unix学习笔记-------利用dup()函数来达到共享同一个文件表项的作用

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dup函数

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源代码如下：

dup()函数的使用：

1.dup()函数的作用是什么？？？

利用dup()函数来达到共享同一个文件表项的作用。

（个人）：

因为一个文件描述符指向一个打开文件文件对象，所以，如果俩个文件描述符的内容是相同的话，那么这两个文件描述符指向的是同一个文件表项。

即：指向的是同一个文件。

所以：dup()的作用就是：把一个文件描述符的内容复制到另一个新的文件描述符上去。

2.函数原型：

DUP(2) Linux Programmer鈥檚Manual DUP(2)

NAME

dup, dup2, dup3 - duplicate a file descriptor

remark: duplicate 报错

英 [ˈdjuːplɪkeɪt] 美 ['duplɪket] 跟读口语练习

vt. 复制；使加倍

n. 副本；复制品

SYNOPSIS

#include <unistd.h>

int dup(int oldfd);

int dup2(int oldfd, int newfd);

#define _GNU_SOURCE

#include <unistd.h>

int dup3(int oldfd, int newfd, int flags);

DESCRIPTION

These system calls create a copy of the file descriptor oldfd.

dup() uses the lowest-numbered unused descriptor for the new descriptor.

dup2() makes newfd be the copy of oldfd, closing newfd first ifnecessary, but note the following:

* If oldfd is not a valid filedescriptor, then the call fails, and newfd is not closed.

* If oldfd is a valid filedescriptor, and newfd has the same value as oldfd, then dup2() does nothing,and returns newfd.

After a successful return from one of these systemcalls, the old and new file descriptors may be used interchangeably. They refer to the same open file description

(see open(2)) and thus share file offset and file status flags; forexample, if the file offset is modified by using lseek(2) on one of thedescriptors, the offset is

also changed for the other.

The two descriptors do not sharefile descriptor flags (the close-on-exec flag). The close-on-exec flag (FD_CLOEXEC; see fcntl(2)) for the duplicatedescriptor is off.

dup3() is the same as dup2(), except that:

* The caller can force theclose-on-exec flag to be set for the new file descriptor by specifyingO_CLOEXEC in flags. See the descriptionof the same flag in open(2)

for reasons why this may be useful.

* If oldfd equals newfd, thendup3() fails with the error EINVAL.

RETURN VALUE

On success, these system calls return the new descriptor. On error, -1 is returned, and errno is setappropriately

#include<unistd.h>

#include<stdio.h>

#include<sys/types.h>

#include<fcntl.h>

int main()

{

//usethe open () function

intfd_t_1;

intfd_t_new;

charbuf[50];

fd_t_1 =open("/home/code/file1/my.txt",O_RDWR,775);

if(fd_t_1<0)

{

printf("openerror\n");

_exit(0);

}

else

printf("thefile descriptor is : %d \n",fd_t_1);

//call the dup()

fd_t_new =dup(fd_t_1);

printf("the new file descriptor is :%d\n",fd_t_new);

//the new descriptor is for

int rd_t=read(fd_t_new,&buf,20);

if(rd_t <0)

{

perror("readerror \n");

exit(0);

}

printf("the contents of reading is :\n");

printf("%s\n",buf);

return 0;

}

运行截图：

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