Files and Directories

Introduction

In the previous chapter we coveredthe basic functions that perform I/O. The discussion centered on I/O for regular files—opening a file, and reading or writing a file. We’ll now look at additionalfeatures of the file system and the properties of a file. We’ll start with the stat functions and go through each member of the stat structure, looking at all the attributes of a file. In this process, we’ll also describe each of the functions that modify these attributes: change the owner, change the permissions, and so on. We’ll also look in more detail at the structure of a UNIX file system and symbolic links. We finish this chapter with the functions that operate on directories, and we develop a function that descendsthrough a directory hierarchy.

stat, fstat, fstatat, and lstat Functions

The discussion in this chapter centers on the four stat functions and the information they return.

#include <sys/stat.h>

int stat(const char *restrict pathname, struct stat *restrict buf);

int fstat(int fd, struct stat *buf);

int lstat(const char *restrict pathname, struct stat *restrict buf);

int fstatat(int fd, const char *restrict pathname, struct stat *restrict buf, int flag);

All four return: 0 if OK, −1 on error

Given a pathname, the stat function returns a structure of information about the named file. The fstat function obtains information about the file that is already open on the descriptor fd. The lstat function is similar to stat, but when the named file is a symbolic link, lstat returns information about the symbolic link, not the file referencedby the symbolic link. (We’ll need lstat in Section 4.22 when we walk down a directory hierarchy. We describe symbolic links in more detail in Section 4.17.)

The fstatat function provides a way to return the file statisticsfor a pathname relative to an open directory represented by the fd argument. The flag argument controls whether symbolic links are followed; when the AT_SYMLINK_NOFOLLOW flag is set, fstatat will not follow symbolic links, but rather returns information about the link itself. Otherwise, the default is to follow symbolic links, returning information about the file to which the symbolic link points. If the fd argument has the value AT_FDCWD and the pathname argument is a relative pathname, then fstatat evaluatesthe pathname argument relative to the current directory. If the pathname argument is an absolute pathname, then the fdargument is ignored. In these two cases, fstatat behaves like either stat or lstat, depending on the value of flag.

The buf argument is a pointer to a structure that we must supply. The functions fill in the structure. The definition of the structure can differ among implementations, but it could look like

struct stat {
mode_t st_mode;/* file type & mode (permissions) */
ino_t st_ino;/* i-node number (serial number) */
dev_t st_dev;/* device number (file system) */
dev_t st_rdev;/* device number for special files */
nlink_t st_nlink;/* number of links */
uid_t st_uid;/* user ID of owner */
gid_t st_gid;/* group ID of owner */
off_t st_size;/* size in bytes, for regular files */
struct timespec st_atim;/* time of last access */
struct timespec st_mtim;/* time of last modification */
struct timespec st_ctim;/* time of last file status change */
blksize_t st_blksize;/* best I/O block size */
blkcnt_t st_blocks;/* number of disk blocks allocated */
};

The st_rdev, st_blksize, and st_blocks fields are not required by POSIX.1. They are defined as part of the XSI option in the Single UNIX Specification.

The timespec structure type defines time in terms of seconds and nanoseconds. It includes at least the following fields:

time_t tv_sec;
long tv_nsec;

Priorto the 2008 version of the standard, the time fields were named st_atime, st_mtime, and st_ctime, and were of type time_t (expressed in seconds). The timespec structure enables higher-resolution timestamps. The old names can be defined in terms of the tv_sec members for compatibility. For example, st_atime can be defined as st_atim.tv_sec.

Note that most members of the stat structure are specified by a primitive system data type (see Section 2.8). We’ll go through each member of this structure to examine the attributes of a file.

The biggest user of the stat functions is probably the ls -l command, to learn all the information about a file.

File Types

We’ve talked about two different types of files so far: regular files and directories. Most files on a UNIX system are either regular files or directories, but there are additionaltypes of files. The types are

1) Regular file. The most common type of file, which contains data of some form. There is no distinction to the UNIX kernel whether this data is text or binary. Any interpretationof the contents of a regular file is left to the application processing the file.

2) Directory file. A file that contains the names of other files and pointers to information on these files. Any process that has read permission for a directory file can read the contents of the directory, but only the kernel can write directly to a directory file. Processes must use the functions described in this chapter to make changes to a directory.

3) Block specialfile. A type of file providing buffered I/O access in fixed-size units to devices such as disk drives.

4) Character specialfile. A type of file providing unbuffered I/O access in variable-sized units to devices. All devices on a system are either block special files or character special files.

5) FIFO. A type of file used for communication between processes. It’s sometimes called a named pipe. We describe FIFOs in Section 15.5.

6) Socket. A type of file used for network communication between processes. A socket can also be used for non-network communication between processes on a single host. We use sockets for interprocess communication in Chapter 16.

7) Symbolic link. A type of file that points to another file. We talk more about symbolic links in Section 4.17.

The type of a file is encoded in the st_mode member of the stat structure. We can determine the file type with the macros shown in Figure 4.1. The argument to each of these macros is the st_mode member from the stat structure.

Macro	Type of file
S_ISREG()	regular file
S_ISDIR()	directory file
S_ISCHR()	character special file
S_ISBLK()	block special file
S_ISFIFO()	pipe or FIFO
S_ISLNK()	symbolic link
S_ISSOCK()	socket

Figure 4.1 File type macros in <sys/stat.h>

POSIX.1 allows implementations to representinterprocess communication (IPC) objects, such as message queues and semaphores, as files. The macros shown in Figure 4.2 allow us to determine the type of IPC object from the stat structure. Instead of taking the st_mode member as an argument, these macros differ from those in Figure 4.1 in that their argument is a pointer to the stat structure.

Macro	Type of object
S_TYPEISMQ()	message queue
S_TYPEISSEM()	semaphore
S_TYPEISSHM()	shared memory object

Figure 4.2 IPC type macros in <sys/stat.h>

Message queues, semaphores, and shared memory objects are discussed in Chapter 15. However, none of the various implementations of the UNIX System discussed in this book represent these objects as files.

Example

The program in Figure 4.3 prints the type of file for each command-line argument.

/**
* 文件名:filedir/filetype.c
* 内容：程序取其命令行参数，然后针对每一个命令行参数打印其文件类型。
* 时间: 2016年 09月 08日星期四 18:53:57 CST
* 作者：firewaywei
*/
#include"apue.h"
int
main(int argc,char*argv[])
{
int i;
struct stat buf;
char*ptr;
for(i =1; i < argc; i++)
{
printf("%s: ", argv[i]);
if(lstat(argv[i],&buf)<0)
{
err_ret("lstat error");
continue;
}
if(S_ISREG(buf.st_mode))
{
ptr ="regular";
}
elseif(S_ISDIR(buf.st_mode))
{
ptr ="directory";
}
elseif(S_ISCHR(buf.st_mode))
{
ptr ="character special";
}
elseif(S_ISBLK(buf.st_mode))
{
ptr ="block special";
}
elseif(S_ISFIFO(buf.st_mode))
{
ptr ="fifo";
}
elseif(S_ISLNK(buf.st_mode))
{
ptr ="symbolic link";
}
elseif(S_ISSOCK(buf.st_mode))
{
ptr ="socket";
}
else
{
ptr ="** unknown mode **";
}
printf("%s\n", ptr);
}
exit(0);
}

Figure 4.3 Print type of file for each command-line argument

Sample output from Figure 4.3 is

$ ./filetype /etc/passwd /etc /dev/log /dev/tty \
> /dev/stdin /dev/ram8
/etc/passwd: regular
/etc: directory
/dev/log: socket
/dev/tty: character special
/dev/stdin: symbolic link
/dev/ram8: block special

(Here, we have explicitlyentered a backslashat the end of the first command line, telling the shell that we want to continue entering the command on another line. The shell then prompted us with its secondary prompt, >, on the next line.) We have specificallyused the lstat function instead of the stat function to detectsymbolic links. If we used the stat function, we would never see symbolic links.

当我们传入的是/etc/passwd文件时，还未执行到程序第19行时，我们用gdb打印buf的信息如下

(gdb) p buf
$11 = {
st_dev = 140737488348000,
st_ino = 140737488347984,
st_nlink = 4131212846,
st_mode = 4195549,
st_uid = 0,
st_gid = 4294967295,
__pad0 = 0,
st_rdev = 140737488348352,
st_size = 140737347998120,
st_blksize = 140737354101952,
st_blocks = 140737354129864,
st_atim = {
tv_sec = 0,
tv_nsec = 1
},
st_mtim = {
tv_sec = 4198701,
tv_nsec = 140737488348096
},
st_ctim = {
tv_sec = 0,
tv_nsec = 4198624
},
__glibc_reserved = {4196432, 140737488348320, 0}
}

等我们执行这条lstat调用之后，再来看buf的信息：

(gdb) p buf
$8 = {
st_dev = 2049,
st_ino = 6294063,
st_nlink = 1,
st_mode = 33188,
st_uid = 0,
st_gid = 0,
__pad0 = 0,
st_rdev = 0,
st_size = 1923,
st_blksize = 4096,
st_blocks = 8,
st_atim = {
tv_sec = 1473379733,
tv_nsec = 332357032
},
st_mtim = {
tv_sec = 1441243055,
tv_nsec = 887790662
},
st_ctim = {
tv_sec = 1441243055,
tv_nsec = 923790841
},
__glibc_reserved = {0, 0, 0}
}

当我们对源码执行gcc -E预编译处理，生成目标文件filetype.i,然后查看该文件，仔细观察S_ISRGE()这个宏被替换如下：

if(((((buf.st_mode))&0170000)==(0100000)))
{
ptr ="regular";
}

==运算符右边的常量表示如下：

二进制	八进制	宏
1000000000000000	0100000	S_IFREG
100000000000000	0040000	S_IFDIR
10000000000000	0020000	S_IFCHR
110000000000000	0060000	S_IFBLK
1000000000000	0010000	S_IFIFO
1010000000000000	0120000	S_IFLNK
1100000000000000	0140000	S_IFSOCK

Historically, early versions of the UNIX System didn’t provide the S_ISxxx macros. Instead, we had to logically AND the st_mode value with the maskS_IFMT and then compare the result with the constants whose names are S_IFxxx. Most systems define this mask and the related constants in the file <sys/stat.h>. If we examine this file, we’ll find the S_ISDIR() macro defined something like

#defineS_ISDIR(mode)(((mode) & S_IFMT) == S_IFDIR)

We’ve said that regularfiles are predominant, but it is interesting to see what percentage of the files on a given system are of each file type. Figure 4.4 shows the counts and percentages for a Linux system that is used as a single-user workstation. This data was obtained from the program shown in Section 4.22.

