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Interlude: Files and Directories
Thus far we have seen the development of two key operating system ab-
stractions: the process, which is a virtualization of the CPU, and the ad-
dress space, which is a virtualization of memory. In tandem, these two
abstractions allow a program to run as if it is in its own private, isolated
world; as if it has its own processor (or processors); as if it has its own
memory. This illusion makes programming the system much easier and
thus is prevalent today not only on desktops and servers but increasingly
on all programmable platforms including mobile phones and the like.
In this section, we add one more critical piece to the virtualization puz-
zle: persistent storage. A persistent-storage device, such as a classic hard
disk drive or a more modern solid-state storage device, stores informa-
tion permanently (or at least, for a long time). Unlike memory, whose
contents are lost when there is a power loss, a persistent-storage device
keeps such data intact. Thus, the OS must take extra care with such a
device: this is where users keep data that they really care about.
CRUX: HOW TO MANAGE A PERSISTENT DEVICE
How should the OS manage a persistent device? What are the APIs?
What are the important aspects of the implementation?
Thus, in the next few chapters, we will explore critical techniques for
managing persistent data, focusing on methods to improve performance
and reliability. We begin, however, with an overview of the API: the in-
terfaces you’ll expect to see when interacting with a UNIX file system.
39.1 Files And Directories
Two key abstractions have developed over time in the virtualization
of storage. The first is the file. A file is simply a linear array of bytes,
each of which you can read or write. Each file has some kind of low-level
name, usually a number of some kind; often, the user is not aware of
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/
foo
bar.txt
bar
foobar
bar.txt
Figure 39.1: An Example Directory Tree
this name (as we will see). For historical reasons, the low-level name of a
file is often referred to as its inode number. We’ll be learning a lot more
about inodes in future chapters; for now, just assume that each file has an
inode number associated with it.
In most systems, the OS does not know much about the structure of
the file (e.g., whether it is a picture, or a text file, or C code); rather, the
responsibility of the file system is simply to store such data persistently
on disk and make sure that when you request the data again, you get
what you put there in the first place. Doing so is not as simple as it seems!
The second abstraction is that of a directory. A directory, like a file,
also has a low-level name (i.e., an inode number), but its contents are
quite specific: it contains a list of (user-readable name, low-level name)
pairs. For example, let’s say there is a file with the low-level name “10”,
and it is referred to by the user-readable name of “foo”. The directory
that “foo” resides in thus would have an entry (“foo”, “10”) that maps
the user-readable name to the low-level name. Each entry in a directory
refers to either files or other directories. By placing directories within
other directories, users are able to build an arbitrary directory tree (or
directory hierarchy), under which all files and directories are stored.
The directory hierarchy starts at a root directory (in UNIX-based sys-
tems, the root directory is simply referred to as /) and uses some kind
of separator to name subsequent sub-directories until the desired file or
directory is named. For example, if a user created a directory foo in the
root directory /, and then created a file bar.txt in the directory foo,
we could refer to the file by its absolute pathname, which in this case
would be /foo/bar.txt. See Figure 39.1 for a more complex directory
tree; valid directories in the example are /, /foo, /bar, /bar/bar,
/bar/foo and valid files are /foo/bar.txt and /bar/foo/bar.txt.
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TIP: THINK CAREFULLY ABOUT NAMING
Naming is an important aspect of computer systems [SK09]. In UNIX
systems, virtually everything that you can think of is named through the
file system. Beyond just files, devices, pipes, and even processes [K84]
can be found in what looks like a plain old file system. This uniformity
of naming eases your conceptual model of the system, and makes the
system simpler and more modular. Thus, whenever creating a system or
interface, think carefully about what names you are using.
Directories and files can have the same name as long as they are in dif-
ferent locations in the file-system tree (e.g., there are two files named
bar.txt in the figure, /foo/bar.txt and /bar/foo/bar.txt).
You may also notice that the file name in this example often has two
parts: bar and txt, separated by a period. The first part is an arbitrary
name, whereas the second part of the file name is usually used to indi-
cate the type of the file, e.g., whether it is C code (e.g., .c), or an image
(e.g., .jpg), or a music file (e.g., .mp3). However, this is usually just a
convention: there is usually no enforcement that the data contained in a
file named main.c is indeed C source code.
Thus, we can see one great thing provided by the file system: a conve-
nient way to name all the files we are interested in. Names are important
in systems as the first step to accessing any resource is being able to name
it. In UNIX systems, the file system thus provides a unified way to access
files on disk, USB stick, CD-ROM, many other devices, and in fact many
other things, all located under the single directory tree.
