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How a C/C code is compiled in a system?
We may all used C or C++ language in our life, either as a product (eg: Games created using Unreal Engine) or even created/developed projects. Well, its most popular and it even exists today as its a foundation for many languages too like Python, Golang. But have you ever wondered how a C or C++ code is compiled in a system? Well, actually I also didn't know at first glance and later found a nice tip from my seniors about it. So let's get into it
Tools needed?
Before knowing about how the stuff works, I recommend you to use GNU's GCC Compiler as it's standard and widely used. You could also use LLVM Clang to try it out too!
In Action!
Alright, to get things started, let's type a normal C++ Hello World code. You'd understand at end, why I started with simple one.
// Hello.cpp#include <iostream>using namespace std;int main(){ cout << "Hello World!"; return 0;}Right, now let's compile the code using g++ Hello.cpp or clang++ Hello.cpp
If you run the output file, you'd get pretty straight output. No doubt.
Hello World!Now, to make things interesting, compile the code again but this time using -S flag
// For GCCgcc -S Hello.c (for C)g++ -S Hello.cpp (for C++)// For Clangclang++ -S Hello.cppYou'd get a file called Hello.s and if you read the contents, it's bit different than usual syntax and if you slightly read a bit more... It's all encoded in assembly format!
Here's an example (do note that it differs across compilers used... Clang, GCC etc)
// From GCC (arrows are presented for better view) .file "Hello.cpp" .text .section .rodata .type _ZStL19piecewise_construct, @object .size _ZStL19piecewise_construct, 1_ZStL19piecewise_construct: <--- Initialize step .zero 1 .local _ZStL8__ioinit .comm _ZStL8__ioinit,1,1.LC0: <--- Which function to execute? .string "Hello World!" .text .globl main .type main, @functionmain: <--- Main operation.LFB1522: .cfi_startproc endbr64 pushq %rbp .cfi_def_cfa_offset 16 .cfi_offset 6, -16 movq %rsp, %rbp .cfi_def_cfa_register 6 leaq .LC0(%rip), %rsi leaq _ZSt4cout(%rip), %rdi call _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc@PLT movl $0, %eax popq %rbp .cfi_def_cfa 7, 8 ret .cfi_endproc.LFE1522: .size main, .-main .type _Z41__static_initialization_and_destruction_0ii, @function_Z41__static_initialization_and_destruction_0ii:.LFB2006: .cfi_startproc endbr64 pushq %rbp .cfi_def_cfa_offset 16 .cfi_offset 6, -16 movq %rsp, %rbp .cfi_def_cfa_register 6 subq $16, %rsp movl %edi, -4(%rbp) movl %esi, -8(%rbp) cmpl $1, -4(%rbp) jne .L5 cmpl $65535, -8(%rbp) jne .L5 leaq _ZStL8__ioinit(%rip), %rdi call _ZNSt8ios_base4InitC1Ev@PLT leaq __dso_handle(%rip), %rdx leaq _ZStL8__ioinit(%rip), %rsi movq _ZNSt8ios_base4InitD1Ev@GOTPCREL(%rip), %rax movq %rax, %rdi call [email protected]: nop leave .cfi_def_cfa 7, 8 ret .cfi_endproc.LFE2006: .size _Z41__static_initialization_and_destruction_0ii, .-_Z41__static_initialization_and_destruction_0ii .type _GLOBAL__sub_I_main, @function_GLOBAL__sub_I_main:.LFB2007: .cfi_startproc endbr64 pushq %rbp .cfi_def_cfa_offset 16 .cfi_offset 6, -16 movq %rsp, %rbp .cfi_def_cfa_register 6 movl $65535, %esi movl $1, %edi call _Z41__static_initialization_and_destruction_0ii popq %rbp .cfi_def_cfa 7, 8 ret .cfi_endproc.LFE2007: .size _GLOBAL__sub_I_main, .-_GLOBAL__sub_I_main .section .init_array,"aw" .align 8 .quad _GLOBAL__sub_I_main .hidden __dso_handle .ident "GCC: (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0" .section .note.GNU-stack,"",@progbits .section .note.gnu.property,"a" .align 8 .long 1f - 0f .long 4f - 1f .long 50: .string "GNU"1: .align 8 .long 0xc0000002 .long 3f - 2f2: .long 0x33: .align 84:You'd see a lot of code just for a Hello World program! (Sounds bit shocking right?! Yeah, I got it too) so let me just explain a bit of it to get it clear...
.file "Hello.cpp" .text .section .rodata .type _ZStL19piecewise_construct, @object .size _ZStL19piecewise_construct, 1This section contains metadata for the C++ file and has some information about the file.
_ZStL19piecewise_construct: .zero 1 .local _ZStL8__ioinit .comm _ZStL8__ioinit,1,1.LC0: .string "Hello World!" .text .globl main .type main, @functionmain:.LFB1522: .cfi_startproc endbr64 pushq %rbp .cfi_def_cfa_offset 16 .cfi_offset 6, -16 movq %rsp, %rbp .cfi_def_cfa_register 6 leaq .LC0(%rip), %rsi leaq _ZSt4cout(%rip), %rdi call In above code, the section _ZSt... denotes initialization of pointers and the main part of code is done on the .LC0 and main sections. LC0 has the information about the function and where to execute it (in this case, it's main function). In main section, you'd see a bit on handling the data and the pointers. Interesting fact is that, it's all encoded in assembly.
Now, let's see a bit complex one (don't panic xD). Addition of 2 numbers!
#include <iostream>using namespace std;int main(){ int a=5, b=10; cout << a+b << "
"; return 0;}Compiling the program with -S flag, you'd get the assembly file. Let's just see the important details about it.
main:.LFB1522: .cfi_startproc endbr64 pushq %rbp .cfi_def_cfa_offset 16 .cfi_offset 6, -16 movq %rsp, %rbp .cfi_def_cfa_register 6 subq $16, %rsp movl $5, -8(%rbp) <-- NOTE HERE movl $10, -4(%rbp) <-- NOTE HERE movl -8(%rbp), %edx movl -4(%rbp), %eax addl %edx, %eax movl %eax, %esi leaq _ZSt4cout(%rip), %rdi call _ZNSolsEi@PLT leaq .LC0(%rip), %rsi movq %rax, %rdi call _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc@PLT movl $0, %eax leave .cfi_def_cfa 7, 8 ret .cfi_endprocIn the above main section, you can see that the registers are being used to store the values 5 and 10... Let's have a close look...
movl $5, -8(%rbp)movl $10, -4(%rbp)So, the values are made to move towards the destination registers and eventually carry out the operation in further parts of assembly code
This is how generally C or C++ is compiled to assembly mode and I've just shown only the upper layer of the glacier (yes, it's bit deep!). I'll also share a blog post soon on how pointers are used in such case to access while running in assembly mode and much more. Until then, see ya! ;)
Original Link: https://dev.to/kcdchennai/how-a-cc-code-is-compiled-in-a-system-5ad4
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