File type	Count	Percentage(%)
regular file	415,803	79.77
directory	62,197	11.93
symbolic link	40,018	8.25
character special	155	0.03
block special	47	0.01
socket	45	0.01
FIFO	0	0.00

Figure 4.4 Counts and percentages of different file types

Set-User-ID and Set-Group-ID

Every process has six or more IDs associatedwith it. These are shown in Figure 4.5.

real user ID	who we really are
real group ID
effective user ID	used for file access permission checks
effective group ID
supplementary group IDs
saved set-user-ID	saved by exec functions
saved set-group-ID

Figure 4.5 User IDs and group IDs associatedwith each process

The real user ID and real group ID identifywho we really are. These two fields are taken from our entryin the password file when we log in. Normally, these values don’t change during a login session, although there are ways for a superuser process to change them, which we describe in Section 8.11.
The effective user ID, effective group ID, and supplementary group IDs determine our file access permissions, as we describe in the next section. (We defined supplementarygroup IDs in Section 1.8.)
The saved set-user-ID and saved set-group-ID contain copies of the effective user ID and the effective group ID, respectively, when a program is executed. We describe the function of these two saved values when we describe the setuid function in Section 8.11.

The saved IDs are required as of the 2001 version of POSIX.1. They were optional in older versions of POSIX. An application can test for the constant _POSIX_SAVED_IDS at compile time or can call sysconf with the _SC_SAVED_IDS argument at runtime, to see whether the implementation supports this feature.

Normally, the effective user ID equals the real user ID, and the effective group ID equals the real group ID.

Every file has an owner and a group owner. The owner is specifiedby the st_uid member of the stat structure; the group owner, by the st_gid member.

When we executea program file, the effective user ID of the process is usually the real user ID, and the effective group ID is usually the real group ID. However, we can also set a specialflag in the file’s mode word (st_mode) that says, ‘‘When this file is executed, set the effective user ID of the process to be the owner of the file (st_uid).’’ Similarly, we can set another bit in the file’s mode word that causes the effective group ID to be the group owner of the file (st_gid). These two bits in the file’s mode word are called the set-user-ID bit and the set-group-ID bit.

For example, if the owner of the file is the superuser and if the file’s set-user-ID bit is set, then while that program file is running as a process, it has superuser privileges. This happensregardless of the real user ID of the process that executes the file. As an example, the UNIX System program that allows anyone to change his or her password, passwd(1), is a set-user-ID program. This is required so that the program can write the new password to the password file, typicallyeither /etc/passwd or /etc/shadow, files that should be writable only by the superuser. Because a process that is running set-user-ID to some other user usually assumes extra permissions, it must be written carefully. We’ll discuss these types of programs in more detail in Chapter 8.

Returning to the stat function, the set-user-ID bit and the set-group-ID bit are contained in the file’s st_mode value. These two bits can be tested against the constants S_ISUID and S_ISGID, respectively.

File Access Permissions

The st_mode value also encodes the access permission bits for the file. When we say file, we mean any of the file types that we described earlier. All the file types — directories, character special files, and so on—have permissions. Many people think of only regular files as having access permissions.

There are nine permission bits for each file, divided into three categories. They are shown in Figure 4.6.

st_mode mask

Meaning

S_IRUSR

S_IWUSR

S_IXUSR

user-read

user-write

user-execute

S_IRGRP

S_IWGRP

S_IXGRP

group-read

group-write

group-execute

S_IROTH

S_IWOTH

S_IXOTH

other-read

other-write

other-execute

Figure 4.6 The nine file access permission bits, from <sys/stat.h>

The term user in the first three rows in Figure 4.6 refers to the owner of the file. The chmod(1) command, which is typicallyused to modify these nine permission bits, allows us to specifyu for user (owner), g for group, and o for other. Some books refer to these three as owner, group, and world; this is confusing, as the chmod command uses o to mean other, not owner. We’ll use the terms user, group, and other, to be consistentwith the chmod command.

The three categoriesin Figure 4.6 — read, write, and execute—are used in various ways by different functions. We’ll summarize them here, and return to them when we describe the actual functions.

●	The first rule is that whenever we want to open any type of file by name, we must have executepermission in each directory mentioned in the name, including the current directory, if it is implied. This is why the executepermission bit for a directory is often called the search bit. For example, to open the file /usr/include/stdio.h, we need executepermission in the directory /, executepermission in the directory /usr, and executepermission in the directory /usr/include. We then need appropriatepermission for the file itself, depending on how we’re trying to open it: read-only, read–write, and so on. If the current directory is /usr/include, then we need executepermission in the current directory to open the file stdio.h. This is an example of the current directory being implied, not specificallymentioned. It is identicalto our opening the file ./stdio.h. Note that read permission for a directory and execute permission for a directory mean different things. Read permission lets us read the directory, obtaining a list of all the filenames in the directory. Execute permission lets us pass through the directory when it is a componentof a pathname that we are trying to access. (We need to search the directory to look for a specificfilename.) Another example of an implicitdirectory reference is if the PATH environment variable, described in Section 8.10, specifiesa directory that does not have execute permission enabled. In this case, the shell will never find executable files in that directory.
●	The read permission for a file determines whether we can open an existingfile for reading: the O_RDONLY and O_RDWR flags for the open function.
●	The write permission for a file determines whether we can open an existingfile for writing: the O_WRONLY and O_RDWR flags for the open function.
●	We must have write permission for a file to specifythe O_TRUNC flag in the open function.
●	We cannot create a new file in a directory unless we have write permission and execute permission in the directory.
●	To delete an existing file, we need write permission and execute permission in the directory containing the file. We do not need read permission or write permission for the file itself.
●	Execute permission for a file must be on if we want to execute the file using any of the seven exec functions (Section 8.10). The file also has to be a regular file.

The file access tests that the kernel performs each time a process opens, creates, or deletes a file depend on the owners of the file (st_uid and st_gid), the effective IDs of the process (effective user ID and effective group ID), and the supplementary group IDs of the process, if supported. The two owner IDs are properties of the file, whereas the two effective IDs and the supplementary group IDs are properties of the process. The tests performed by the kernel are as follows:

If the effective user ID of the process is 0 (the superuser), access is allowed. This gives the superuser free rein throughoutthe entirefile system.
If the effective user ID of the process equals the owner ID of the file (i.e., the process owns the file), access is allowed if the appropriateuser access permission bit is set. Otherwise, permission is denied. By appropriateaccess permission bit, we mean that if the process is opening the file for reading, the user-read bit must be on. If the process is opening the file for writing, the user-write bit must be on. If the process is executing the file, the user-execute bit must be on.
If the effective group ID of the process or one of the supplementarygroup IDs of the process equals the group ID of the file, access is allowed if the appropriategroup access permission bit is set. Otherwise, permission is denied.
If the appropriate other access permission bit is set, access is allowed. Otherwise, permission is denied.

These four steps are tried in sequence. Note that if the process owns the file (step 2), access is granted or denied based only on the user access permissions; the group permissions are never looked at. Similarly, if the process does not own the file but belongs to an appropriategroup, access is granted or denied based only on the group access permissions; the other permissions are not looked at.

Ownershipof New Files and Directories

When we described the creation of a new file in Chapter 3 using either open or creat, we never said which values were assignedto the user ID and group ID of the new file. We’ll see how to create a new directory in Section 4.21 when we describe the mkdir function. The rules for the ownershipof a new directory are identicalto the rules in this section for the ownershipof a new file.

The user ID of a new file is set to the effective user ID of the process. POSIX.1 allows an implementation to choose one of the following options to determine the group ID of a new file:

The group ID of a new file can be the effective group ID of the process.
The group ID of a new file can be the group ID of the directory in which the file is being created.

FreeBSD 8.0 and Mac OS X 10.6.8 always copy the new file’s group ID from the directory. Several Linux file systems allow the choice between the two options to be selected using a mount(1) command option. The default behavior for Linux 3.2.0 and Solaris 10 is to determine the group ID of a new file depending on whether the set-group-ID bit is set for the directory in which the file is created. If this bit is set, the new file’s group ID is copied from the directory; otherwise, the new file’s group ID is set to the effective group ID of the process.

Using the second option—inheritingthe directory’s group ID—assuresus that all files and directories created in that directory will have the same group ID as the directory. This group ownership of files and directories will then propagatedown the hierarchy from that point. This is used in the Linux directory /var/mail, for example.

As we mentioned earlier, this option for group ownership is the default for FreeBSD 8.0 and Mac OS X 10.6.8, but an option for Linux and Solaris. Under Solaris 10, and by default under Linux 3.2.0, we have to enable the set-group-ID bit, and the mkdir function has to propagatea directory’s set-group-ID bit automatically for this to work. (This is described in Section 4.21.)

access and faccessat Functions

As we described earlier, when we open a file, the kernel performs its access tests based on the effective user and group IDs. Sometimes, however, a process wants to test accessibility based on the real user and group IDs. This is useful when a process is running as someone else, using either the set-user-ID or the set-group-ID feature. Even though a process might be set-user-ID to root, it might still want to verifythat the real user can access a given file. The access and faccessat functions base their tests on the real user and group IDs. (Replace effective with real in the four steps at the end of Section 4.5.)

