"C Language" Reading Notes (3) [compilation and execution] "C Language" Reading Notes [programs and programming languages]

This section describes [programming languages] In the Reading Notes of C language. Currently, C language is widely used in many fields, such as games, embedded systems, and Smart appliances. Why not write data directly in assembly or machine language? The reason is that assembly and machine language are affected by the computer architecture. Written directly by means of an architectural assembly or machine instruction.ProgramIt can only run on a computer with this architecture.   The advantage of C language is that Computers of various architectures have their own C compilers, which can compile C programs into machine commands of different architectures, which means programs written in C Language You can compile and run the program on different computers with only a few modifications or even no modifications. .

Hello, world we started from simple hello, world -- some people said, and again, this "Hello, world", many bloggers are not bothered to write such a thing? Well, maybe I wrote something different from what they said? Or you may understand something different from others. First, write a program.

Source program (or source file): a text file created and saved by a programmer in a text editor. The source program is actually a bit sequence consisting of 0 and 1. These 8 digits are in a group and become bytes. Each byte corresponds to a text character through the ASCII character set (most of which are.

The following is the hexadecimal number representation corresponding to the above program

Enter man ASCII on the terminal to view the ASCII table

Procedure:

View hexadecimal format

Reply to text format

 

Let's see what the corresponding binary is like? -- This is also the storage status on the disk.

 

Summary: all the information in the system, including disk files, stored programs, user data stored in the memory, and data transmitted over the network, is represented by a string of bits. Thinking: the same set of byte sequences may represent an integer, floating point number, string, or machine instruction in different contexts. -- This depends on the context of the data object. What should we do? As a programmer, we need to know the machine representation of numbers, because they are different from common integers and real numbers. Usage reference: Use Vim in Linux with xxd to view and edit binary files

The Source translation program can be understood by humans. In order to run on the system, we need to convert the C statement into a series of low-level machine language commands through other programs. Then these commands are packaged in a format called "executable Target Program" and Binary Disk Files . In Linux, the conversion from the source file to the target file (executable target program file) is completed by the compiler driver. For example:

We use the GCC tool to observe each stage. ( Gcc : GNU Compiler Collection , GNU Compiler set )   First, a GCC Command Option diagram is provided.  

  Preprocessing phase

View some source code of Main. I

Conclusion: The pre-processor (CPP) modifies the original C program based on the command starting with "#" And returns another C program (or C program), which usually uses "I" as the file extension.

Compilation phase

 

View the source code of Main. S.

 

Summary: translate a C language text file into an assembly language text file. -- Assembly language is very useful because it provides a general output language for different compilers in different advanced languages.

 

Assembly stage  

Open the source code. Note that it is a binary disk file, and pay attention to the operation steps.

This is garbled and requires: %! Xxd operation

View commands

 

Summary: The assembler (AS) translates main. s into machine language instructions and packs these commandsRelocationFormat of the Target Program and save the result to main. o file, Main. O is a binary file, and its Byte encoding is a machine language instruction rather than a character (So garbled characters may appear in the text compiler ).

 

Link stage  

Perform operations in the assembly phase to view some script codes.

 

Summary: The printf function is called in the program. It is a function of the C standard library. The printf function exists in a separate pre-compiled target file named printf. O. The connector (LD) is responsible for merging the pre-compiled target file of printf. O, and the result is the. out file.

Note: A. out is an executable target file, while Main. O is a relocated target program file. -- The system is responsible for executing the executable target file after it is loaded into the system memory. The target program file that can be relocated is provided to the linker for merging.

To sum up this article, I learned the compilation process of the GCC compiler and became familiar with the entire process. If you have good knowledge, I hope you can give me some advice. You need to know what is going on, and listen to the next decomposition.

Recommended Vim configuration sharing

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