Compilation and link of C language program

Preface ：

Once the “hello word” Shocking world , Let countless apes flock . Now let me talk about my understanding of this note .

The process of the birth of notes

I take c/c++ give an example , Use the compiler as the panel , The linker is the string , Create a zither . Last use c The code waves the strings for everyone , Feel the birth of this wonderful note .

Detailed explanation of zither panel - compiler

The functional core of the compiler ：“c The language code -->（ precompile ）–> Assembly code -->( Compile optimization )–> Machine code ”

1. precompile

Preprocessing instructions contain macro definitions , Conditional compilation , The header file contains , Special symbols, etc ; The compiler will first process the preprocessing instructions , abbreviation precompile , precompiled The output file in , Constant only ; Like numbers 、 character string 、 Definition of variables , as well as C Keywords of language , Such as main, if , else , for , while , { , } , + , - , * , \ wait .

• Preprocessing ：#pragma #error
#pragma Instructions can be followed by parameters , And according to the parameter content , Number, etc , Its functions vary greatly . The most commonly used instruction is #pragma once, Used to prevent repeated references to header files .

• Macro definition ：#define 【SrcName】 【DesName】
  Direct will DesName Replace with SrcName, For subsequent use of macros, use SrcName.

• Conditional compilation ：#if #else #elif #endif #ifdef #ifndef
  Conditional compilation allows programmers to selectively compile code segments . therefore , The compiler will filter useless code segments according to conditional compilation statements , Do not participate in the compilation process （ Even if the error is not reported ）.

• The header file contains ：*#include <> #include ""*
  Use #include The main purpose of is to make different source programs use common variables , Avoid repeating definitions .
  <> It mainly includes “/ usr/ include” System files under the path .
  “” It mainly includes programmer's customized header files , The storage path is generally at the same level as the system path .

• Special symbols line FILE
  #line Will be replaced with line number ,FILE Replaced file name, etc ;

2. compile , Optimize

The precompiled output file is used as compilation , Optimization of the Input file . Through grammar , lexing , take c Code conversion to equivalent assembly code ;

• compile
  According to the preprocessed output file , Will conform to the grammar , Lexical statements are translated into equivalent assembly language ;

• Optimize
  Compiler optimization is the most complex compilation stage , In terms of details , It will delete invalid code , Delete public expression , Reduce cycle strength and other ways to optimize code ; In general , Optimization can be divided into assembly code optimization and machine code optimization , The former is related to computers , The latter is closely related to the hardware structure .

3. assembly

Assembly process actually refers to the process of translating assembly language code into target machine instructions . The generated relocatable file consists of data segment and code segment . It can create an executable or shared object file through links .

Detailed explanation of zither strings - The linker

The linker associates machine language code with related files （ Symbol table parsing , relocation ）, The process of generating executable files ; For example, a function or variable in a header file is called in a function , Just link that header file and its specific implementation file .

1. Static links
The linker ： Copy the code in the object file to the final executable ;
Static link library ： A collection of object codes ;
Static link is to copy the relevant code to the corresponding location of the source code , Participate in program generation .

2. Dynamic links
In the process of program generation , Only a few symbols required for normal compilation are provided . During program execution , Mapping all contents in the dynamic link library to the address space of the corresponding process , Ensure the normal execution and calling of the program .

Zither playing - Note birth

The four processes of note birth are summarized as follows ： Precompiling (.c) —> compile 、 The optimizer （.s、.asm）—> Sink programming (.obj、.o、.a、.ko) —> Link program （.exe、.elf、.axf etc. ）. Understanding of the compilation process , I'm still at the threshold , Need to learn more . Welcome to consult each other , Common progress

about GCC compiler , The compilation instructions corresponding to the above process are as follows ：