39.2 The File System Interface
Let’s now discuss the file system interface in more detail. We’ll start
with the basics of creating, accessing, and deleting files. You may think
this is straightforward, but along the way we’ll discover the mysterious
call that is used to remove files, known as unlink(). Hopefully, by the
end of this chapter, this mystery won’t be so mysterious to you!
39.3 Creating Files
We’ll start with the most basic of operations: creating a file. This can be
accomplished with the open system call; by calling open() and passing
it the O CREAT flag, a program can create a new file. Here is some exam-
ple code to create a file called “foo” in the current working directory:
int fd = open(“foo”, O_CREAT|O_WRONLY|O_TRUNC,
S_IRUSR|S_IWUSR);
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ASIDE: THE CREAT() SYSTEM CALL
The older way of creating a file is to call creat(), as follows:
// option: add second flag to set permissions
int fd = creat(“foo”);
You can think of creat() as open() with the following flags: O CREAT
| O WRONLY | O TRUNC. Because open() can create a file, the usage
of creat() has somewhat fallen out of favor (indeed, it could just be
implemented as a library call to open()); however, it does hold a special
place in UNIX lore. Specifically, when Ken Thompson was asked what he
would do differently if he were redesigning UNIX, he replied: “I’d spell
creat with an e.”
The routine open() takes a number of different flags. In this exam-
ple, the second parameter creates the file (O CREAT) if it does not exist,
ensures that the file can only be written to (O WRONLY), and, if the file
already exists, truncates it to a size of zero bytes thus removing any exist-
ing content (O TRUNC). The third parameter specifies permissions, in this
case making the file readable and writable by the owner.
One important aspect of open() is what it returns: a file descriptor. A
file descriptor is just an integer, private per process, and is used in UNIX
systems to access files; thus, once a file is opened, you use the file de-
scriptor to read or write the file, assuming you have permission to do so.
In this way, a file descriptor is a capability [L84], i.e., an opaque handle
that gives you the power to perform certain operations. Another way to
think of a file descriptor is as a pointer to an object of type file; once you
have such an object, you can call other “methods” to access the file, like
read() and write() (we’ll see how to do so below).
As stated above, file descriptors are managed by the operating system
on a per-process basis. This means some kind of simple structure (e.g., an
array) is kept in the proc structure on UNIX systems. Here is the relevant
piece from the xv6 kernel [CK+08]:
struct proc {
…
struct file *ofile[NOFILE]; // Open files
…
};
A simple array (with a maximum of NOFILE open files) tracks which
files are opened on a per-process basis. Each entry of the array is actually
just a pointer to a struct file, which will be used to track information
about the file being read or written; we’ll discuss this further below.
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TIP: USE STRACE (AND SIMILAR TOOLS)
The strace tool provides an awesome way to see what programs are up
to. By running it, you can trace which system calls a program makes, see
the arguments and return codes, and generally get a very good idea of
what is going on.
The tool also takes some arguments which can be quite useful. For ex-
ample, -f follows any fork’d children too; -t reports the time of day
at each call; -e trace=open,close,read,write only traces calls to
those system calls and ignores all others. There are many other flags; read
the man pages and find out how to harness this wonderful tool.
39.4 Reading And Writing Files
Once we have some files, of course we might like to read or write them.
Let’s start by reading an existing file. If we were typing at a command
line, we might just use the program cat to dump the contents of the file
to the screen.
prompt> echo hello > foo
prompt> cat foo
hello
prompt>
In this code snippet, we redirect the output of the program echo to
the file foo, which then contains the word “hello” in it. We then use cat
to see the contents of the file. But how does the cat program access the
file foo?
To find this out, we’ll use an incredibly useful tool to trace the sys-
tem calls made by a program. On Linux, the tool is called strace; other
systems have similar tools (see dtruss on a Mac, or truss on some older
UNIX variants). What strace does is trace every system call made by a
program while it runs, and dump the trace to the screen for you to see.
Here is an example of using strace to figure out what cat is doing
(some calls removed for readability):
prompt> strace cat foo
…
open(“foo”, O_RDONLY|O_LARGEFILE) = 3
read(3, “hello\n”, 4096) = 6
write(1, “hello\n”, 6) = 6
hello
read(3, “”, 4096) = 0
close(3) = 0
…
prompt>
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The first thing that cat does is open the file for reading. A couple
of things we should note about this; first, that the file is only opened for
reading (not writing), as indicated by the O RDONLY flag; second, that
the 64-bit offset be used (O LARGEFILE); third, that the call to open()
succeeds and returns a file descriptor, which has the value of 3.
