My C language learning records (blue bridge) -- files and file input and output

File and file input and output

What is a document

There are different types of documents , In programming , It mainly uses two kinds of documents ：

Program files ： Include source files （ The suffix is .c）, Target file （ The suffix is .o） And executable files . The content of this file is program code .

Data files ： The content of the document is not a program , It's data for reading and writing when the program is running , Including the data output to the disk during the program operation and the data for reading during the program operation , Such as the grade data of the whole class 、 Data of goods transaction, etc .

What we discuss in this section is Data files . In order to simplify users' understanding of input and output devices , Users do not have to distinguish between various input and output devices , operating system Treat all kinds of equipment as documents . For example, the terminal keyboard is an input file , The display and printer are output files .

Classification of documents

According to the organization of data , Data files can be divided into ASCII Files and binaries . Data is stored in binary form in memory , If output to external memory without conversion , Binary files , It can be considered as the image of memory data , Called image file . If it is required to use... In external memory ASCII Code storage , It needs to be converted before storage .ASCII Files are also called text files , One character per byte ASCII code .

use ASCII When outputting in code form, bytes and characters correspond one by one , A byte represents a character , So it's easy to process characters one by one , It's also easy to output characters . But it usually takes up more storage space , And it takes time to switch . Output values in binary form , It can save external storage space and conversion time , Output the contents of the storage unit in the memory intact to the external memory disk , At this time, a byte does not necessarily represent a character .

Open and close files

You should “ open ” This document , After use “ close ” file . actually , The so-called opening a file refers to establishing a corresponding information area for the file （ Used to store information about documents ） And file buffers （ Used to temporarily store input and output data ）.

fopen()  Function to open a data file

fopen()  The function is called in **fopen( file name , How to use files );**.

for example ：

fopen("a1","r");

To open a computer with a name of “ a1 ” The file of , The way to use files is “ Read in ”（r representative read, Read in ）.fopen The return value of the function points to a1 Pointer to file （ namely a1 The starting address of the file information area ）. Will usually fopen The return value of the function is assigned to a pointer variable pointing to the file . Such as ：

FILE *fp;

fp = fopen("a1","r");

such fp On and file a1 It's connected , Or say fp Yes a1 file .

fclose()  Function to close the data file

After using a file, you should close it , To prevent it from being misused .“ close ” Is to undo the file information area and file buffer , Make the file pointer variable no longer point to the file .

fclose()  How to call function ：**fclose( The file pointer );**.

for example ：

fclose(fp);

If you don't close the file, you will lose data , You should get into the habit of closing all files before the program terminates .

Read and write data files in sequence

After opening the file , You can read and write it . When writing in sequence , The data written first is stored in the front of the file , The data written later is stored in the later position in the file . In sequential reading , The order of reading data is consistent with the physical order of data in the file . Sequential reading and writing needs to be realized by library functions .

The function of reading or outputting a character to a text file is shown in table ：

Function name	Call form	function	Return value
fgetc	fgetc(fp)	from fp The file pointed to reads a character	success , Bring back the characters you read ; If it fails, the end of file flag EOF（ namely -1）
fputc	fputc(ch,fp)	Alphabet character ch Write file pointer variable fp In the file pointed to	Output success , The return value is the output character ; Output failed , Then return to EOF（ namely -1）

explain

fgetc First letter of f For documents （file）, In the middle of the get Said to get , the last one c According to the character char.fputc Also similar .

Enter some characters from the keyboard , Send them to disk one by one , Until the user enters a “ # ” until .

Their thinking

use fgetc Function to enter characters one by one from the keyboard , And then use fputc Function to write to a disk file .

establish 12-1.c File and enter the following code ：

#include<stdio.h>

#include<stdlib.h>   // stdlib The header file is standard library Standard library header file

int main(){

    FILE * fp;

    char ch,filename[10];

    printf("Please enter the file name:");

    scanf("%s",filename);

    // If the file doesn't exist ,fopen The file will be created

    if((fp=fopen(filename,"w"))==NULL){    // Open the output file and make fp Point to this file

        printf("Unable to open this file\n");     // If there is an error opening , It outputs “ Cannot be opened ” Information about

        exit(0);     // To terminate the program

    }

    ch=getchar();     // Used to receive the last entered carriage return

    printf("Please enter a string  in the disk（Ends with a #）：");

    ch=getchar();     // Receive the first character entered from the keyboard

    while(ch!='#'){     // When the input # End cycle at

        fputc(ch,fp);

        putchar(ch);

        ch=getchar();

