In depth analysis - file operation

List of articles

Depth analysis ： data
Depth analysis ： recursive
Depth analysis ： Structure
Depth analysis ： Dynamic memory management
Depth analysis ： File operations

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Preface

Files are a very important part of today's computer system . Files are used to store programs 、 file 、 data 、 graphics 、 Photo 、 video 、 letter 、 Tables and many other kinds of information . As a programmer , Must be able to write programs to create files and read and write data from files . This article will introduce relevant contents .

One 、 Example questions ： Information storage of address book

1. Address book requirements

The address book needs a space to store information , It can be accessed at any time when necessary

Our commonly used address book can store information after use , When we need to retrieve information again , The contents still exist , At this time, we need a file to store relevant information . The content of the file is not necessarily a program , It's the data that the program reads and writes when it runs , For example, the file from which the program needs to read data , Or output content file .

Two 、 File storage

1. standard I/O

I/O Two levels of ( There are two levels of handling file access ) ：

1. Bottom I/O（ low-level I/O ）： Use the basics provided by the operating system I/O .
2. Standard advanced I/O（ standard high-level I/O ）： Use C Library standard package stdio.h Header file definition .

standard I/O The advantages of ：

1. standard I/O There are many specialized functions that simplify dealing with different I/O The problem of .
2. Better portability .
3. Input and output are buffered . in other words , Transfer one large piece of information at a time instead of one byte of information （ Usually at least 512 byte ）.
4. Have higher data transmission rate .

2. Document classification

In programming , There are two kinds of documents we usually talk about ： Program files 、 Data files

Program files : Include source files （ The suffix is .c）, Target file （windows Environment suffix is .obj）, Executable program （windows Environment suffix is .exe）.

Data files : The content of the file is not necessarily a program , It's the data that the program reads and writes when it runs , For example, the file from which the program needs to read data , Or output content file .

3. file type

According to the organization of data , Data files are called text files or binary files

text file : If it's required to use ASCII In the form of code , You need to convert before storing . With ASCII A file stored in the form of characters is a text file .

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Binary : Data is stored in memory in binary form , If the output without conversion is to external memory , Binary files .

4. File buffer

ANSIC The standard is “ Buffer file system ” Processing of data files , The so-called buffer file system means that the system automatically opens up a block in memory for each file being used in the program “ File buffer ”. Data output from memory to disk is first sent to a buffer in memory , After the buffer is filled, it is sent to the disk together . If you read data from disk to computer , Then read the data from the disk file and input it into the memory buffer （ Fill the buffer ）, And then send the data from the buffer to the program data area one by one （ Program variables, etc ）. The size of the buffer depends on C The compiler system decides .

5. The file pointer

Buffer file system , The key concept is “ File type pointer ”, abbreviation “ The file pointer ”

FILE *pf;// File pointer variable 

Each used file has a corresponding file information area in memory , It is used to store information about files （ Such as the name of the document , Document status and The current location of the file, etc ）. This information is stored in a structure variable . The structure type is system declared , The name FILE.

Different C Compiler FILE Types don't contain exactly the same content , But it's the same . Every time you open a file , The system will automatically create a FILE Structural variables , And fill in the information , The user does not have to turn off Heart details .

Definition pf It's a point FILE Pointer variable of type data . You can make pf Point to the file information area of a file （ It's a structural variable ）. Through this article The information in the file information area can access the file . in other words , Through the file pointer variable, you can find the file associated with it .

3、 ... and 、 Opening and closing of files

1. File switch function

- fopen

Call structure

FILE *fopen(" file name . suffix ", " Open mode ");

Return a pointer , You need to determine whether it is a null pointer

- fclose

Call structure

int fclose( File address );

Closing successfully returns 0, Otherwise return to EOF(-1)

Invoke the sample

FILE *P_File = fopen(" file name . suffix ", " Open mode ");
if (P_File == NULL) {
    
	perror(" fail to open file :");
}
fclose(P_File);

