Compilation and linking

One 、 Compilation process

Two 、 Preprocessing

C The one-step operation of the language compiler before compiling the source code .

function	effect
Header file expansion	Replace the header file name with the contents of the header file used in the program
Macro replace	Symbols in the scanner , Replace it with the content defined by the macro
To comment on	Remove the comments from the program
Conditional compilation	Filter out pseudo commands in conditional compilation

3、 ... and 、 compile

The process of compiling is essentially the process of translating high-level language into machine language .

1. morphology 、 grammar 、 Semantic analysis . After confirming that all instructions comply with the rules , Translate it into equivalent assembly code .
2. Code optimization . Delete public expression , Cycle optimization 、 Copying spread 、 Deletion of useless assignment .
3. Target code optimization . Use the values of relevant variables stored in each hardware register of the machine , To reduce the number of accesses to memory .

Four 、 assembly

The assembly process is the process of translating the compiled assembly language code into the target machine instructions .
What is stored in the assembly object file is the machine language code of the object equivalent to the source program . The object file consists of segments . Usually there are at least two segments in a target file .

Paragraph name	effect
Code segment	Program instructions
Data segment	All kinds of global variables or static data to be used in the program

5、 ... and 、 link

The function of link is to complete the connection of various called functions and libraries in the file , And package them together to form an executable .

1. Symbol resolution
   Make sure that the variables used have their unique definitions .
2. relocation
   Through the linker, the data and code in different target files are reallocated to memory addresses , bring CPU Can find instructions from memory .