#include <unistd.h>

int access(const char *pathname, int mode);

int faccessat(int fd, const char *pathname, int mode, int flag);

Both return: 0 if OK, −1 on error

The mode is either the value F_OK to test if a file exists, or the bitwise OR of any of the flags shown in Figure 4.7.

mode	Description
R_OK	test for read permission
W_OK	test for write permission
X_OK	test for execute permission

Figure 4.7 The mode flags for access function, from <unistd.h>

The faccessat function behaves like access when the pathname argument is absolute or when the fd argument has the value AT_FDCWD and the pathname argument is relative. Otherwise, faccessat evaluatesthe pathname relative to the open directory referencedby the fd argument.

The flag argument can be used to change the behavior of faccessat. If the AT_EACCESS flag is set, the access checks are made using the effective user and group IDs of the calling process instead of the real user and group IDs.

Example

Figure 4.8 shows the use of the access function.

/**
* 文件名:filedir/access.c
* 内容：access函数的使用方法
* 时间: 2016年 09月 11日星期日 12:56:09 CST
* 作者：firewaywei
*/
#include"apue.h"
#include<fcntl.h>
int
main(int argc,char*argv[])
{
if(argc !=2)
{
err_quit("usage: a.out <pathname>");
}
if(access(argv[1], R_OK)<0)
{
err_ret("access error for %s", argv[1]);
}
else
{
printf("read access OK\n");
}
if(open(argv[1], O_RDONLY)<0)
{
err_ret("open error for %s", argv[1]);
}
else
{
printf("open for reading OK\n");
}
exit(0);
}

Figure 4.8 Example of access function

Here is a sample session with this program:

$ ls -l access
-rwxrwxr-x 1 fireway fireway 13672 9月 11 12:58 access
$ ./access access
read access OK
open for reading OK
$ ls -l /etc/shadow
-rw-r----- 1 root shadow 1099 9月 3 2015 /etc/shadow
$ ./access /etc/shadow
access error for /etc/shadow: Permission denied
open error for /etc/shadow: Permission denied
$ su   become superuser
密码： enter superuser password
# chown root access change file’s user ID to root
# ls -l access
-rwxrwxr-x 1 root fireway 13672 9月 11 12:58 access
r# chmod u+s access and turn on set-user-ID bit
# ls -l access   check owner and SUID bit
-rwsrwxr-x 1 root fireway 13672 9月 11 12:58 access
# exit   go back to normal user
exit
$ ./access /etc/shadow
access error for /etc/shadow: Permission denied
open for reading OK

In this example, the set-user-ID program can determine that the real user cannot normally read the file, even though the open function will succeed.

ubuntu安装好后，root初始密码（默认密码）不知道，需要设置。
1) 先用安装时候的用户登录进入系统
2) 输入：sudo passwd 按回车
3) 输入新密码，重复输入密码，最后提示passwd：password updated sucessfully。此时已完成root密码的设置
4) 输入：su root。切换用户到root试试

In the precedingexample and in Chapter 8, we’ll sometimes switch to become the superuser to demonstratehow something works. If you’re on a multiuser system and do not have superuser permission, you won’t be able to duplicatethese examples completely.

umask Function

Now that we’ve described the nine permission bits associatedwith every file, we can describe the file mode creation maskthat is associatedwith every process.

The umask function sets the file mode creation maskfor the process and returns the previous value. (This is one of the few functions that doesn’t have an error return.)

#include <sys/stat.h>

mode_t umask(mode_t cmask);

Returns: previous file mode creation mask

The cmask argument is formed as the bitwiseOR of any of the nine constants from Figure 4.6: S_IRUSR, S_IWUSR, and so on.

The file mode creation mask is used whenever the process creates a new file or a new directory. (Recallfrom Sections 3.3 and 3.4 our description of the open and creat functions. Both accept a mode argument that specifiesthe new file’s access permission bits.) We describe how to create a new directory in Section 4.21. Any bits that are on in the file mode creation mask are turned off in the file’s mode.

Example

The program in Figure 4.9 creates two files: one with a umask of 0 and one with a umask that disables all the group and other permission bits.

/**
* 文件名:filedir/umask.c
* 内容：创建了两个文件，创建第一个时，umask值为0，创建第二个时，
* umask值禁止所有组和其他用户的访问权限。
* 时间: 2016年 09月 11日星期日 22:02:50 CST
* 作者：firewaywei
*/
#include"apue.h"
#include<fcntl.h>
#define RWRWRW (S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH)
int
main(void)
{
umask(0);
if(creat("foo", RWRWRW)<0)
{
err_sys("creat error for foo");
}
umask(S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);
if(creat("bar", RWRWRW)<0)
{
err_sys("creat error for bar");
}
exit(0);
}

Figure 4.9 Example of umask function

If we run this program, we can see how the permission bits have been set.

$ umask first print the current file mode creation mask
002
$ ./a.out
$ ls -l foo bar
-rw------- 1 fireway fireway 0 9月 11 22:05 bar
-rw-rw-rw- 1 fireway fireway 0 9月 11 22:05 foo
$ umask see if the file mode creation mask changed
002

Most users of UNIX systems never deal with their umask value. It is usually set once, on login, by the shell’s start-up file, and never changed. Nevertheless, when writing programs that create new files, if we want to ensure that specificaccess permission bits are enabled, we must modify the umask value while the process is running. For example, if we want to ensure that anyone can read a file, we should set the umask to 0. Otherwise, the umask value that is in effect when our process is running can cause permission bits to be turned off.

In the precedingexample, we use the shell’s umask command to print the file mode creation mask both before we run the program and after it completes. This shows us that changing the file mode creation mask of a process doesn’t affectthe mask of its parent (often a shell). All of the shells have a built-in umask command that we can use to set or print the current file mode creation mask.

Users can set the umask value to control the default permissions on the files they create. This value is expressed in octal, with one bit representingone permission to be masked off, as shown in Figure 4.10. Permissions can be denied by setting the correspondingbits. Some common umask values are 002 to prevent others from writing your files, 022 to prevent group members and others from writing your files, and 027 to prevent group members from writing your files and others from reading, writing, or executing your files.

Mask bit	Meaning
0400	user-read
0200	user-write
0100	user-execute
0040	group-read
0020	group-write
0010	group-execute
0004	other-read
0002	other-write
0001	other-execute

Figure 4.10 The umask file access permission bits

The Single UNIX Specification requires that the umask command support a symbolic mode of operation. Unlike the octalformat, the symbolic format specifieswhich permissions are to be allowed (i.e., clear in the file creation mask) instead of which ones are to be denied (i.e., set in the file creation mask). Compare both forms of the command, shown below.

$ umask first print the current file mode creation mask
0002
$ umask -S print the symbolic form
u=rwx,g=rwx,o=rx
$ umask 027 change the file mode creation mask
$ umask -S print the symbolic form
u=rwx,g=rx,o=

chmod, fchmod, and fchmodat Functions

The chmod, fchmod, and fchmodat functions allow us to change the file access permissions for an existing file.

#include <sys/stat.h>

int chmod(const char *pathname, mode_t mode);

int fchmod(int fd, mode_t mode);

int fchmodat(int fd, const char *pathname, mode_t mode, int flag);

All three return: 0 if OK, −1 on error

The chmod function operates on the specifiedfile, whereasthe fchmod function operates on a file that has already been opened. The fchmodat function behaves like chmod when the pathname argument is absolute or when the fd argument has the value AT_FDCWD and the pathname argument is relative. Otherwise, fchmodat evaluatesthe pathname relative to the open directory referenced by the fd argument. The flag argument can be used to change the behavior of fchmodat—when the AT_SYMLINK_NOFOLLOW flag is set, fchmodat doesn’t follow symbolic links.

To change the permission bits of a file, the effective user ID of the process must be equal to the owner ID of the file, or the process must have superuser permissions.

The mode is specifiedas the bitwiseOR of the constants shown in Figure 4.11.

mode

Description

S_ISUID

S_ISGID

S_ISVTX

set-user-ID on execution

set-group-ID on execution

saved-text (sticky bit)

S_IRWXU

S_IRUSR

S_IWUSR

S_IXUSR

read, write, and execute by user (owner)

read by user (owner)

write by user (owner)

execute by user (owner)

S_IRWXG

S_IRGRP

S_IWGRP

S_IXGRP

read, write, and execute by group

read by group

write by group

execute by group

S_IRWXO

S_IROTH

S_IWOTH

S_IXOTH

read, write, and execute by other (world)

read by other (world)

write by other (world)

execute by other (world)

Figure 4.11 The mode constants for chmod functions, from <sys/stat.h>

Note that nine of the entriesin Figure 4.11 are the nine file access permission bits from Figure 4.6. We’ve added the two set-ID constants (S_ISUID and S_ISGID), the saved-text constant (S_ISVTX), and the three combined constants (S_IRWXU, S_IRWXG, and S_IRWXO).

The saved-text bit (S_ISVTX) is not part of POSIX.1. It is defined in the XSI option in the Single UNIX Specification. We describe its purpose in the next section.

Example

Recallthe final state of the files foo and bar when we ran the program in Figure 4.9 to demonstratethe umask function:

$ ls -l foo bar
-rw------- 1 fireway fireway 0 9月 11 22:05 bar
-rw-rw-rw- 1 fireway fireway 0 9月 11 22:05 foo

The program shown in Figure 4.12 modifies the mode of these two files.

