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diff --git a/content/blog/2018-11-28-cpp-compiler.md b/content/blog/2018-11-28-cpp-compiler.md new file mode 100644 index 0000000..24f8ce8 --- /dev/null +++ b/content/blog/2018-11-28-cpp-compiler.md @@ -0,0 +1,138 @@ ++++ +date = 2018-11-28 +title = "The C++ Compiler" +description = "Learn basics about the C++ compilation process." ++++ + +# A Brief Introduction + +[C++](https://en.wikipedia.org/wiki/C%2B%2B) is a general-purpose +programming language with object-oriented, generic, and functional +features in addition to facilities for low-level memory manipulation. + +The source code, shown in the snippet below, must be compiled before it +can be executed. There are many steps and intricacies to the compilation +process, and this post was a personal exercise to learn and remember as +much information as I can. + +```cpp +#include <iostream> + +int main() +{ + std::cout << "Hello, world!\n"; +} +``` + +# Compilation Process + +## An Overview + +Compiling C++ projects is a frustrating task most days. Seemingly +nonexistent errors keeping your program from successfully compiling can +be annoying (especially since you know you wrote it perfectly the first +time, right?). + +I'm learning more and more about C++ these days and decided to write +this concept down so that I can cement it even further in my own head. +However, C++ is not the only compiled language. Check out [the Wikipedia +entry for compiled +languages](https://en.wikipedia.org/wiki/Compiled_language) for more +examples of compiled languages. + +I'll start with a wonderful, graphical way to conceptualize the C++ +compiler. View [The C++ Compilation +Process](https://web.archive.org/web/20190419035048/http://faculty.cs.niu.edu/~mcmahon/CS241/Notes/compile.html) +by Kurt MacMahon, an NIU professor, to see the graphic and an +explanation. The goal of the compilation process is to take the C++ code +and produce a shared library, dynamic library, or an executable file. + +## Compilation Phases + +Let's break down the compilation process. There are four major steps to +compiling C++ code. + +### Step 1 + +The first step is to expand the source code file to meet all +dependencies. The C++ preprocessor includes the code from all the header +files, such as `#include <iostream>`. Now, what does that +mean? The previous example includes the `iostream` header. +This tells the computer that you want to use the `iostream` +standard library, which contains classes and functions written in the +core language. This specific header allows you to manipulate +input/output streams. After all this, you'll end up which a temporary +file that contains the expanded source code. + +In the example of the C++ code above, the `iostream` class +would be included in the expanded code. + +### Step 2 + +After the code is expanded, the compiler comes into play. The compiler +takes the C++ code and converts this code into the assembly language, +understood by the platform. You can see this in action if you head over +to the [GodBolt Compiler Explorer](https://godbolt.org), which shows C++ +being converted into assembly dynamically. + +For example, the `Hello, world!` code snippet above compiles +into the following assembly code: + +```asm +.LC0: + .string "Hello, world!\n" +main: + push rbp + mov rbp, rsp + mov esi, OFFSET FLAT:.LC0 + mov edi, OFFSET FLAT:_ZSt4cout + call std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) + mov eax, 0 + pop rbp + ret +__static_initialization_and_destruction_0(int, int): + push rbp + mov rbp, rsp + sub rsp, 16 + mov DWORD PTR [rbp-4], edi + mov DWORD PTR [rbp-8], esi + cmp DWORD PTR [rbp-4], 1 + jne .L5 + cmp DWORD PTR [rbp-8], 65535 + jne .L5 + mov edi, OFFSET FLAT:_ZStL8__ioinit + call std::ios_base::Init::Init() [complete object constructor] + mov edx, OFFSET FLAT:__dso_handle + mov esi, OFFSET FLAT:_ZStL8__ioinit + mov edi, OFFSET FLAT:_ZNSt8ios_base4InitD1Ev + call __cxa_atexit +.L5: + nop + leave + ret +_GLOBAL__sub_I_main: + push rbp + mov rbp, rsp + mov esi, 65535 + mov edi, 1 + call __static_initialization_and_destruction_0(int, int) + pop rbp + ret +``` + +### Step 3 + +Third, the assembly code generated by the compiler is assembled into the +object code for the platform. Essentially, this is when the compiler +takes the assembly code and assembles it into machine code in a binary +format. After researching this online, I figured out that a lot of +compilers will allow you to stop compilation at this step. This would be +useful for compiling each source code file separately. This saves time +later if a single file changes; only that file needs to be recompiled. + +### Step 4 + +Finally, the object code file generated by the assembler is linked +together with the object code files for any library functions used to +produce a shared library, dynamic library, or an executable file. It +replaces all references to undefined symbols with the correct addresses. |