02. Use of gcc compiler

One 、GCC The build process

One C/C++ The document goes through Preprocessing (preprocessing)、 compile (compilation)、 assembly (assembly) and link (linking) etc. 4 Step to become an executable file .

gcc The compiler can control the whole process by specifying different compilation options  

（1） Preprocessing

C/C++ In the source file , With “#” The first command is called a preprocessing command , If it contains a command “#include”、 Macro definition command “#define”、 Conditional compilation command “#if”、“#ifdef” etc. . Preprocessing is about to include (include) Insert the file into the original file 、 Expand the macro definition 、 Select the code to use according to the conditional compilation command , Finally, output these things to a “.i” Waiting for further processing in the file .

（2） compile

Compiling is to put C/C++ Code ( Such as the above “.i” file )“ translate ” Into assembly code , The tools used are cc1( Its name is cc1,x86 Have their own cc1 command ,ARM The board also has its own cc1 command ).

（3） assembly

Assembly is to translate the assembly code output in the second step into machine code in a certain format , stay Linux The general performance of the system is ELF Target file (OBJ file ), The tools used are as.x86 Have their own as command ,ARM The version also has its own as command , It could be xxxx-as（ such as arm-linux-as）.

“ Disassembly ” It refers to converting machine code into assembly code , This is often used in debugging programs .

（4） link

The link will be generated in the previous step OBJ Of file and system libraries OBJ file 、 Linking library files , Finally, an executable file that can run on a specific platform is generated , The tools used are ld or collect2.

Two 、  Execute the whole compilation process in one step      

          The program code is as follows ：

#include <stdio.h>

#define MAX 20
#define MIN 10

//#define _DEBUG

int main(int argc,char *argv[])
{
	printf("Hello world\r\n");
	printf("MAX = %d,MIN = %d,MAX + MIN = %d\r\n",MAX,MIN,MAX + MIN);

#ifdef _DEBUG
	printf("Hello world\r\n");
    printf("MAX = %d,MIN = %d,MAX - MIN = %d\r\n",MAX,MIN,MAX - MIN);
#endif

	return 0;
}

Preprocessing ： 

gcc -E -o hello.i hello.c 

It means that you will hello.c Document preprocessing , All information is stored in hello.i in .

        Open generated hello.i, It is found that the header file to be included has been included in the preprocessing stage , The macro definition is expanded . Now we'll put the... In the program _DEBUG Macro definition open , It is found that the content of conditional compilation appears .

compile ： 

gcc -S -o hello.s hello.i 

  The preprocessed content can be compiled and generated by using the above command .s file , The contents of the document are as follows ：

assembly ： 

gcc -c -o hello.o hello.s

Use the above command to .s Files can be programmed .o file , That is, the assembly process .

link ：

gcc -o hello hello.o

The last step is to add no parameters , The system can be .o Documents and what you wrote .o The file is linked into executable APP

3、 ... and 、 Common compilation options

Common options	describe
-E	Preprocessing , During the development process, you want to quickly determine that a macro can be used “-E  -dM”
-c	Pretreatment 、 compile 、 The compilation has been done , But don't link
-o	Specify output file
-I	Specify the header directory
-L	Specify the link time library file directory
-l	Specify which library file to link

Usually use the command first ：

        gcc -c -o main.o main.c Command preprocessing 、 compile 、 The assembly is complete , Finally, use it by yourself gcc -o xxx.o Link . 

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Be careful ：

        gcc The compiler will not check your errors during and processing , It checks for errors in the compilation step .