/**
* 文件名:filedir/changemod.c
* 内容：修改两个文件的模式
* 时间: 2016年 09月 12日星期一 08:15:10 CST
* 作者：firewaywei
*/
#include"apue.h"
int
main(void)
{
struct stat statbuf;
/* turn on set-group-ID and turn off group-execute */
if(stat("foo",&statbuf)<0)
{
err_sys("stat error for foo");
}
if(chmod("foo",(statbuf.st_mode &~S_IXGRP)| S_ISGID)<0)
{
err_sys("chmod error for foo");
}
/* set absolute mode to "rw-r--r--" */
if(chmod("bar", S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)<0)
{
err_sys("chmod error for bar");
}
exit(0);
}

Figure 4.12 Example of chmod function

After running the program in Figure 4.12, we see that the final state of the two files is

$ ls -l foo bar
-rw-r--r-- 1 fireway fireway 0 9月 11 22:05 bar
-rw-rwSrw- 1 fireway fireway 0 9月 11 22:05 foo

In this example, we have set the permissions of the file bar to an absolute value, regardless of the current permission bits. For the file foo, we set the permissions relative to their current state. To do this, we first call stat to obtain the current permissions and then modify them. We have explicitlyturned on the set-group-ID bit and turned off the group-execute bit. Note that the ls command lists the group-execute permission as S to signifythat the set-group-ID bit is set without the group-execute bit being set.

On Solaris, the ls command displays an l instead of an S to indicatethat mandatoryfile and record locking has been enabled for this file. This behavior applies only to regular files, but we’ll discuss this more in Section 14.3.

Finally, note that the time and date listed by the ls command did not change after we ran the program in Figure 4.12. We’ll see in Section 4.19 that the chmod function updates only the time that the i-node was last changed. By default, the ls -l lists the time when the contents of the file were last modified.

The chmod functions automaticallyclear two of the permission bits under the following conditions:

●

On systems, such as Solaris, that placespecial meaning on the sticky bit when used with regular files, if we try to set the sticky bit (S_ISVTX) on a regular file and do not have superuser privileges, the sticky bit in the mode is automaticallyturned off. (We describe the sticky bit in the next section.) To prevent malicioususers from setting the sticky bit and adverselyaffecting system performance, only the superuser can set the sticky bit of a regular file.

In FreeBSD 8.0 and Solaris 10, only the superuser can set thesticky bit on a regular file. Linux 3.2.0 and Mac OS X 10.6.8 placeno such restrictionon the setting of the sticky bit, because the bit has no meaning when applied to regular files on these systems. Although the bit also has no meaning when applied to regular files on FreeBSD, everyone except the superuser is prevented from setting it on a regular file.

●

The group ID of a newly created file might potentiallybe a group that the calling process does not belong to. Recall from Section 4.6 that it’s possible for the group ID of the new file to be the group ID of the parent directory. Specifically, if the group ID of the new file does not equal either the effective group ID of the process or one of the process’s supplementarygroup IDs and if the process does not have superuser privileges, then the set-group-ID bit is automatically turned off. This prevents a user from creating a set-group-ID file owned by a group that the user doesn’t belong to.

FreeBSD 8.0 fails an attempt to set the set-group-ID in this case. The other systems silentlyturn the bit off, but don’t fail the attempt to change the file access permissions.

FreeBSD 8.0, Linux 3.2.0, Mac OS X 10.6.8, and Solaris 10 add another security feature to try to prevent misuseof some of the protection bits. If a process that does not have superuser privilegeswrites to a file, the set-user-ID and set-group-ID bits are automatically turned off. If malicioususers find a set-group-ID or a set-user-ID file they can write to, even though they can modify the file, they lose the special privileges of the file.

Sticky Bit

The S_ISVTX bit has an interesting history. On versions of the UNIX System that predated demand paging, this bit was known as the sticky bit. If it was set for an executableprogram file, then the first time the program was executed, a copy of the program’s text was saved in the swap area when the process terminated. (The text portion of a program is the machine instructions.) The program would then load into memory more quickly the next time it was executed, because the swap area was handled as a contiguousfile, as compared to the possibly random location of data blocks in a normal UNIX file system. The sticky bit was often set for common application programs, such as the text editor and the passes of the C compiler. Naturally, there was a limit to the number of stickyfiles that could be contained in the swap area before running out of swap space, but it was a useful technique. The name sticky came about because the text portion of the filestuck around in the swap area until the system was rebooted. Later versions of the UNIX System referred to this as the saved-text bit; hencethe constant S_ISVTX. With today’s newer UNIX systems, most of which have a virtual memory system and a faster file system, the need for this technique has disappeared.

On contemporarysystems, the use of the sticky bit has been extended. The Single UNIX Specification allows the sticky bit to be set for a directory. If the bit is set for a directory, a file in the directory can be removed or renamed only if the user has write permission for the directory and meets one of the following criteria:

Owns the file
Owns the directory
Is the superuser

The directories /tmp and /var/tmp are typical candidatesfor thesticky bit—they are directories in which any user can typicallycreate files. The permissions for these two directories are often read, write, and execute for everyone (user, group, and other). But users should not be able to delete or rename files owned by others.

The saved-text bit is not part of POSIX.1. It is part of the XSI option defined in the Single UNIX Specification, and is supported by FreeBSD 8.0, Linux 3.2.0, Mac OS X 10.6.8, and Solaris 10.
Solaris 10 places specialmeaning on the sticky bit if it is set on a regular file. In this case, if none of the execute bits is set, the operating system will not cache the contents of the file.

chown, fchown, fchownat, and lchown Functions

The chown functions allow us to change a file’s user ID and group ID, but if either of the arguments owner or group is −1, the correspondingID is left unchanged.

#include <unistd.h>

int chown(const char *pathname, uid_t owner, gid_t group);

int fchown(int fd, uid_t owner, gid_t group);

int fchownat(int fd, const char *pathname, uid_t owner, gid_t group, int flag);

int lchown(const char *pathname, uid_t owner, gid_t group);

All four return: 0 if OK, −1 on error

These four functions operate similarlyunless the referencedfile is a symboliclink. In that case, lchown and fchownat (with the AT_SYMLINK_NOFOLLOW flag set) change the owners of the symbolic link itself, not the file pointed to by the symbolic link.

The fchown function changes the ownershipof the open file referenced by the fd argument. Since it operates on a file that is already open, it can’t be used to change the ownership of a symboliclink.

The fchownat function behaves like either chown or lchown when the pathname argument is absolute or when the fd argument has the value AT_FDCWD and the pathname argument is relative. In these cases, fchownat acts like lchown if the AT_SYMLINK_NOFOLLOW flag is set in the flag argument, or it acts like chown if the AT_SYMLINK_NOFOLLOW flag is clear. When the fd argument is set to the file descriptorof an open directory and the pathname argument is a relative pathname, fchownat evaluates the pathname relative to the open directory.

Historically, BSD-based systems have enforced the restrictionthat only the superuser can change the ownership of a file. This is to prevent users from giving away their files to others, thereby defeatingany disk space quota restrictions. System V, however, has allowed all users to change the ownership of any files they own.

POSIX.1 allows either form of operation, depending on the value of _POSIX_CHOWN_RESTRICTED.
With Solaris 10, this functionality is a configuration option, whose default value is to enforce the restriction. FreeBSD 8.0, Linux 3.2.0, and Mac OS X 10.6.8 always enforce the chown restriction.

Recallfrom Section 2.6 that the _POSIX_CHOWN_RESTRICTED constant can optionally be defined in the header <unistd.h>, and can always be queried using either the pathconf function or the fpathconf function. Also recallthat this option can depend on the referencedfile; it can be enabled or disabled on a per file system basis. We’ll use the phrase ‘‘if _POSIX_CHOWN_RESTRICTED is in effect,’’ to mean ‘‘if it applies to the particular file that we’re talking about,’’ regardless of whether this actual constant is defined in the header.

If _POSIX_CHOWN_RESTRICTED is in effect for the specifiedfile, then

1) Only a superuser process can change the user ID of the file.

2) A nonsuperuser process can change the group ID of the file if the process owns the file (the effective user ID equals the user ID of the file), owner is specified as −1 or equals the user ID of the file, and group equals either the effective group ID of the process or one of the process’s supplementarygroup IDs.

This means that when _POSIX_CHOWN_RESTRICTED is in effect, you can’t change the user ID of your files. You can change the group ID of files that you own, but only to groups that you belong to.

If these functions are called by a process other than a superuser process, on successful return, both the set-user-ID and the set-group-ID bits are cleared.

File Size

The st_size member of the stat structure contains the size of the file in bytes. This field is meaningful only for regular files, directories, and symboliclinks.

FreeBSD 8.0, Mac OS X 10.6.8, and Solaris 10 also define the file size for a pipe as the number of bytes that are availablefor reading from the pipe. We’ll discuss pipes in Section 15.2.

For a regular file, a file size of 0 is allowed. We’ll get an end-of-file indicationon the first read of the file. For a directory, the file size is usually a multipleof a number, such as 16 or 512. We talk about reading directories in Section 4.22.

For a symbolic link, the file size is the number of bytes in the filename. For example, in the following case, the file size of 7 is the length of the pathname usr/lib:

lrwxrwxrwx 1 fireway fireway 13 9月 26 18:29 apue.link -> src.3e.tar.gz
lrwxrwxrwx 1 fireway fireway 7 9月 26 18:31 apue.link2 -> apue.3e/

(Note that symbolic links do not contain the normal C null byte at the end of the name, as the length is always specifiedby st_size.)

Most contemporaryUNIX systems provide the fields st_blksize and st_blocks. The first is the preferredblock size for I/O for the file, and the latter is the actual number of 512-byte blocks that are allocated. Recall from Section 3.9 that we encounteredthe minimum amount of time required to read a file when we used st_blksize for the read operations. The standard I/O library, which we describe in Chapter 5, also tries to read or write st_blksize bytes at a time, for efficiency.

Be aware that different versions of the UNIX System use units other than 512-byte blocks for st_blocks. Use of this value is nonportable.