Why does the first call to open() return 3, not 0 or perhaps 1 as you
might expect? As it turns out, each running process already has three
files open, standard input (which the process can read to receive input),
standard output (which the process can write to in order to dump infor-
mation to the screen), and standard error (which the process can write
error messages to). These are represented by file descriptors 0, 1, and 2,
respectively. Thus, when you first open another file (as cat does above),
it will almost certainly be file descriptor 3.
After the open succeeds, cat uses the read() system call to repeat-
edly read some bytes from a file. The first argument to read() is the file
descriptor, thus telling the file system which file to read; a process can of
course have multiple files open at once, and thus the descriptor enables
the operating system to know which file a particular read refers to. The
second argument points to a buffer where the result of the read()will be
placed; in the system-call trace above, strace shows the results of the read
in this spot (“hello”). The third argument is the size of the buffer, which
in this case is 4 KB. The call to read() returns successfully as well, here
returning the number of bytes it read (6, which includes 5 for the letters
in the word “hello” and one for an end-of-line marker).
At this point, you see another interesting result of the strace: a single
call to the write() system call, to the file descriptor 1. As we mentioned
above, this descriptor is known as the standard output, and thus is used
to write the word “hello” to the screen as the program cat is meant to
do. But does it call write() directly? Maybe (if it is highly optimized).
But if not, what cat might do is call the library routine printf(); in-
ternally, printf() figures out all the formatting details passed to it, and
eventually writes to standard output to print the results to the screen.
The cat program then tries to read more from the file, but since there
are no bytes left in the file, the read() returns 0 and the program knows
that this means it has read the entire file. Thus, the program calls close()
to indicate that it is done with the file “foo”, passing in the corresponding
file descriptor. The file is thus closed, and the reading of it thus complete.
Writing a file is accomplished via a similar set of steps. First, a file
is opened for writing, then the write() system call is called, perhaps
repeatedly for larger files, and then close(). Use strace to trace writes
to a file, perhaps of a program you wrote yourself, or by tracing the dd
utility, e.g., dd if=foo of=bar.
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ASIDE: DATA STRUCTURE — THE OPEN FILE TABLE
Each process maintains an array of file descriptors, each of which refers
to an entry in the system-wide open file table. Each entry in this table
tracks which underlying file the descriptor refers to, the current offset,
and other relevant details such as whether the file is readable or writable.
39.5 Reading And Writing, But Not Sequentially
Thus far, we’ve discussed how to read and write files, but all access
has been sequential; that is, we have either read a file from the beginning
to the end, or written a file out from beginning to end.
Sometimes, however, it is useful to be able to read or write to a spe-
cific offset within a file; for example, if you build an index over a text
document, and use it to look up a specific word, you may end up reading
from some random offsets within the document. To do so, we will use
the lseek() system call. Here is the function prototype:
off_t lseek(int fildes, off_t offset, int whence);
The first argument is familiar (a file descriptor). The second argu-
ment is the offset, which positions the file offset to a particular location
within the file. The third argument, called whence for historical reasons,
determines exactly how the seek is performed. From the man page:
If whence is SEEK_SET, the offset is set to offset bytes.
If whence is SEEK_CUR, the offset is set to its current
location plus offset bytes.
If whence is SEEK_END, the offset is set to the size of
the file plus offset bytes.
As you can tell from this description, for each file a process opens, the
OS tracks a “current” offset, which determines where the next read or
write will begin reading from or writing to within the file. Thus, part
of the abstraction of an open file is that it has a current offset, which
is updated in one of two ways. The first is when a read or write of N
bytes takes place, N is added to the current offset; thus each read or write
implicitly updates the offset. The second is explicitly with lseek, which
changes the offset as specified above.
The offset, as you might have guessed, is kept in that struct file
we saw earlier, as referenced from the struct proc. Here is a (simpli-
fied) xv6 definition of the structure:
struct file {
int ref;
char readable;
char writable;
struct inode *ip;
uint off;
};
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ASIDE: CALLING LSEEK() DOES NOT PERFORM A DISK SEEK
The poorly-named system call lseek() confuses many a student try-
ing to understand disks and how the file systems atop them work. Do
not confuse the two! The lseek() call simply changes a variable in OS
memory that tracks, for a particular process, at which offset its next read
or write will start. A disk seek occurs when a read or write issued to the
disk is not on the same track as the last read or write, and thus neces-
sitates a head movement. Making this even more confusing is the fact
that calling lseek() to read or write from/to random parts of a file, and
then reading/writing to those random parts, will indeed lead to more
disk seeks. Thus, calling lseek() can lead to a seek in an upcoming
read or write, but absolutely does not cause any disk I/O to occur itself.
