C language: 10. Input stream, output stream, error stream

1、 I / O stream · brief introduction

linux Put all the programs , All devices are treated as files .

linux Start a c Language application , It will open by default 3 File stdin、stdout、stderr

Standard input stream stdio

Any application may interact with a file or device , If the program needs to read device information , So it's through stdin Standard input stream to read .
The standard input stream defaults to keyboard .

Standard output stream stdout

stdout It's actually a document , This file can output the output stream to the specified device , For example, network card 、 The printer .
When the standard output stream does nothing , The default is the terminal of the display .（ Print data to the display by default ）

Standard error flow stderr

2、 Input stream 、 Output stream 、 Error flow demonstration

#include <stdio.h>

int main()
{
    
	// Output stream 
	//printf("input the value");// The bottom implementation of this line of code is the following code 
	fprintf(stdout, "input the value\n");
	
	// Input stream 
	int a;
	fscanf(stdin, "%d", &a);//scanf("%d", &a); //fscanf In fact, that is scanf The prototype of the 
	
	// Error flow 
	if (a < 0){
    
		fprintf(stderr, "the value must > 0\n");
		return 1;
	}
	return 0;
}

3、 Redirection mechanism

linux Output stream redirection

Additional ：
Use command >> file name The output stream content is appended to the file . Such as ls >> a.txt It means that you will ls The execution result of the command is output to a.txt in , Every execution will go a.txt Add content to .

ls >> a.txt And ls 1>> a.txt Effect of equivalent

Cover ：
Use command > file name Overwrite the contents of the output stream to the file . Such as ls > a.txt It means that you will ls The execution result of the command is overwritten a.txt in , Every time you execute a.txt Only the results of the last command execution will be preserved in .

linux Input stream redirection

Suppose there is main.c and input.txt Two documents , The contents are as follows ：
/main.c

#include <stdio.h>

int main()
{
    
        printf("please input num 1\n");
        int num1;
        scanf("%d", &num1);
        printf("please input num 2\n");
        int num2;
        scanf("%d", &num2);
        printf("num1 + num2 = %d\n", num1 + num2);
        return 0;
}

/input.txt

2
2

Execute the command below
gcc main.c -o main.out
./main.out < input.txt

The results are as follows ：

linux Error stream redirection

./a.out 1>t.txt 2>f.txt <input.txt
1>t.txt： Redirect the standard output stream to t.txt file
2>f.txt： Redirect the standard error stream to f.txt file
<input.txt： Redirect the standard input stream to input.txt file

end

Whole linux The operating system is composed of several gadgets ;
In use c Language production system tools , Applet time , Cooperation between multiple tools , If they are connected and run, they must be able to judge the right or wrong execution of the program ;
At this point, the standard input stream 、 Output stream 、 The error flow comes in handy ;