Holes in a File

In Section 3.6, we mentioned that a regular file can contain ‘‘holes.’’ We showed an example of this in Figure 3.2. Holes are created by seeking pastthe current end of file and writing some data. As an example, consider the following:

$ ls -l core
-rw-r--r-- 1 sar 8483248 Nov 18 12:18 core
$ du -s core
272cor7

The size of the file core is slightlymore than 8 MB, yet the du command reports that the amount of disk space used by the file is 272 512-byte blocks (139,264 bytes). Obviously, this file has many holes.

The du command on many BSD-derived systems reports the number of 1,024-byte blocks. Solaris reports the number of 512-byte blocks. On Linux, the units reported depend on the whether the POSIXLY_CORRECT environment is set. When it is set, the du command reports 1,024-byte block units; when it is not set, the command reports 512-byte block units.

As we mentioned in Section 3.6, the read function returns data bytes of 0 for any byte positions that have not been written. If we execute the following command, we can see that the normal I/O operations read up through the size of the file:

$ wc -c core
8483248 core

The wc(1) command with the -c option counts the number of characters (bytes) in the file.

If we make a copy of this file, using a utilitysuch as cat(1), all these holes are written out as actual data bytes of 0:

$ cat core > core.copy
$ ls -l core*sar8483248Nov 1812:18 core
-rw-r--r--1
-rw-rw-r--1sar8483248Nov 1812:27 core.copy
$ du -s core*
272core
16592core.copy

Here, the actual number of bytes used by the new file is 8,495,104 (512 × 16,592). The difference between this size and the size reported by ls is caused by the number of blocks used by the file system to hold pointers to the actual data blocks.

Interested readers should refer to Section 4.2 of Bach [1986], Sections 7.2 and 7.3 of McKusick et al. [1996] (or Sections 8.2 and 8.3 in McKusick and Neville-Neil [2005]), Section 15.2 of McDougall and Mauro [2007], and Chapter 12 in Singh [2006] for additional details on the physical layout of files.

File Truncation

Sometimes we would like to truncatea file by choppingoff data at the end of the file. Emptying a file, which we can do with the O_TRUNC flag to open, is a specialcase of truncation.

#include <unistd.h>

int truncate(const char *pathname, off_t length);

int ftruncate(int fd, off_t length);

Both return: 0 if OK, −1 on error

These two functions truncate an existing file to length bytes. If the previous size of the file was greater than length, the data beyond length is no longer accessible. Otherwise, if the previous size was less than length, the file size will increase and the data between the old end of file and the new end of file will read as 0 (i.e., a hole is probably created in the file).

BSD releases prior to 4.4BSD could only make a file smaller with truncate.
Solaris also includes an extension to fcntl (F_FREESP) that allows us to free any part of a file, not just a chunk at the end of the file.

We use ftruncate in the program shown in Figure 13.6 when we need to empty a file after obtaining a lock on the file.

File Systems

To appreciatethe concept of links to a file, we need a conceptualunderstanding of the structure of the UNIX file system. Understanding the difference between an i-node and a directory entry that points to an i-node is also useful.

Variousimplementations of the UNIX file system are in use today. Solaris, for example, supports several types of disk file systems: the traditional BSD-derivedUNIX file system (called UFS), a file system (called PCFS) to read and write DOS-formatted diskettes, and a file system (called HSFS) to read CD file systems. We saw one difference between file system types in Figure 2.20. UFS is based on the Berkeley fast file system, which we describe in this section.

Each file system type has its own characteristic features — and some of these features can be confusing. For example, most UNIX file systems support case-sensitive filenames. Thus, if you create one file named file.txt and another named file.TXT, then two distinctfiles are created. On Mac OS X, however, the HFS file system is case-preservingwith case-insensitive comparisons. Thus, if you create file.txt, when you try to create file.TXT, you will overwrite file.txt. However, only the name used when the file was created is stored in the file system (the case-preserving aspect). In fact, any permutationof uppercase and lowercase letters in the sequencef, i, l, e, ., t, x, t will match when searching for the file (the case-insensitive comparison aspect). As a consequence, besides file.txt and file.TXT, we can access the file with the names File.txt, fILE.tXt, and FiLe.TxT.

We can think of a disk drive being divided into one or more partitions. Each partitioncan contain a file system, as shown in Figure 4.13. The i-nodes are fixed-length entriesthat contain most of the information about a file.

Figure 4.13 Disk drive, partitions, and a file system

If we examine the i-node and data block portion of a cylindergroup in more detail, we could have the arrangementshown in Figure 4.14.

Figure 4.14 Cylinder group’s i-nodes and data blocks in more detail

Note the following points from Figure 4.14.

•	Two directory entriespoint to the same i-node entry. Every i-node has a link count that contains the number of directory entries that point to it. Only when the link count goes to 0 can the file be deleted (thereby releasing the data blocks associated with the file). This is why the operation of ‘‘unlinking a file’’ does not always mean "deleting the blocks associated with the file." This is why the function that removes a directory entry is called unlink, not delete. In the stat structure, the link count is contained in the st_nlink member. Its primitive system data type is nlink_t. These types of links are called hard links. Recall from Section 2.5.2 that the POSIX.1 constant LINK_MAX specifies the maximum value for a file’s link count.
•	The other type of link is called a symbolic link. With a symbolic link, the actual contents of the file—the data blocks—store the name of the file that the symbolic link points to. In the following example, the filename in the directory entry is the three-character string lib and the 7 bytes of data in the file are usr/lib: lrwxrwxrwx 1 root 7 Sep 25 07:14 lib -> usr/lib The file type in the i-node would be S_IFLNK so that the system knows that this is a symbolic link.
•	The i-node contains all the information about the file: the file type, the file’s access permission bits, the size of the file, pointers to the file’s data blocks, and so on. Most of the information in the stat structure is obtained from the i-node. Only two items of interest are stored in the directory entry: the filename and the i-node number. The other items—the length of the filename and the length of the directory record—are not of interest to this discussion. The data type for the i-node number is ino_t.
•	Because the i-node number in the directory entry points to an i-node in the same file system, a directory entry can’t refer to an i-node in a different file system. This is why the ln(1) command (make a new directory entry that points to an existing file) can’t cross file systems. We describe the link function in the next section.
•	When renaming a file without changing file systems, the actual contents of the file need not be moved—all that needs to be done is to add a new directory entry that points to the existing i-node and then unlink the old directory entry. The link count will remain the same. For example, to rename the file /usr/lib/foo to /usr/foo, the contents of the file foo need not be moved if the directories /usr/lib and /usr are on the same file system. This is how the mv(1) command usually operates.

We’ve talked about the concept of a link count for a regular file, but what about the link count field for a directory? Assume that we make a new directory in the working directory, as in

$ mkdir testdir

Figure 4.15 shows the result. Note that in this figure, we explicitly show the entries for dot and dot-dot.

Figure 4.15 Sample cylinder group after creating the directory testdir

The i-node whose number is 2549 has a type field of ‘‘directory’’ and a link count equal to 2. Any leaf directory (a directory that does not contain any other directories) always has a link count of 2. The value of 2 comes from the directory entry that names the directory (testdir) and from the entry for dot in that directory. The i-node whose number is 1267 has a type field of ‘‘directory’’ and a link count that is greater than or equal to 3. We know that this link count is greater than or equal to 3 because, at a minimum, the i-node is pointed to from the directory entry that names it (which we don’t show in Figure 4.15), from dot, and from dot-dot in the testdir directory. Note that every subdirectory in a parent directory causes the parent directory’s link count to be increased by 1.

This format is similar to the classic format of the UNIX file system, which is described in detail in Chapter 4 of Bach [1986]. Refer to Chapter 7 of McKusick et al. [1996] or Chapter 8 of McKusick and Neville-Neil [2005] for additional information on the changes made with the Berkeley fast file system. See Chapter 15 of McDougall and Mauro [2007] for details on UFS, the Solaris version of the Berkeley fast file system. For information on the HFS file system format used in Mac OS X, see Chapter 12 of Singh [2006].

link, linkat, unlink, unlinkat, and remove Functions

As we saw in the previous section, a file can have multiple directory entries pointing to its i-node. We can use either the link function or the linkat function to create a link to an existing file.

#include <unistd.h>

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

int linkat(int efd, const char *existingpath, int nfd, const char *newpath, int ßag);

Both return: 0 if OK, −1 on error

These functions create a new directory entry, newpath, that references the existing file existingpath. If the newpath already exists, an error is returned. Only the last component of the newpath is created. The rest of the path must already exist.

With the linkat function, the existing file is specified by both the efd and existingpath arguments, and the new pathname is specified by both the nfd and newpath arguments. By default, if either pathname is relative, it is evaluated relative to the corresponding file descriptor. If either file descriptor is set to AT_FDCWD, then the corresponding pathname, if it is a relative pathname, is evaluated relative to the current directory. If either pathname is absolute, then the corresponding file descriptor argument is ignored.

When the existing file is a symbolic link, the ßag argument controls whether the linkat function creates a link to the symbolic link or to the file to which the symbolic link points. If the AT_SYMLINK_FOLLOW flag is set in the ßag argument, then a link is created to the target of the symbolic link. If this flag is clear, then a link is created to the symbolic link itself.

The creation of the new directory entry and the increment of the link count must be an atomic operation. (Recall the discussion of atomic operations in Section 3.11.)

Most implementations require that both pathnames be on the same file system, although POSIX.1 allows an implementation to support linking across file systems. If an implementation supports the creation of hard links to directories, it is restricted to only the superuser. This constraint exists because such hard links can cause loops in the file system, which most utilities that process the file system aren’t capable of handling. (We show an example of a loop introduced by a symbolic link in Section 4.17.) Many file system implementations disallow hard links to directories for this reason.

To remove an existing directory entry, we call the unlink function.

