link, linkat, unlink, unlinkat, and remove Functions

Linux 4.2.0

link, linkat - make a new name for a file

在之前的章节，我们知道一个文件可以有多个

dirctory entries

指向它的

i-node

。我们可以使用

link ， linkat

来创建已经存在文件的链接(link)

原型如下：

#include <unistd.h>

int link(const char *oldpath, const char *newpath);

#include <fcntl.h>           /* Definition of AT_* constants */
#include <unistd.h>

int linkat(int olddirfd, const char *oldpath,
int newdirfd, const char *newpath, int flags);
//Return： 0 if OK, -1 on error

link

link() creates a new link (also known as

a hard link

) to an existing file.

If

newpath

exists, it will not be overwritten.

This new name may be used exactly as the old one for any operation;both names refer to the same file (and so have the same permissions and ownership) and it is impossible to tell which name was the “original”.

linkat

The

linkat()

system call operates in exactly the same way as

link()

, except for the differences described here.：

If the pathname given in

oldpath

is relative, then it is interpreted relative to the directory referred to by the file descriptor

olddirfd

(rather than relative to the current working directory of the calling process, as is done by link() for a relative pathname).

If oldpath is relative and olddirfd is the special value AT_FDCWD, then oldpath is interpreted relative to the current working directory of the calling process (

like link()

).

If oldpath is absolute, then olddirfd is ignored.

The interpretation of

newpath

is as for

oldpath

, except that a relative pathname is interpreted relative to the directory referred to by the file descriptor

newdirfd

.

flags argument

如果flag是

AT_SYMLINK_FOLLOW

,创建的

link

是用于

symbolic link

指向的目标。如果

flag

被清除，则

link

用于

symblic link

本身。

Besides

directory entry

的新建和

link count

的增加必须是原子操作。

大部分实现都需要路径名在同一个文件系统上，尽管POSIX.1允许实现支持

link

能跨越文件系统。支持创建

directories

的

hard link(硬链接)

的实现，都只能让超级用户进行该项操作。这条限制(constraint)是因为

hard link

可能在文件系统中造成循环(loops)，这是文件系统不法处理的。（在4.17节我们会展示symbolic link的loop实例）。因此，许多文件系统不允许

（目录）directories

的硬链接(hard link)。

unlink, unlinkat - delete a name and possibly the file it refers to

原型：

#include <unistd.h>
int unlink(const char *pathname);

#include <fcntl.h>           /* Definition of AT_* constants */
#include <unistd.h>
int unlinkat(int dirfd, const char *pathname, int flags);
//both Return： 0 if OK, -1 on error

这些系统调用会移除目录条目(

directory entry

)并且减少

pathename

引用的文件的链接数目

link count

。如果文件还有其他的链接（link），文件的内容还可以通过其他链接访问。如果产生

error

，文件不会发生变化。

As mentioned earlier, to unlink a file, we must have

write permission

and

execute permission

in the directory containing the directory entry, as it is the directory entry that we will be removing. Also, as mentioned in Section 4.10, if

the sticky bit

is set in this directory we must have

write permission

for the directory and meet one of the following criteria:

• Own the file

• Own the directory

• Have superuser privileges

仅仅当

link count

达到0，文件的内容才会被删除。另一种阻止文件内容被删除的条件就是，一些进程

open

了该文件，文件的内容也不会被删除。当某文件被关闭了(closed)，内核会先确定打开(

open

)该文件的进程数量，如果为0，此时内核会继续确定

link count

，如果还为0，文件内容被删除。

unlinat

pathname

为相对路径，

unlinkat

会将其看做

fd

开的目录的相对路径。

如果

fd

值为

AT_FDCWD

时，

pathname

就是相对路径，相对于调用进程的当前工作目录。如果

pathname

是绝对路径，

fd

直接被忽略。

flag

用于改变

unlinkat

默认行为。

1.

AT_REMOVEDIR

: 用于删除目录，类似于使用

rmdir

命令。

2. clear：

unlinkat

操作和

unlink

一致。

Example

#include <unistd.h>
#include <fcntl.h>

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>

int main()
{
if(open("tempfile", O_RDWR|O_CREAT, S_IRWXO) < 0)
{
fprintf(stderr, "open file error %s\n", strerror(errno));
exit(-1);
}
if(unlink("tempfile") < 0)
{
fprintf(stderr, "unlink file error %s\n", strerror(errno));
exit(-1);
}

printf("file unlinked\n");
sleep(15);
printf("done\n");

return 0;
}

sleep(15),用于睡眠15s，定义在unistd中。

This property of unlink is often used by a program to ensure that a temporary file it creates won’t be left around in case the program crashes. The process creates a file using either

open

or

creat

and then immediately calls

unlink

. The file is not deleted, however, because it is still open. Only when the process either closes the file or terminates, which causes the kernel to close all its open files, is the file deleted.

If

pathname

is a

symbolic link

,

unlink

removes the symbolic link, not the file referenced by the link. There is no function to remove the file referenced by a symbolic link given the name of the link.

The superuser can call

unlink

with pathname specifying a directory if the file system supports it, but the function

rmdir

should be used instead to unlink a directory. We describe the

rmdir

function in Section 4.21.

remove

We can also

unlink a file or a directory

with the

remove

function. For a file, remove is identical to unlink. For a directory, remove is identical to

rmdir

.

remove - remove a file or directory

#include <stdio.h>

int remove(const char *pathname);
//Returns: 0 if OK, -1 on error