Program translation and execution, from editing, preprocessing, compilation, assembly, linking to execution

1、 Program translation environment and execution environment

stay C In any implementation of language , There are two environments .

1. Translation environment , In this environment, the source code is converted into executable machine instructions .
2. Running environment , Actually execute the code .

2、 Compile and link

2.1、 A hidden process

Here is a program we are familiar with .

#include<stdio.h>
int main()
{
    
	printf("Hello World\n");
	return 0;
}

We can easily print successfully on our screen by compiling and running in the compiler ,
But in fact , In fact, the compilation stage also includes Preprocessing 、 compile 、 assembly 、 link .
Pictured ：

In general

• The source file of the program passes compile Process is converted into object code , Generate target file （.obj（win In the environment ） or .o（linux Environmental Science ））.
• If there are multiple source files , The linker in the link will bind them together , Finally, an executable file is formed .
• Links combine multiple source files and link libraries , Finally, an executable file is generated . Link libraries provide us with other library functions that need to be introduced .

2.2、 Details of the compilation process

2.2.1、 Preprocessing

   In the preprocessing process, we mainly deal with those in the source code file with # Precompiled instructions at the beginning , Such as #include、#define
The following rules .

• Will all " #define " Delete , And expand all the macro definitions （ All replacement ）.
• Process conditional precompile instructions , Such as #if、#ifdef、#elif、#else、#endif.
• Handle " #include " Precompiling instructions , Insert the included file into the location of the precompiled instruction . Note that this process is recursive , That is, one file may contain another file .
• Delete all comments .
• Keep all #pragma Compiler Directives , Because compilers need to use them .

After pretreatment .i The file no longer contains any macro definitions .

 

2.2.2、 compile

During compilation , Will experience 4 A process , Lexical analysis 、 Syntax analysis 、 Semantic analysis 、 Symbol summary , Convert the source code to Assembly code Then store it in .s In file .

In every process ：

• Lexical analysis ： Through a call lex The program divides the character sequence of the source code into a series of marks , This token has an identifier 、 keyword 、 Literal （ Like numbers 、 character string ）.
• Syntax analysis ： Parsing tokens through a parser , Form a grammar tree , Complete the syntax analysis of some expressions .
• Semantic analysis ： Analyze whether the grammar is meaningful through semantic analyzer （ For example, pointer multiplication is meaningless , But it is legal in grammatical analysis , So semantic analysis is about meaning ）.
• Symbol summary ： It will summarize the global variables and functions in the source code , Form a table , Easy to use in subsequent links .
  For example, yes. Test.c Source file , take Test.c Inside Add and main Summary .
  

Yes add.c, take add.c Inside Add Summary .

2.2.3、 assembly

Compilation will Assembly code convert to Binary instructions , Generate target file （.obj（win In the environment ） or .o（linux Environmental Science ））.

It is worth noting that this process will form a symbol table , During the previous compilation , We summarize global variables and functions , Here we add an address to the members in the summary , It forms a symbol table .

The symbol table will be linked in the following .

2.2.4、 link

There are two actions in the linking process , One is the consolidated segment table , Another is the merging and relocation of symbol tables .

1. Merge segment tables
Formed after compilation .o file , The data in it is linux The environment is made up of elf Stored in this format , such elf Format will divide the file into many segments , Each segment stores different data .

If more than one .o file （ Target file ）, As mentioned earlier, it will link with the link library to form an executable file （.exe）, The executable file is also elf This format , So the previous ones .o In the document was elf The same segments divided by format need to be merged .

2. Merging and relocating symbol tables
In this operation , Merged the address of the symbol table in the assembly , And relocating the addresses of individual members .
such as ：

This will ensure that the program runs , The function we use is valid , But if it doesn't exist Add function , Then the merged address will be invalid , Linked errors will occur after the program is executed .
such as ：

3、 perform

The process of program execution ：

1. The program must be loaded into memory . In an operating system environment ： This is usually done by the operating system . In an independent environment , Program
   The loading of must be manually scheduled , It can also be done by putting executable code into read-only memory .
2. The execution of the procedure begins . And then I call main function .
3. Start executing program code . At this point, the program will use a runtime stack （stack）（ Here you can refer to Function stack frame creation and destruction ）, Store function local variables and return address . Programs can also use static （static） Memory , Variables stored in static memory retain their values throughout the execution of the program .
4. To terminate the program . Normal termination main function ; It could be an accidental termination .

End of this chapter .