As you can see in the structure, the OS can use this to determine
whether the opened file is readable or writable (or both), which under-
lying file it refers to (as pointed to by the struct inode pointer ip),
and the current offset (off). There is also a reference count (ref), which
we will discuss further below.
These file structures represent all of the currently opened files in the
system; together, they are sometimes referred to as the open file table.
The xv6 kernel just keeps these as an array as well, with one lock per
entry, as shown here:
struct {
struct spinlock lock;
struct file file[NFILE];
} ftable;
Let’s make this a bit clearer with a few examples. First, let’s track a
process that opens a file (of size 300 bytes) and reads it by calling the
read() system call repeatedly, each time reading 100 bytes. Here is a
trace of the relevant system calls, along with the values returned by each
system call, and the value of the current offset in the open file table for
this file access:
Return Current
System Calls Code Offset
fd = open(“file”, O RDONLY); 3 0
read(fd, buffer, 100); 100 100
read(fd, buffer, 100); 100 200
read(fd, buffer, 100); 100 300
read(fd, buffer, 100); 0 300
close(fd); 0 –
There are a couple of items of interest to note from the trace. First,
you can see how the current offset gets initialized to zero when the file is
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opened. Next, you can see how it is incremented with each read() by
the process; this makes it easy for a process to just keep calling read()
to get the next chunk of the file. Finally, you can see how at the end, an
attempted read() past the end of the file returns zero, thus indicating to
the process that it has read the file in its entirety.
Second, let’s trace a process that opens the same file twice and issues a
read to each of them.
OFT[10] OFT[11]
Return Current Current
System Calls Code Offset Offset
fd1 = open(“file”, O RDONLY); 3 0 –
fd2 = open(“file”, O RDONLY); 4 0 0
read(fd1, buffer1, 100); 100 100 0
read(fd2, buffer2, 100); 100 100 100
close(fd1); 0 – 100
close(fd2); 0 – –
In this example, two file descriptors are allocated (3 and 4), and each
refers to a different entry in the open file table (in this example, entries 10
and 11, as shown in the table heading; OFT stands for Open File Table).
If you trace through what happens, you can see how each current offset
is updated independently.
In one final example, a process uses lseek() to reposition the current
offset before reading; in this case, only a single open file table entry is
needed (as with the first example).
Return Current
System Calls Code Offset
fd = open(“file”, O RDONLY); 3 0
lseek(fd, 200, SEEK SET); 200 200
read(fd, buffer, 50); 50 250
close(fd); 0 –
Here, the lseek() call first sets the current offset to 200. The subse-
quent read() then reads the next 50 bytes, and updates the current offset
accordingly.
39.6 Shared File Table Entries: fork() And dup()
In many cases (as in the examples shown above), the mapping of file
descriptor to an entry in the open file table is a one-to-one mapping. For
example, when a process runs, it might decide to open a file, read it, and
then close it; in this example, the file will have a unique entry in the open
file table. Even if some other process reads the same file at the same time,
each will have its own entry in the open file table. In this way, each logical
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int main(int argc, char *argv[]) {
int fd = open(“file.txt”, O_RDONLY);
assert(fd >= 0);
int rc = fork();
if (rc == 0) {
rc = lseek(fd, 10, SEEK_SET);
printf(“child: offset %d\n”, rc);
} else if (rc > 0) {
(void) wait(NULL);
printf(“parent: offset %d\n”,
(int) lseek(fd, 0, SEEK_CUR));
}
return 0;
}
Figure 39.2: Shared Parent/Child File Table Entries (fork-seek.c)
reading or writing of a file is independent, and each has its own current
offset while it accesses the given file.
However, there are a few interesting cases where an entry in the open
file table is shared. One of those cases occurs when a parent process creates
a child process with fork(). Figure 39.2 shows a small code snippet in
which a parent creates a child and then waits for it to complete. The child
adjusts the current offset via a call to lseek() and then exits. Finally the
parent, after waiting for the child, checks the current offset and prints out
its value.
When we run this program, we see the following output:
prompt> ./fork-seek
child: offset 10
parent: offset 10
prompt>
Figure 39.3 shows the relationships that connect each process’s private
descriptor array, the shared open file table entry, and the reference from
it to the underlying file-system inode. Note that we finally make use of
the reference count here. When a file table entry is shared, its reference
count is incremented; only when both processes close the file (or exit) will
the entry be removed.