    }

    fclose(fp);

    putchar(10);

    return 0;

}

Enter the following command to compile and run ：

gcc -o 12-1 12-1.c

./12-1

Enter the following command to view the file file_name The content of ：

cat /home/project/file_name

The program runs as follows ：

 

Program analysis ：

• exit  Exist in the standard C In the library function of , The function is to terminate the program , When using this function, the beginning of the program should contain  stdlib.h  The header file .
• The execution process is as follows ： First read a character from the keyboard , Check if it is “ # ”, If it is , Indicates the end of the string , Do not execute loop body . If not , Then execute the loop body , Output this character to file.date . Then the character is displayed on the screen , Then read a character from the keyboard . So again and again , Until it appears “ # ” Until the character . At this time, the program has “ hello shiyanlou ” Write with “ file.date ” In named file .

Read and write data files randomly

It's easier to read and write files in sequence , It's also easy to operate , But it's not efficient . For example, the documents contain information about millions of people in a city , We want to find someone , In order to read and write, you need to read in the first data one by one , The waiting time is unbearable .

To solve this problem , Move the position pointer inside the file to the position where you need to read and write , Read and write again , This kind of reading and writing is called random reading and writing . Random access is not read and write according to the physical location of data in the file , It can access data from any location , Obviously, this method is much more efficient than sequential access .

The key to random reading and writing is to move the position pointer as required , That is, the location of the file .

There are two main functions to move the position pointer inside the file , namely  rewind()  and  fseek().

rewind()  The call form of the function is ：**rewind( The file pointer );**, Its function is to move the position pointer inside the file to the beginning of the file .

Here is the main introduction fseek function .fseek Function is used to move the position pointer inside the file , Its calling form is ：**fseek（ The file pointer , Displacement , The starting point ）;**.

among ：“ The file pointer ” Point to the moved file ;“ Displacement ” Represents the number of bytes moved , The required displacement is long Type data , So that when the file length is greater than 64KB There will be no error when , When the displacement is represented by a constant , Suffix is required “ L ”;“ The starting point ” Indicates where to calculate the displacement , There are three starting points specified ： File header , Current location and end of file .

Its expression is shown in the following table ：

The starting point	A symbol	Digital representation
File header	SEEK_SET	0
The current position	SEEK_CUR	1
end of file	SEEK_END	2

for example ：

fseek(fp,100L,0);

The meaning is to move the position pointer away from the beginning of the file 100 Bytes .

It should also be noted that fseek Functions are generally used in binary files . In the text file due to the need for conversion , Therefore, errors often occur in the calculated position .

Homework

Copy the contents of the file

Introduce

Achieve one C Language program , It can support the simplest replication function .

Knowledge point

• Linux Next C Language programming
• C Language foundation
• C Program input and output processing
• C Language file operation

For example, make a copy  /etc/protocols  File to  /tmp  Catalog , The effect of program execution after compilation is as follows ：

# compile

$ gcc copy.c -o copy

# function

$ ./copy /etc/protocols /tmp/protocols

Note that the copied file and the target file should be passed in as parameters of the compiled executable file , instead of  scanf()  Read , If you use  scanf()  Reading will cause the test system to wait for timeout .

The fragments of program parameter processing are as follows ：

#include <stdlib.h>

#include <stdio.h>

int main(int argc, char *argv[]){

    //...

}

The goal is

• In the experimental building environment on the right WebIDE Create a new document in , Name it  copy.c;
• Copy the program fragment to the document  copy.c  in , And supplement documents ;
• Use a file to complete , Do not use separate header files ;
• copy.c  You can compile and run in the environment , The compiled executable accepts two parameters , They are the source and target file paths of the copy ;
• There are two kinds of errors to deal with ： The copied source file does not exist , Or the copied target file already exists . When an error occurs, you need to print a with  error  Output information of .

Refer to the answer

#include<stdio.h>

#include<stdlib.h>

int main(int argc,char **argv){ //tips: main Functions should be able to receive parameters on the command line

    if(argc<3)

        return 1;

    // Get the file name

    char* file1 = argv[1];

    char* file2 = argv[2];

    FILE  *fp1,*fp2;

    char ch;

    // Read source file

    if((fp1=fopen(file1,"r"))==NULL)

       printf("File isn't exist!\n");

    // Create destination file

    if((fp2=fopen(file2,"w"))==NULL)

       printf("File can't be created!\n");

    // Read and write files

    while((ch=fgetc(fp1))!=EOF)

        fputc(ch,fp2);

    fclose(fp1);

    fclose(fp2);

    return 0;

}

copy