GCC [6] - 4 stages of compilation

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Preface

GCC Compilation is divided into 4 Stages ： Preprocessing 、 compile 、 assembly 、 link
gcc General format of instruction

gcc 【 Options 】 Files to compile 【 Options 】【 The output file 】

Options	explain
-E	control GCC The compiler only preprocesses the source code
-S	control GCC The compiler only deals with the compilation phase of the specified file
-c	control GCC The compiler only deals with the formulated files to the assembly stage , And generate the corresponding target file
-o outfile	Specifies the filename of the output file

eg: Here is a very simple paragraph hello.c Code , Take this code for example

#include <stdio.h>
#define NAME "mao"
void main()
{
    
	printf("hello:%s\n",NAME);
}

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One 、 Pretreatment stage

Preprocessing The main work of this stage is to delete all comments in the program 、 Deal with # The first order . Such as ： Expansion of header file 、 Replacement of macro definitions .
For the above code #include <stdio.h> Indicates the import header file , In the pretreatment stage stdio.h The file will be loaded into my hello.c in , And for the... Used in the code NAME, It will be replaced with the real content defined by the macro mao, You can use the following command to perform preprocessing ： Generate .i file ,linux The suffix of the file after preprocessing in the system is usually .i

gcc -E hello.c -o hello.i

Two 、 Compile phase

gcc In the compilation stage, various checks will be carried out on the preprocessed code （ Lexical analysis , Syntax analysis , Semantic analysis ）, To determine whether our code conforms to the specification , If you generate assembly code （.s Final document ）
Be careful ：gcc -S You can't only compile pre processed .i file , It means control GCC The compiler only processes the specified file to the compilation stage

• If the file to be compiled is preprocessed .i file , be GCC The compiler only needs to compile this file .
• If the file to be compiled is .c perhaps .cpp Source file , be GCC The compiler preprocesses and compiles it ( Two steps ).

gcc -S hello.i -o hello.s

3、 ... and 、 Assembly stage

hold .s The file is translated into binary .o file （ Target file ）
Be careful ：gcc -c It's not just about editing .s file , Its meaning is to control GCC The compiler only processes the specified file to the assembly stage

• If the specified file is .c file , be gcc -c The instruction will execute on the file Preprocessing 、 compile 、 assembly .
• If the specified file is preprocessed .i file , be gcc -c Instruction to compile and assemble the file .
• If the specified file is a compiled file , be gcc -c Instruction only performs assembly operations on this file .

gcc -c hello.s -o hello.o

Four 、 Link phase

The assembly phase compiles the code into binary files , And other components of the system （ For example, standard library 、 Dynamic link library, etc ） Combined to work properly , For example, call print Function print , In the preprocessing stage, it is only “stdio.h” The declaration in the header file is introduced , There is no function implementation , Then how to call it ？
This is the job of linking , Before linking, we deal with files , And linking is the process of packaging multiple files , It combines all object files and system components into an executable .

gcc hello.o -o hello

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