Sharing open file table entries across parent and child is occasionally
useful. For example, if you create a number of processes that are cooper-
atively working on a task, they can write to the same output file without
any extra coordination. For more on what is shared by processes when
fork() is called, please see the man pages.
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Parent
File
Descriptors
3:
Child
File
Descriptors
3:
Open File Table
refcnt: 2
off: 10
inode: Inode #1000
(file.txt)
Figure 39.3: Processes Sharing An Open File Table Entry
One other interesting, and perhaps more useful, case of sharing occurs
with the dup() system call (and its cousins, dup2() and dup3()).
The dup() call allows a process to create a new file descriptor that
refers to the same underlying open file as an existing descriptor. Figure
39.4 shows a small code snippet that shows how dup() can be used.
The dup() call (and, in particular, dup2()) is useful when writing
a UNIX shell and performing operations like output redirection; spend
some time and think about why! And now, you are thinking: why didn’t
they tell me this when I was doing the shell project? Oh well, you can’t get
everything in the right order, even in an incredible book about operating
systems. Sorry!
int main(int argc, char *argv[]) {
int fd = open(“README”, O_RDONLY);
assert(fd >= 0);
int fd2 = dup(fd);
// now fd and fd2 can be used interchangeably
return 0;
}
Figure 39.4: Shared File Table Entry With dup() (dup.c)
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39.7 Writing Immediately With fsync()
Most times when a program calls write(), it is just telling the file
system: please write this data to persistent storage, at some point in the
future. The file system, for performance reasons, will buffer such writes
in memory for some time (say 5 seconds, or 30); at that later point in
time, the write(s) will actually be issued to the storage device. From the
perspective of the calling application, writes seem to complete quickly,
and only in rare cases (e.g., the machine crashes after the write() call
but before the write to disk) will data be lost.
However, some applications require something more than this even-
tual guarantee. For example, in a database management system (DBMS),
development of a correct recovery protocol requires the ability to force
writes to disk from time to time.
To support these types of applications, most file systems provide some
additional control APIs. In the UNIX world, the interface provided to ap-
plications is known as fsync(int fd). When a process calls fsync()
for a particular file descriptor, the file system responds by forcing all dirty
(i.e., not yet written) data to disk, for the file referred to by the specified
file descriptor. The fsync() routine returns once all of these writes are
complete.
Here is a simple example of how to use fsync(). The code opens
the file foo, writes a single chunk of data to it, and then calls fsync()
to ensure the writes are forced immediately to disk. Once the fsync()
returns, the application can safely move on, knowing that the data has
been persisted (if fsync() is correctly implemented, that is).
int fd = open(“foo”, O_CREAT|O_WRONLY|O_TRUNC,
S_IRUSR|S_IWUSR);
assert(fd > -1);
int rc = write(fd, buffer, size);
assert(rc == size);
rc = fsync(fd);
assert(rc == 0);
Interestingly, this sequence does not guarantee everything that you
might expect; in some cases, you also need to fsync() the directory that
contains the file foo. Adding this step ensures not only that the file itself
is on disk, but that the file, if newly created, also is durably a part of the
directory. Not surprisingly, this type of detail is often overlooked, leading
to many application-level bugs [P+13,P+14].
39.8 Renaming Files
Once we have a file, it is sometimes useful to be able to give a file a
different name. When typing at the command line, this is accomplished
with mv command; in this example, the file foo is renamed bar:
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prompt> mv foo bar
Using strace, we can see that mv uses the system call rename(char
*old, char *new), which takes precisely two arguments: the original
name of the file (old) and the new name (new).
One interesting guarantee provided by the rename() call is that it is
(usually) implemented as an atomic call with respect to system crashes;
if the system crashes during the renaming, the file will either be named
the old name or the new name, and no odd in-between state can arise.
Thus, rename() is critical for supporting certain kinds of applications
that require an atomic update to file state.
Let’s be a little more specific here. Imagine that you are using a file ed-
itor (e.g., emacs), and you insert a line into the middle of a file. The file’s
name, for the example, is foo.txt. The way the editor might update the
file to guarantee that the new file has the original contents plus the line
inserted is as follows (ignoring error-checking for simplicity):
int fd = open(“foo.txt.tmp”, O_WRONLY|O_CREAT|O_TRUNC,
S_IRUSR|S_IWUSR);
write(fd, buffer, size); // write out new version of file
fsync(fd);
close(fd);
rename(“foo.txt.tmp”, “foo.txt”);
What the editor does in this example is simple: write out the new
version of the file under a temporary name (foo.txt.tmp), force it to
disk with fsync(), and then, when the application is certain the new
file metadata and contents are on the disk, rename the temporary file to
the original file’s name. This last step atomically swaps the new file into
place, while concurrently deleting the old version of the file, and thus an
atomic file update is achieved.