2. How to open the file

How files are used	meaning	If the specified file does not exist
“r”（ read-only ）	To enter data , Open an existing text file	error
“w”（ Just write ）	To output data , Open a text file	Create a new file
“a”（ Additional ）	Add data to the end of the text file	error
“rb”（ read-only ）	To enter data , Open a binary file	error
“wb”（ Just write ）	To output data , Open a binary file	Create a new file
“ab”（ Additional ）	Add data to the end of a binary file	error
“r+”（ Reading and writing ）	For reading and writing , Open a text file	error
“w+”（ Reading and writing ）	For reading and writing , Suggest a new file	Create a new file
“a+”（ Reading and writing ）	Open a file , Read and write at the end of the file	Create a new file
“rb+”（ Reading and writing ）	Open a binary file for reading and writing	error
“wb+”（ Reading and writing ）	For reading and writing , Create a new binary file	Create a new file
“ab+”（ Reading and writing ）	Open a binary file , Read and write at the end of the file	Create a new file

Four 、 Sequential reading and writing of files

1. Character input and output （fputc & fgetc）

- fputc

Call structure

int fputc(int , FILE *stream);

Write data to file ( Write from the end )

Invoke the sample

fputc('1', P_File);
fputc('2', P_File);
fputc('3', P_File);

- fgetc

Call structure

int fgetc(FILE *stream);

Read data to file ( Read in sequence from the beginning , Read once and walk backward one space )

1. After reading, return to EOF
2. Normal reading returns ASCII Code value

Invoke the sample

int ret = fgetc(P_File);
printf("%c", ret);

2. Text line I / O function （fputs & fgets）

- fputs

Call structure

int fputs(const char *str, File *stream);

Write data to file ( Write from the end )

Invoke the sample

fputs("abcdef\n", P_File);
fputs("123456\n", P_File);

- fgets

Call structure

char *fgets(char *str, int n, FILE *stream)

The number of reads n Include '/0' The location of , So read at most n - 1 individual

Invoke the sample

fgets(arr, 5, P_File);
printf("%s\n", arr);
fgets(arr, 20, P_File);
printf("%s\n", arr);

3. Format input and output functions （fprintf & fscanf）

- fprintf

Call structure

int fprintf(File *stream,  Back with printf Agreement );

Write data to file ( Write from the end )

Invoke the sample

fprintf(P_File, "%s\n", "hello world!");

- fscanf

Call structure

int fscanf(File *stream,  Back with scanf Agreement );

Read ( From the beginning )

Invoke the sample

fscanf(P_File, "%s", arr);
printf("%s\n", arr);

4. Binary I / O functions （fwrite & fread）

- fwrite

Call structure

int fwrite( The address of the element to be written ,  The size of each element ,  Number of elements , File *stream);

Write... In binary ( The string is written in binary and text, and the result is the same )

Invoke the sample

fwrite(arr, sizeof(char), 20, P_File);

- fread

Call structure

int fread( The address of the element to be read ,  The size of each element ,  Number of elements , File *stream);

Return the number of read complete elements , If the number read < The actual number of readings , Is the last read

Invoke the sample

fread(arr, sizeof(char), 20, P_File);

5、 ... and 、 Random reading and writing of documents and end determination

1. Random reading and writing of documents

- fseek

Call structure

int fseek(" file ",  Offset ,  The starting position of the offset );

Determine the writing position

1. SEEK_CUR - The current position
2. SEEK_SET - Starting position
3. SEEK_END - Ending position ( When pointing to the end , The offset can only be negative )

- fwind

Call structure

int fwind(" file ");

Return the file pointer to the starting position

2. Determination of the end of the document

- feof

Call structure

int feof( File address );

Is to judge the end of reading failure , Or the end of the file

1. Whether the reading of text file is finished , Determine whether the return value is EOF, Or is it NULL etc. . Each function has a specific end flag .
2. Judgment of reading end of binary file , Judge whether the return value is less than the actual number to be read .

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summary

For most programs , Relevant operations of documents are essential . standard I/O The package automatically creates input and output buffers to speed up data transmission . A variety of input and output methods make the program more flexible and convenient , Greatly improve the efficiency of the program , At the same time, it also has good portability .