1_2.md

Chapter 1: A Tour of Computer System

1.2 Programs Are Translated by Other Programs into Different Forms

本节主要介绍了，源文件被什么程序通过什么处理过程逐渐成为一个可执行程序的。

Why we need to translate a program

• The hello program begins life as a high-level C program because it can be read and understood by human beings in that form.

• In order to run hello.c on the system, the individual C statements must be translated by other programs into a sequence of low-level ==machine-language== instructions.

• These instructions are then packaged in a form called an ==executable object== program and stored as a binary disk file. Object programs are also referred to as executable object files.

• On a Unix system, the translation from source file to object file is performed by a ==compiler driver==.

How to translate a program from a source file to an executable object file


• 4 phases from source file (hello.c) to an executable object file (hello):

  • ==Preprocessing== (hello.c -> hello.i) - 预处理

    • modifies the original C program according to directives that begin with the ==‘#’== character.

    • The result is another C program, typically with the ==.i== suffix.

      gcc -E hello.c 	// show hello.i in console
      or
      gcc -save-temps -S hello.c // save all temp files (.i, .s)
      

  • ==Compilation== (hello.i -> hello.s) - 编译

    • The ==compiler (cc1)== translates the text file hello.i into the text file hello.s, which contains an ==assembly-language== program.

      main:
      	subq	$8, %rsp
      	movl	$.LC0, %edi
      	call	puts
      	movl	$0, %eax
      	addq	%8, %rsp
      	ret

      gcc -S hello.c // keep .s file
      

    • Understanding assembly-language is key to machine-level execution model.

  • ==Assembly== (hello.s -> hello.o) - 汇编

    • the ==assembler (as)== translates hello.s into machine-language instructions, packages them in a form known as a ==relocatable object== program, and stores the result in the object file hello.o.

      gcc -c hello.c	// keep object file
      

    • Disassembling object file:

      objdump -d hello.o
      

  • ==Linking== (hello.o -> hello) - 链接

    • hello program calls the printf function, which is part of the standard C library provided by every C compiler.

    • The printf function resides in a separate precompiled object file called printf.o, which must somehow be merged with our hello.o program.

    • The ==linker (ld)== handles this merging.