#include <unistd.h>

int unlink(const char *pathname);

int unlinkat(int fd, const char *pathname, int ßag);

Both return: 0 if OK, −1 on error

These functions remove the directory entry and decrement the link count of the file referenced by pathname. If there are other links to the file, the data in the file is still accessible through the other links. The file is not changed if an error occurs.

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

Only when the link count reaches 0 can the contents of the file be deleted. One other condition prevents the contents of a file from being deleted: as long as some process has the file open, its contents will not be deleted. When a file is closed, the kernel first checks the count of the number of processes that have the file open. If this count has reached 0, the kernel then checks the link count; if it is 0, the file’s contents are deleted.

If the pathname argument is a relative pathname, then the unlinkat function evaluates the pathname relative to the directory represented by the fd file descriptor argument. If the fd argument is set to the value AT_FDCWD, then the pathname is evaluated relative to the current working directory of the calling process. If the pathname argument is an absolute pathname, then the fd argument is ignored.

The ßag argument gives callers a way to change the default behavior of the unlinkat function. When the AT_REMOVEDIR flag is set, then the unlinkat function can be used to remove a directory, similar to using rmdir. If this flag is clear, then unlinkat operates like unlink.

Example

The program shown in Figure 4.16 opens a file and then unlinks it. The program then goes to sleep for 15 seconds before terminating.

Figure 4.16 Open a file and then unlink it

Running this program gives us

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.

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.

#include <stdio.h>

int remove(const char *pathname);

Returns: 0 if OK, −1 on error

ISO C specifies the remove function to delete a file. The name was changed from the historical UNIX name of unlink because most non-UNIX systems that implement the C standard didn’t support the concept of links to a file at the time.

rename and renameat Functions

A file or a directory is renamed with either the rename or renameat function.

#include <stdio.h>

int rename(const char *oldname, const char *newname);

int renameat(int oldfd, const char *oldname, int newfd, const char *newname);

Both return: 0 if OK, −1 on error

The rename function is defined by ISO C for files. (The C standard doesn’t deal with directories.) POSIX.1 expanded the definition to include directories and symbolic links.

There are several conditions to describe for these functions, depending on whether oldname refers to a file, a directory, or a symbolic link. We must also describe what happens if newname already exists.

If oldname specifies a file that is not a directory, then we are renaming a file or a symbolic link. In this case, if newname exists, it cannot refer to a directory. If newname exists and is not a directory, it is removed, and oldname is renamed to newname. We must have write permission for the directory containing oldname and the directory containing newname, since we are changing both directories.
If oldname specifies a directory, then we are renaming a directory. If newname exists, it must refer to a directory, and that directory must be empty. (When we say that a directory is empty, we mean that the only entries in the directory are dot and dot-dot.) If newname exists and is an empty directory, it is removed, and oldname is renamed to newname. Additionally, when we’re renaming a directory, newname cannot contain a path prefix that names oldname. For example, we can’t rename /usr/foo to /usr/foo/testdir, because the old name (/usr/foo) is a path prefix of the new name and cannot be removed.
If either oldname or newname refers to a symbolic link, then the link itself is processed, not the file to which it resolves.
We can’t rename dot or dot-dot. More precisely, neither dot nor dot-dot can appear as the last component of oldname or newname.
As a special case, if oldname and newname refer to the same file, the function returns successfully without changing anything.

If newname already exists, we need permissions as if we were deleting it. Also, because we’re removing the directory entry for oldname and possibly creating a directory entry for newname, we need write permission and execute permission in the directory containing oldname and in the directory containing newname.

The renameat function provides the same functionality as the rename function, except when either oldname or newname refers to a relative pathname. If oldname specifies a relative pathname, it is evaluated relative to the directory referenced by oldfd. Similarly, newname is evaluated relative to the directory referenced by newfd if newname specifies a relative pathname. Either the oldfd or newfd arguments (or both) can be set to AT_FDCWD to evaluate the corresponding pathname relative to the current directory.

Symbolic Links

A symbolic link is an indirect pointer to a file, unlike the hard links described in the previous section, which pointed directly to the i-node of the file. Symbolic links were introduced to get around the limitations of hard links.

Hard links normally require that the link and the file reside in the same file system.
Only the superuser can create a hard link to a directory (when supported by the underlying file system).

There are no file system limitations on a symbolic link and what it points to, and anyone can create a symbolic link to a directory. Symbolic links are typically used to "move" a file or an entire directory hierarchy to another location on a system.

When using functions that refer to a file by name, we always need to know whether the function follows a symbolic link. If the function follows a symbolic link, a pathname argument to the function refers to the file pointed to by the symbolic link. Otherwise, a pathname argument refers to the link itself, not the file pointed to by the link. Figure 4.17 summarizes whether the functions described in this chapter follow a symbolic link. The functions mkdir, mkfifo, mknod, and rmdir do not appear in this figure, as they return an error when the pathname is a symbolic link. Also, the functions that take a file descriptor argument, such as fstat and fchmod, are not listed, as the function that returns the file descriptor (usually open) handles the symbolic link. Historically, implementations have differed in whether chown follows symbolic links. In all modern systems, however, chown does follow symbolic links.

Symbolic links were introduced with 4.2BSD. Initially, chown didn’t follow symbolic links, but this behavior was changed in 4.4BSD. System V included support for symbolic links in SVR4, but diverged from the original BSD behavior by implementing chown to follow symbolic links. In older versions of Linux (those before version 2.1.81), chown didn’t follow symbolic links. From version 2.1.81 onward, chown follows symbolic links. With FreeBSD 8.0, Mac OS X 10.6.8, and Solaris 10, chown follows symbolic links. All of these platforms provide implementations of lchown to change the ownership of symbolic links themselves.

Figure 4.17 Treatment of symbolic links by various functions

One exception to the behavior summarized in Figure 4.17 occurs when the open function is called with both O_CREAT and O_EXCL set. In this case, if the pathname refers to a symbolic link, open will fail with errno set to EEXIST. This behavior is intended to close a security hole so that privileged processes can’t be fooled into writing to the wrong files.

Example

It is possible to introduce loops into the file system by using symbolic links. Most functions that look up a pathname return an errno of ELOOP when this occurs. Consider the following commands:

This creates a directory foo that contains the file a and a symbolic link that points to foo. We show this arrangement in Figure 4.18, drawing a directory as a circle and a file as a square.

Figure 4.18 Symbolic link testdir that creates a loop

If we write a simple program that uses the standard function ftw(3) on Solaris to descend through a file hierarchy, printing each pathname encountered, the output is

In Section 4.22, we provide our own version of the ftw function that uses lstat instead of stat, to prevent it from following symbolic links.

Note that on Linux, the ftw and nftw functions record all directories seen and avoid processing a directory more than once, so they don’t display this behavior.

A loop of this form is easy to remove. We can unlink the file foo/testdir, as unlink does not follow a symbolic link. But if we create a hard link that forms a loop of this type, its removal is much more difficult. This is why the link function will not form a hard link to a directory unless the process has superuser privileges.

Indeed, Rich Stevens did this on his own system as an experiment while writing the original version of this section. The file system got corrupted and the normal fsck(1) utility couldn’t fix things. The deprecated tools clri(8) and dcheck(8) were needed to repair the file system.
The need for hard links to directories has long since passed. With symbolic links and the mkdir function, there is no longer any need for users to create hard links to directories.

When we open a file, if the pathname passed to open specifies a symbolic link, open follows the link to the specified file. If the file pointed to by the symbolic link doesn’t exist, open returns an error saying that it can’t open the file. This response can confuse users who aren’t familiar with symbolic links. For example,

The file myfile does exist, yet cat says there is no such file, because myfile is a symbolic link and the file pointed to by the symbolic link doesn’t exist. The -l option to ls gives us two hints: the first character is an l, which means a symbolic link, and the sequence -> also indicates a symbolic link. The ls command has another option (-F) that appends an at-sign (@) to filenames that are symbolic links, which can help us spot symbolic links in a directory listing without the -l option.

Creating and Reading Symbolic Links

A symbolic link is created with either the symlink or symlinkat function.

#include <unistd.h>

int symlink(const char *actualpath, const char *sympath);

int symlinkat(const char *actualpath, int fd, const char *sympath);

Both return: 0 if OK, −1 on error

A new directory entry, sympath, is created that points to actualpath. It is not required that actualpath exist when the symbolic link is created. (We saw this in the example at the end of the previous section.) Also, actualpath and sympath need not reside in the same file system.

The symlinkat function is similar to symlink, but the sympath argument is evaluated relative to the directory referenced by the open file descriptor for that directory (specified by the fd argument). If the sympath argument specifies an absolute pathname or if the fd argument has the special value AT_FDCWD, then symlinkat behaves the same way as symlink.

Because the open function follows a symbolic link, we need a way to open the link itself and read the name in the link. The readlink and readlinkat functions do this.

#include <unistd.h>

ssize_t readlink(const char* restrict pathname, char *restrict buf, size_t bufsize);

ssize_t readlinkat(int fd, const char* restrict pathname, char *restrict buf, size_t bufsize);

Both return: number of bytes read if OK, −1 on error

These functions combine the actions of open, read, and close. If successful, they return the number of bytes placed into buf. The contents of the symbolic link that are returned in buf are not null terminated.

The readlinkat function behaves the same way as the readlink function when the pathname argument specifies an absolute pathname or when the fd argument has the special value AT_FDCWD. However, when the fd argument is a valid file descriptor of an open directory and the pathname argument is a relative pathname, then readlinkat evaluates the pathname relative to the open directory represented by fd.

File Times

In Section 4.2, we discussed how the 2008 version of the Single UNIX Specification increased the resolution of the time fields in the stat structure from seconds to seconds plus nanoseconds. The actual resolution stored with each file’s attributes depends on the file system implementation. For file systems that store timestamps in second granularity, the nanoseconds fields will be filled with zeros. For file systems that store timestamps in a resolution higher than seconds, the partial seconds value will be converted into nanoseconds and returned in the nanoseconds fields.

