Copy of file descriptor

Copy of file descriptors
dup and dup2 Two very useful system calls , All descriptors are used to copy a file , Make the new file descriptor also identify the file identified by the old file descriptor .

int dup(int oldfd); 
int dup2(int oldfd, int newfd);

dup and dup2 Often used to redirect processes stdin、stdout and stderr.
review ：ls > test.txt

Include header file #include <unistd.h>

dup Function introduction
int dup(int oldfd);

• function ： Copy oldfd File descriptor , And assign a new file descriptor , The new file descriptor is the smallest available in the calling process file descriptor table File descriptor for .
• Parameters ： File descriptor to copy oldfd.
• Return value ： success ： New file descriptor . Failure ： return －1, The error code is stored in errno in .

dup2 Function introduction
int dup2(int oldfd, int newfd)

• function ： Copy an open file descriptor oldfd, And assign a new file descriptor newfd,newfd Also identify oldfd The identified file .
• Be careful ：newfd Is a non negative integer smaller than the maximum allowable value of the file descriptor , If newfd Is an open file descriptor , The file will be closed , And then copy .
• Parameters ： File descriptor to copy oldfd New file descriptor allocated newfd
• Return value ： success ： return newfd Failure ： return -1, The error code is stored in errno in

Case study 1：dup Realization printf Output redirection function

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
    
	// If you need to realize the function of output redirection 
	// First of all, it looks like printf Functions are operation file descriptors 1 The corresponding file ,
	// The default is the operation terminal , As long as you can put 1 The file corresponding to the identification changes , You can achieve output redirection 
	// So after creating the file descriptor corresponding to the file , take 1 File descriptor closed , Then passed dup
	// The new file descriptor copied by the function is 1, such printf Function pair 1 operation , It is written in the file 
	int fd_file;
	fd_file = open("test.txt", O_WRONLY | O_CREAT | O_TRUNC, 0664);
	if(fd_file == -1)
	{
    
		perror("fail to open");
		exit(1);
	}

	close(1);

	int fd = dup(fd_file);
	printf("hello world\n");
	printf("fd = %d\n", fd);

	return 0;
}

Execution results ：

Case study 2：dup Realize output reset backward , Also want standard output , How to achieve

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
    
	int fd1;
	int fd2;
	fd2 = dup(1);
	printf("new:fd2 = %d\n",fd2);

	fd1 = open("test.txt", O_RDWR | O_CREAT, 0664);
	
	close(1);
	int fd3 = dup(fd1);
	printf("hello world\n");
	printf("fd = %d\n", fd3);

	close(1);
	int fd4 = dup(fd2);
	printf("nihao beijing\n");
	printf("fd = %d\n", fd4);

	return 0;
}

Execution results ：

Case study 3：dup2 Realization printf Output redirection

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(void)
{
    
	int fd1;
	int fd2;
	
	fd1 = open("test.txt", O_CREAT | O_WRONLY, 0664);
	if (fd1 < 0)
	{
    
		perror("fail to open");
		exit(1);
	}

    // First close 1 File descriptor , And then fd1 Copy to 1, signify 1 and fd1 All identification test.txt file , The return value follows 1 It's the same 
	fd2 = dup2(fd1, 1);
    printf("hello world\n");
	printf("fd2 = %d\n", fd2);

	return 0;
}

Execution results ：

Case study 4： dup2 Realize output reset backward , Restore the standard output

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
    
	int fd1;
    // If you use dup2, You need to assign an initial value to the variable corresponding to the second parameter 
	int fd2 = 3;
	
    // take 1 Copy one as fd2, therefore fd2 It identifies the standard output 
	dup2(1, fd2);
	printf("fd2 = %d\n", fd2);
	
	fd1 = open("test.txt", O_CREAT | O_RDWR, 0664);

    // Output redirection ： Close the file descriptor 1, take fd1 Copy one as 1, therefore 1 At this time, the symbol is test.txt file 
	dup2(fd1, 1);
	printf("hello world\n");

    // Realize standard output again ： Close the file descriptor 1, take fd2 Copy one as 1, therefore 1 At this time, the standard output is marked 
	dup2(fd2, 1);
	printf(" Hello Beijing \n");
	return 0;
}

Execution results ：