39.9 Getting Information About Files
Beyond file access, we expect the file system to keep a fair amount
of information about each file it is storing. We generally call such data
about files metadata. To see the metadata for a certain file, we can use the
stat() or fstat() system calls. These calls take a pathname (or file
descriptor) to a file and fill in a stat structure as seen in Figure 39.5.
You can see that there is a lot of information kept about each file, in-
cluding its size (in bytes), its low-level name (i.e., inode number), some
ownership information, and some information about when the file was
accessed or modified, among other things. To see this information, you
can use the command line tool stat. In this example, we first create
a file (called file) and then use the stat command line tool to learn
some things about the file.
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struct stat {
dev_t st_dev; // ID of device containing file
ino_t st_ino; // inode number
mode_t st_mode; // protection
nlink_t st_nlink; // number of hard links
uid_t st_uid; // user ID of owner
gid_t st_gid; // group ID of owner
dev_t st_rdev; // device ID (if special file)
off_t st_size; // total size, in bytes
blksize_t st_blksize; // blocksize for filesystem I/O
blkcnt_t st_blocks; // number of blocks allocated
time_t st_atime; // time of last access
time_t st_mtime; // time of last modification
time_t st_ctime; // time of last status change
};
Figure 39.5: The stat structure.
Here is the output on Linux:
prompt> echo hello > file
prompt> stat file
File: ‘file’
Size: 6 Blocks: 8 IO Block: 4096 regular file
Device: 811h/2065d Inode: 67158084 Links: 1
Access: (0640/-rw-r—–) Uid: (30686/remzi)
Gid: (30686/remzi)
Access: 2011-05-03 15:50:20.157594748 -0500
Modify: 2011-05-03 15:50:20.157594748 -0500
Change: 2011-05-03 15:50:20.157594748 -0500
Each file system usually keeps this type of information in a structure
called an inode1. We’ll be learning a lot more about inodes when we
talk about file system implementation. For now, you should just think
of an inode as a persistent data structure kept by the file system that has
information like we see above inside of it. All inodes reside on disk; a
copy of active ones are usually cached in memory to speed up access.
39.10 Removing Files
At this point, we know how to create files and access them, either se-
quentially or not. But how do you delete files? If you’ve used UNIX, you
probably think you know: just run the program rm. But what system call
does rm use to remove a file?
1Some file systems call these structures similar, but slightly different, names, such as
dnodes; the basic idea is similar however.
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Let’s use our old friend strace again to find out. Here we remove
that pesky file foo:
prompt> strace rm foo
…
unlink(“foo”) = 0
…
We’ve removed a bunch of unrelated cruft from the traced output,
leaving just a single call to the mysteriously-named system call unlink().
As you can see, unlink() just takes the name of the file to be removed,
and returns zero upon success. But this leads us to a great puzzle: why
is this system call named unlink? Why not just remove or delete?
To understand the answer to this puzzle, we must first understand more
than just files, but also directories.
39.11 Making Directories
Beyond files, a set of directory-related system calls enable you to make,
read, and delete directories. Note you can never write to a directory di-
rectly. Because the format of the directory is considered file system meta-
data, the file system considers itself responsible for the integrity of direc-
tory data; thus, you can only update a directory indirectly by, for exam-
ple, creating files, directories, or other object types within it. In this way,
the file system makes sure that directory contents are as expected.
To create a directory, a single system call, mkdir(), is available. The
eponymous mkdir program can be used to create such a directory. Let’s
take a look at what happens when we run the mkdir program to make a
simple directory called foo:
prompt> strace mkdir foo
…
mkdir(“foo”, 0777) = 0
…
prompt>
When such a directory is created, it is considered “empty”, although it
does have a bare minimum of contents. Specifically, an empty directory
has two entries: one entry that refers to itself, and one entry that refers
to its parent. The former is referred to as the “.” (dot) directory, and the
latter as “..” (dot-dot). You can see these directories by passing a flag (-a)
to the program ls:
prompt> ls -a
./ ../
prompt> ls -al
total 8
drwxr-x— 2 remzi remzi 6 Apr 30 16:17 ./
drwxr-x— 26 remzi remzi 4096 Apr 30 16:17 ../
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TIP: BE WARY OF POWERFUL COMMANDS
The program rm provides us with a great example of powerful com-
mands, and how sometimes too much power can be a bad thing. For
example, to remove a bunch of files at once, you can type something like:
prompt> rm *
where the * will match all files in the current directory. But sometimes
you want to also delete the directories too, and in fact all of their contents.
