Table of contents
File redirection.
dup
File descriptor copy. Use the existing file descriptor to copy and generate a new file descriptor, and the two file descriptors before and after the function call point to the same file. Save it for later use.
man 2 dup
parameter oldfd
There is already a file descriptor.
return value
new file descriptor.
test code 1
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
int main(int argc, char *argv[])
{
int fd = open(argv[1], O_RDWR); //得到一个文件描述符
int fd1 = dup(fd); //文件描述符拷贝
printf("fd=%d\n", fd);
printf("fd1=%d\n", fd1);
close(fd);
close(fd1);
return 0;
}Test Results
dup2
File descriptor copy. The redirected file descriptor points to. Through this function, the command line "redirection" function can be realized. Make the file descriptor originally pointing to a file point to other specified files.
man 2 dup2
parameter oldfd
An existing file descriptor.
parameter newfd
The file descriptor to be redirected.
return value
Success: newfd
Failed: -1
Test code 2
Read two files, modify the redirection of the second file descriptor to the first file descriptor, then write content to the second file descriptor, modify the file descriptor of 1 and redirect it to the first file, Terminal output content.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
int main(int argc, char *argv[])
{
int fd1 = open(argv[1], O_RDWR);
int fd2 = open(argv[2], O_RDWR);
int flag;
flag= dup2(fd1, fd2); //fd2文件描述符重定向到fd1文件描述符
flag=write(fd2,"hello world!",12);
if(flag<0){
perror("write error");
}
dup2(fd1,STDOUT_FILENO); //终端输出重定向到fd1
printf("\n你好，世界！\n");
close(fd1);
close(fd2);
return 0;
}Test Results
Test code 3
The function of dup is realized by fcntl.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
int main(int argc, char *argv[])
{
int fd1 = open(argv[1], O_RDWR);
printf("fd1:%d\n", fd1);
int fd2=fcntl(fd1,F_DUPFD,0); //已被占用的文件描述符，返回最小可用的
printf("fd2:%d\n",fd2);
int fd3=fcntl(fd1,F_DUPFD,7); //未被占用的文件描述符，返回该值
printf("fd3:%d\n",fd3);
close(fd1);
close(fd2);
close(fd3);
return 0;
}Test Results