Three time fields are maintained for each file. Their purpose is summarized in Figure 4.19.

Figure 4.19 The three time values associated with each file

Note the difference between the modification time (st_mtim) and the changed-status time (st_ctim). The modification time indicates when the contents of the file were last modified. The changed-status time indicates when the i-node of the file was last modified. In this chapter, we’ve described many operations that affect the i-node without changing the actual contents of the file: changing the file access permissions, changing the user ID, changing the number of links, and so on. Because all the information in the i-node is stored separately from the actual contents of the file, we need the changed-status time, in addition to the modification time.

Note that the system does not maintain the last-access time for an i-node. This is why the functions access and stat, for example, don’t change any of the three times.

The access time is often used by system administrators to delete files that have not been accessed for a certain amount of time. The classic example is the removal of files named a.out or core that haven’t been accessed in the past week. The find(1) command is often used for this type of operation.

The modification time and the changed-status time can be used to archive only those files that have had their contents modified or their i-node modified.

The ls command displays or sorts only on one of the three time values. By default, when invoked with either the -l or the -t option, it uses the modification time of a file. The -u option causes the ls command to use the access time, and the -c option causes it to use the changed-status time.

Figure 4.20 summarizes the effects of the various functions that we’ve described on these three times. Recall from Section 4.14 that a directory is simply a file containing directory entries: filenames and associated i-node numbers. Adding, deleting, or modifying these directory entries can affect the three times associated with that directory. This is why Figure 4.20 contains one column for the three times associated with the file or directory and another column for the three times associated with the parent directory of the referenced file or directory. For example, creating a new file affects the directory that contains the new file, and it affects the i-node for the new file. Reading or writing a file, however, affects only the i-node of the file and has no effect on the directory.

Figure 4.20 Effect of various functions on the access, modification, and changed-status times

(The mkdir and rmdir functions are covered in Section 4.21. The utimes, utimensat, and futimens functions are covered in the next section. The seven exec functions are described in Section 8.10. We describe the mkfifo and pipe functions in Chapter 15.)

futimens, utimensat, and utimes Functions

Several functions are available to change the access time and the modification time of a file. The futimens and utimensat functions provide nanosecond granularity for specifying timestamps, using the timespec structure (the same structure used by the stat family of functions; see Section 4.2).

#include <sys/stat.h>

int futimens(int fd, const struct timespec times[2]);

int utimensat(int fd, const char *path, const struct timespec times[2], int ßag);

Both return: 0 if OK, −1 on error

In both functions, the first element of the times array argument contains the access time, and the second element contains the modification time. The two time values are calendar times, which count seconds since the Epoch, as described in Section 1.10. Partial seconds are expressed in nanoseconds.

Timestamps can be specified in one of four ways:

The privileges required to execute these functions depend on the value of the times argument.

If times is a null pointer or if either tv_nsec field is set to UTIME_NOW, either the effective user ID of the process must equal the owner ID of the file, the process must have write permission for the file, or the process must be a superuser process.
If times is a non-null pointer and either tv_nsec field has a value other than UTIME_NOW or UTIME_OMIT, the effective user ID of the process must equal the owner ID of the file, or the process must be a superuser process. Merely having write permission for the file is not adequate.
If times is a non-null pointer and both tv_nsec fields are set to UTIME_OMIT, no permissions checks are performed.

With futimens, you need to open the file to change its times. The utimensat function provides a way to change a file’s times using the file’s name. The pathname argument is evaluated relative to the fd argument, which is either a file descriptor of an open directory or the special value AT_FDCWD to force evaluation relative to the current directory of the calling process. If pathname specifies an absolute pathname, then the fd argument is ignored.

The ßag argument to utimensat can be used to further modify the default behavior. If the AT_SYMLINK_NOFOLLOW flag is set, then the times of the symbolic link itself are changed (if the pathname refers to a symbolic link). The default behavior is to follow a symbolic link and modify the times of the file to which the link refers.

Both futimens and utimensat are included in POSIX.1. A third function, utimes, is included in the Single UNIX Specification as part of the XSI option.

#include <sys/time.h>

int utimes(const char *pathname, const struct timeval times[2]);

Returns: 0 if OK, −1 on error

The utimes function operates on a pathname. an array of two timestamps—access time and expressed in seconds and microseconds:

struct timeval {
time_t tv_sec;/* seconds */
long tv_usec;/* microseconds */
};

Note that we are unable to specify a value for the changed-status time, st_ctim—the time the i-node was last changed—as this field is automatically updated when the utime function is called.

On some versions of the UNIX System, the touch(1) command uses one of these functions. Also, the standard archive programs, tar(1) and cpio(1), optionally call these functions to set a file’s times to the time values saved when the file was archived.

Example

The program shown in Figure 4.21 truncates files to zero length using the O_TRUNC option of the open function, but does not change their access time or modification time. To do this, the program first obtains the times with the stat function, truncates the file, and then resets the times with the futimens function.

Figure 4.21 Example of futimens function

We can demonstrate the program in Figure 4.21 on Linux with the following commands:

As we would expect, the last-modification times and the last-access times have not changed. The changed-status times, however, have changed to the time that the program was run.

mkdir, mkdirat, and rmdir Functions

Directories are created with the mkdir and mkdirat functions, and deleted with the rmdir function.

#include <sys/stat.h>

int mkdir(const char *pathname, mode_t mode);

int mkdirat(int fd, const char *pathname, mode_t mode);

Both return: 0 if OK, −1 on error

These functions create a new, empty directory. The entries for dot and dot-dot are created automatically. The specified file access permissions, mode, are modified by the file mode creation mask of the process.

A common mistake is to specify the same mode as for a file: read and write permissions only. But for a directory, we normally want at least one of the execute bits enabled, to allow access to filenames within the directory. (See Exercise 4.16.)

The user ID and group ID of the new directory are established according to the rules we described in Section 4.6.

Solaris 10 and Linux 3.2.0 also have the new directory inherit the set-group-ID bit from the parent directory. Files created in the new directory will then inherit the group ID of that directory. With Linux, the file system implementation determines whether this behavior is supported. For example, the ext2, ext3, and ext4 file systems allow this behavior to be controlled by an option to the mount(1) command. With the Linux implementation of the UFS file system, however, the behavior is not selectable; it inherits the set-group-ID bit to mimic the historical BSD implementation, where the group ID of a directory is inherited from the parent directory.
BSD-based implementations don’t propagate the set-group-ID bit; they simply inherit the group ID as a matter of policy. Because FreeBSD 8.0 and Mac OS X 10.6.8 are based on 4.4BSD, they do not require inheriting the set-group-ID bit. On these platforms, newly created files and directories always inherit the group ID of the parent directory, regardless of whether the set-group-ID bit is set.
Earlier versions of the UNIX System did not have the mkdir function; it was introduced with 4.2BSD and SVR3. In the earlier versions, a process had to call the mknod function to create a new directory — but use of the mknod function was restricted to superuser processes. To circumvent this constraint, the normal command that created a directory, mkdir(1), had to be owned by root with the set-user-ID bit on. To create a directory from a process, the mkdir(1) command had to be invoked with the system(3) function.

The mkdirat function is similar to the mkdir function. When the fd argument has the special value AT_FDCWD, or when the pathname argument specifies an absolute pathname, mkdirat behaves exactly like mkdir. Otherwise, the fd argument is an open directory from which relative pathnames will be evaluated.

An empty directory is deleted with the rmdir function. Recall that an empty directory is one that contains entries only for dot and dot-dot.

#include <unistd.h>

int rmdir(const char *pathname);

Returns: 0 if OK, −1 on error

If the link count of the directory becomes 0 with this call, and if no other process has the directory open, then the space occupied by the directory is freed. If one or more processes have the directory open when the link count reaches 0, the last link is removed and the dot and dot-dot entries are removed before this function returns. Additionally, no new files can be created in the directory. The directory is not freed, however, until the last process closes it. (Even though some other process has the directory open, it can’t be doing much in the directory, as the directory had to be empty for the rmdir function to succeed.)

Reading Directories

Directories can be read by anyone who has access permission to read the directory. But only the kernel can write to a directory, to preserve file system sanity. Recall from Section 4.5 that the write permission bits and execute permission bits for a directory determine if we can create new files in the directory and remove files from the directory — they don’t specify if we can write to the directory itself.

The actual format of a directory depends on the UNIX System implementation and the design of the file system. Earlier systems, such as Version 7, had a simple structure: each directory entry was 16 bytes, with 14 bytes for the filename and 2 bytes for the i-node number. When longer filenames were added to 4.2BSD, each entry became variable length, which means that any program that reads a directory is now system dependent. To simplify the process of reading a directory, a set of directory routines were developed and are part of POSIX.1. Many implementations prevent applications from using the read function to access the contents of directories, thereby further isolating applications from the implementation-specific details of directory formats.

#include <dirent.h>

DIR *opendir(const char *pathname);

DIR *fdopendir(int fd);

Both return: pointer if OK, NULL on error

struct dirent *readdir(DIR *dp);

Returns: pointer if OK, NULL at end of directory or error

void rewinddir(DIR *dp);
int closedir(DIR *dp);
Returns: 0 if OK, −1 on error

long telldir(DIR *dp);

Returns: current location in directory associated with dp

void seekdir(DIR *dp, long loc);

The fdopendir function first appeared in version 4 of the Single UNIX Specification. It provides a way to convert an open file descriptor into a DIR structure for use by the directory handling functions.

The telldir and seekdir functions are not part of the base POSIX.1 standard. They are included in the XSI option in the Single UNIX Specification, so all conforming UNIX System implementations are expected to provide them.

Recall our use of several of these functions in the program shown in Figure 1.3, our bare-bones implementation of the ls command.