You can do this by telling rm to recursively descend into each directory,
and remove its contents too:
prompt> rm -rf *
Where you get into trouble with this small string of characters is when
you issue the command, accidentally, from the root directory of a file sys-
tem, thus removing every file and directory from it. Oops!
Thus, remember the double-edged sword of powerful commands; while
they give you the ability to do a lot of work with a small number of
keystrokes, they also can quickly and readily do a great deal of harm.
39.12 Reading Directories
Now that we’ve created a directory, we might wish to read one too.
Indeed, that is exactly what the program ls does. Let’s write our own
little tool like ls and see how it is done.
Instead of just opening a directory as if it were a file, we instead use
a new set of calls. Below is an example program that prints the contents
of a directory. The program uses three calls, opendir(), readdir(),
and closedir(), to get the job done, and you can see how simple the
interface is; we just use a simple loop to read one directory entry at a time,
and print out the name and inode number of each file in the directory.
int main(int argc, char *argv[]) {
DIR *dp = opendir(“.”);
assert(dp != NULL);
struct dirent *d;
while ((d = readdir(dp)) != NULL) {
printf(“%lu %s\n”, (unsigned long) d->d_ino,
d->d_name);
}
closedir(dp);
return 0;
}
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The declaration below shows the information available within each
directory entry in the struct dirent data structure:
struct dirent {
char d_name[256]; // filename
ino_t d_ino; // inode number
off_t d_off; // offset to the next dirent
unsigned short d_reclen; // length of this record
unsigned char d_type; // type of file
};
Because directories are light on information (basically, just mapping
the name to the inode number, along with a few other details), a program
may want to call stat() on each file to get more information on each,
such as its length or other detailed information. Indeed, this is exactly
what ls does when you pass it the -l flag; try strace on ls with and
without that flag to see for yourself.
39.13 Deleting Directories
Finally, you can delete a directory with a call to rmdir() (which is
used by the program of the same name, rmdir). Unlike file deletion,
however, removing directories is more dangerous, as you could poten-
tially delete a large amount of data with a single command. Thus, rmdir()
has the requirement that the directory be empty (i.e., only has “.” and “..”
entries) before it is deleted. If you try to delete a non-empty directory, the
call to rmdir() simply will fail.
39.14 Hard Links
We now come back to the mystery of why removing a file is performed
via unlink(), by understanding a new way to make an entry in the
file system tree, through a system call known as link(). The link()
system call takes two arguments, an old pathname and a new one; when
you “link” a new file name to an old one, you essentially create another
way to refer to the same file. The command-line program ln is used to
do this, as we see in this example:
prompt> echo hello > file
prompt> cat file
hello
prompt> ln file file2
prompt> cat file2
hello
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Here we created a file with the word “hello” in it, and called the file
file
2. We then create a hard link to that file using the ln program. After
this, we can examine the file by either opening file or file2.
The way link() works is that it simply creates another name in the
directory you are creating the link to, and refers it to the same inode num-
ber (i.e., low-level name) of the original file. The file is not copied in any
way; rather, you now just have two human-readable names (file and
file2) that both refer to the same file. We can even see this in the direc-
tory itself, by printing out the inode number of each file:
prompt> ls -i file file2
67158084 file
67158084 file2
prompt>
By passing the -i flag to ls, it prints out the inode number of each file
(as well as the file name). And thus you can see what link really has done:
just make a new reference to the same exact inode number (67158084 in
this example).
By now you might be starting to see why unlink() is called unlink().
When you create a file, you are really doing two things. First, you are
making a structure (the inode) that will track virtually all relevant infor-
mation about the file, including its size, where its blocks are on disk, and
so forth. Second, you are linking a human-readable name to that file, and
putting that link into a directory.
After creating a hard link to a file, to the file system, there is no dif-
ference between the original file name (file) and the newly created file
name (file2); indeed, they are both just links to the underlying meta-
data about the file, which is found in inode number 67158084.
