.align 8
4:

Step 3: Assemble

g++ -c main.cpp

Outputs: main.o
It’s a binary file, so we cannot read it.

Step 4: Linking

g++ main.cpp

Outputs: a.out which is executable.

Using Multiple files

Take following 3 files:

// main.cpp
#include <iostream>
#include "add.h"
int main()
{
    std::cout << add(2,3) << std::endl;
    return 0;
}
// --------------------------------------
// add.h
int add(int, int);
// --------------------------------------
// add.cpp
int add(int x, int y)
{
    return x+y;
}

g++ main.cpp add.cpp -o main # Generates executable file 'main' which outputs 5 when executed

# OR do it in steps
g++ -c main.cpp add.cpp # Genetates two object codes 'main.o' and 'add.o'
g++ main.o add.o -o main # Generates executable file 'main' which outputs 5 when executed