The dirent structure defined in <dirent.h> is implementation dependent. Implementations define the structure to contain at least the following two members:

ino_td_ino;/* i-node number */
chard_name[];/* null-terminated filename */

The d_ino entry is not defined by POSIX.1, because it is an implementation feature, but it is defined as part of the XSI option in POSIX.1. POSIX.1 defines only the d_name entry in this structure.

Note that the size of the d_name entry isn’t specified, but it is guaranteed to hold at least NAME_MAX characters, not including the terminating null byte (recall Figure 2.15.) Since the filename is null terminated, however, it doesn’t matter how d_name is defined in the header, because the array size doesn’t indicate the length of the filename.

The DIR structure is an internal structure used by these seven functions to maintain information about the directory being read. The purpose of the DIR structure is similar to that of the FILE structure maintained by the standard I/O library, which we describe in Chapter 5.

The pointer to a DIR structure returned by opendir and fdopendir is then used with the other five functions. The opendir function initializes things so that the first readdir returns the first entry in the directory. When the DIR structure is created by fdopendir, the first entry returned by readdir depends on the file offset associated with the file descriptor passed to fdopendir. Note that the ordering of entries within the directory is implementation dependent and is usually not alphabetical.

Example

We’ll use these directory routines to write a program that traverses a file hierarchy. The goal is to produce a count of the various types of files shown in Figure 4.4. The program shown in Figure 4.22 takes a single argument — the starting pathname—and recursively descends the hierarchy from that point. Solaris provides a function, ftw(3), that performs the actual traversal of the hierarchy, calling a user-defined function for each file. The problem with this function is that it calls the stat function for each file, which causes the program to follow symbolic links. For example, if we start at the root and have a symbolic link named /lib that points to /usr/lib, all the files in the directory /usr/lib are counted twice. To correct this problem, Solaris provides an additional function, nftw(3), with an option that stops it from following symbolic links. Although we could use nftw, we’ll write our own simple file walker to show the use of the directory routines.

In SUSv4, nftw is included as part of the XSI option. Implementations are included in FreeBSD 8.0, Linux 3.2.0, Mac OS X 10.6.8, and Solaris 10. (In SUSv4, the ftw function has been marked as obsolescent.) BSD-based systems have a different function, fts(3), that provides similar functionality. It is available in FreeBSD 8.0, Linux 3.2.0, and Mac OS X 10.6.8.

Figure 4.22 Recursively descend a directory hierarchy, counting file types

To illustrate the ftw and nftw functions, we have provided more generality in this program than needed. For example, the function myfunc always returns 0, even though the function that calls it is prepared to handle a nonzero return.

For additional information on descending through a file system and using this technique in many standard UNIX System commands—find, ls, tar, and so on — refer to Fowler, Korn, and Vo [1989].

chdir, fchdir, and getcwd Functions

Every process has a current working directory. This directory is where the search for all relative pathnames starts (i.e., with all pathnames that do not begin with a slash). When a user logs in to a UNIX system, the current working directory normally starts at the directory specified by the sixth field in the /etc/passwd file — the user ’s home directory. The current working directory is an attribute of a process; the home directory is an attribute of a login name.

We can change the current working directory of the calling process by calling the chdir or fchdir function.

#include <unistd.h>

int chdir(const char *pathname);

int fchdir(int fd);

Both return: 0 if OK, −1 on error

We can specify the new current working directory either as a pathname or through an open file descriptor.

Example

Because it is an attribute of a process, the current working directory cannot affect processes that invoke the process that executes the chdir. (We describe the relationship between processes in more detail in Chapter 8.) As a result, the program in Figure 4.23 doesn’t do what we might expect.

Figure 4.23 Example of chdir function

If we compile this program, call the executable mycd, and run it, we get the following:

The current working directory for the shell that executed the mycd program didn’t change. This is a side effect of the way that the shell executes programs. Each program is run in a separate process, so the current working directory of the shell is unaffected by the call to chdir in the program. For this reason, the chdir function has to be called directly from the shell, so the cd command is built into the shells.

Because the kernel must maintain knowledge of the current working directory, we should be able to fetch its current value. Unfortunately, the kernel doesn’t maintain the full pathname of the directory. Instead, the kernel keeps information about the directory, such as a pointer to the directory’s v-node.

The Linux kernel can determine the full pathname. Its components are distributed throughout the mount table and the dcache table, and are reassembled, for example, when you read the /proc/self/cwd symbolic link.

What we need is a function that starts at the current working directory (dot) and works its way up the directory hierarchy, using dot-dot to move up one level. At each level, the function reads the directory entries until it finds the name that corresponds to the i-node of the directory that it just came from. Repeating this procedure until the root is encountered yields the entire absolute pathname of the current working directory. Fortunately, a function already exists that does this work for us.

#include <unistd.h>

char *getcwd(char *buf, size_t size);

Returns: buf if OK, NULL on error

We must pass to this function the address of a buffer, buf, and its size (in bytes). The buffer must be large enough to accommodate the absolute pathname plus a terminating null byte, or else an error will be returned. (Recall the discussion of allocating space for a maximum-sized pathname in Section 2.5.5.)

Some older implementations of getcwd allow the first argument buf to be NULL. In this case, the function calls malloc to allocate size number of bytes dynamically. This is not part of POSIX.1 or the Single UNIX Specification and should be avoided.

Example

The program in Figure 4.24 changes to a specific directory and then calls getcwd to print the working directory. If we run the program, we get

Figure 4.24 Example of getcwd function

Note that chdir follows the symbolic link—as we expect it to, from Figure 4.17 — but when it goes up the directory tree, getcwd has no idea when it hits the /var/spool directory that it is pointed to by the symbolic link /usr/spool. This is a characteristic of symbolic links.

The getcwd function is useful when we have an application that needs to return to the location in the file system where it started out. We can save the starting location by calling getcwd before we change our working directory. After we complete our processing, we can pass the pathname obtained from getcwd to chdir to return to our starting location in the file system.

The fchdir function provides us with an easy way to accomplish this task. Instead of calling getcwd, we can open the current directory and save the file descriptor before we change to a different location in the file system. When we want to return to where we started, we can simply pass the file descriptor to fchdir.

Device Special Files

The two fields st_dev and st_rdev are often confused. We’ll need to use these fields in Section 18.9 when we write the ttyname function. The rules for their use are simple.

•

We can usually access the major and minor device numbers through two macros defined by most implementations: major and minor. Consequently, we don’t care how the two numbers are stored in a dev_t object.

Early systems stored the device number in a 16-bit integer, with 8 bits for the major number and 8 bits for the minor number. FreeBSD 8.0 and Mac OS X 10.6.8 use a 32-bit integer, with 8 bits for the major number and 24 bits for the minor number. On 32-bit systems, Solaris 10 uses a 32-bit integer for dev_t, with 14 bits designated as the major number and 18 bits designated as the minor number. On 64-bit systems, Solaris 10 represents dev_t as a 64-bit integer, with 32 bits for each number. On Linux 3.2.0, although dev_t is a 64-bit integer, only 12 bits are used for the major number and 20 bits are used for the minor number.

POSIX.1 states that the dev_t type exists, but doesn’t define what it contains or how to get at its contents. The macros major and minor are defined by most implementations. Which header they are defined in depends on the system. They can be found in <sys/types.h> on BSD-based systems. Solaris defines their function prototypes in <sys/mkdev.h>, because the macro definitions in <sys/sysmacros.h> are considered obsolete in Solaris. Linux defines these macros in <sys/sysmacros.h>, which is included by <sys/types.h>.

•

The st_dev value for every filename on a system is the device number of the file system containing that filename and its corresponding i-node.

•

Only character special files and block special files have an st_rdev value. This value contains the device number for the actual device.

Example

The program in Figure 4.25 prints the device number for each command-line argument. Additionally, if the argument refers to a character special file or a block special file, the st_rdev value for the special file is printed.

Figure 4.25 Print st_dev and st_rdev values

Running this program on Linux gives us the following output:

The first two arguments to the program are directories (/ and /home/sar), and the next two are the device names /dev/tty[01]. (We use the shell’s regular expression language to shorten the amount of typing we need to do. The shell will expand the string /dev/tty[01] to /dev/tty0 /dev/tty1.)

We expect the devices to be character special files. The output from the program shows that the root directory has a different device number than does the /home/sar directory, which indicates that they are on different file systems. Running the mount(1) command verifies this.

We then use ls to look at the two disk devices reported by mount and the two terminal devices. The two disk devices are block special files, and the two terminal devices are character special files. (Normally, the only types of devices that are block special files are those that can contain random-access file systems—disk drives, floppy disk drives, and CD-ROMs, for example. Some older UNIX systems supported magnetic tapes for file systems, but this was never widely used.)

Note that the filenames and i-nodes for the two terminal devices (st_dev) are on device 0/5—the devtmpfs pseudo file system, which implements the /dev—but that their actual device numbers are 4/0 and 4/1.

Summary of File Access Permission Bits

We’ve covered all the file access permission bits, some of which serve multiple purposes. Figure 4.26 summarizes these permission bits and their interpretation when applied to a regular file and a directory.

Figure 4.26 Summary of file access permission bits

The final nine constants can also be grouped into threes, as follows:

S_IRWXU = S_IRUSR | S_IWUSR | S_IXUSR
S_IRWXG = S_IRGRP | S_IWGRP | S_IXGRP
S_IRWXO = S_IROTH | S_IWOTH | S_IXOTH

Summary

This chapter has centered on the stat function. We’ve gone through each member in the stat structure in detail. This, in turn, led us to examine all the attributes of UNIX files and directories. We’ve looked at how files and directories might be laid out in a file system, and we’ve seen how to navigate the file system namespace. A thorough understanding of all the properties of files and directories and all the functions that operate on them is essential to UNIX programming.

参考

《Advanced Programming in the UNIX Envinronment, 2013》Chapter 4. Files and Directories

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