Thus, to remove a file from the file system, we call unlink(). In the
example above, we could for example remove the file named file, and
still access the file without difficulty:
prompt> rm file
removed ‘file’
prompt> cat file2
hello
The reason this works is because when the file system unlinks file, it
checks a reference count within the inode number. This reference count
(sometimes called the link count) allows the file system to track how
many different file names have been linked to this particular inode. When
unlink() is called, it removes the “link” between the human-readable
2Note again how creative the authors of this book are. We also used to have a cat named
“Cat” (true story). However, she died, and we now have a hamster named “Hammy.” Update:
Hammy is now dead too. The pet bodies are piling up.
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name (the file that is being deleted) to the given inode number, and decre-
ments the reference count; only when the reference count reaches zero
does the file system also free the inode and related data blocks, and thus
truly “delete” the file.
You can see the reference count of a file using stat() of course. Let’s
see what it is when we create and delete hard links to a file. In this exam-
ple, we’ll create three links to the same file, and then delete them. Watch
the link count!
prompt> echo hello > file
prompt> stat file
… Inode: 67158084 Links: 1 …
prompt> ln file file2
prompt> stat file
… Inode: 67158084 Links: 2 …
prompt> stat file2
… Inode: 67158084 Links: 2 …
prompt> ln file2 file3
prompt> stat file
… Inode: 67158084 Links: 3 …
prompt> rm file
prompt> stat file2
… Inode: 67158084 Links: 2 …
prompt> rm file2
prompt> stat file3
… Inode: 67158084 Links: 1 …
prompt> rm file3
39.15 Symbolic Links
There is one other type of link that is really useful, and it is called a
symbolic link or sometimes a soft link. Hard links are somewhat limited:
you can’t create one to a directory (for fear that you will create a cycle in
the directory tree); you can’t hard link to files in other disk partitions
(because inode numbers are only unique within a particular file system,
not across file systems); etc. Thus, a new type of link called the symbolic
link was created [MJLF84].
To create such a link, you can use the same program ln, but with the
-s flag. Here is an example:
prompt> echo hello > file
prompt> ln -s file file2
prompt> cat file2
hello
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As you can see, creating a soft link looks much the same, and the orig-
inal file can now be accessed through the file name file as well as the
symbolic link name file2.
However, beyond this surface similarity, symbolic links are actually
quite different from hard links. The first difference is that a symbolic
link is actually a file itself, of a different type. We’ve already talked about
regular files and directories; symbolic links are a third type the file system
knows about. A stat on the symlink reveals all:
prompt> stat file
… regular file …
prompt> stat file2
… symbolic link …
Running ls also reveals this fact. If you look closely at the first char-
acter of the long-form of the output from ls, you can see that the first
character in the left-most column is a – for regular files, a d for directo-
ries, and an l for soft links. You can also see the size of the symbolic link
(4 bytes in this case) and what the link points to (the file named file).
prompt> ls -al
drwxr-x— 2 remzi remzi 29 May 3 19:10 ./
drwxr-x— 27 remzi remzi 4096 May 3 15:14 ../
-rw-r—– 1 remzi remzi 6 May 3 19:10 file
lrwxrwxrwx 1 remzi remzi 4 May 3 19:10 file2 -> file
The reason that file2 is 4 bytes is because the way a symbolic link is
formed is by holding the pathname of the linked-to file as the data of the
link file. Because we’ve linked to a file named file, our link file file2
is small (4 bytes). If we link to a longer pathname, our link file would be
bigger:
prompt> echo hello > alongerfilename
prompt> ln -s alongerfilename file3
prompt> ls -al alongerfilename file3
-rw-r—– 1 remzi remzi 6 May 3 19:17 alongerfilename
lrwxrwxrwx 1 remzi remzi 15 May 3 19:17 file3 ->
alongerfilename
Finally, because of the way symbolic links are created, they leave the
possibility for what is known as a dangling reference:
prompt> echo hello > file
prompt> ln -s file file2
prompt> cat file2
hello
prompt> rm file
prompt> cat file2
cat: file2: No such file or directory
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As you can see in this example, quite unlike hard links, removing the
original file named file causes the link to point to a pathname that no
longer exists.
39.16 Permission Bits And Access Control Lists
The abstraction of a process provided two central virtualizations: of
the CPU and of memory. Each of these gave the illusion to a process that
it had its own private CPU and its own private memory; in reality, the OS
underneath used various techniques to share limited physical resources
among competing entities in a safe and secure manner.
The file system also presents a virtual view of a disk, transforming it
from a bunch of raw blocks into much more user-friendly files and di-
rectories, as described within this chapter. However, the abstraction is
notably different from that of the CPU and memory, in that files are com-
monly shared among different users and processes and are not (always)
private. Thus, a more comprehensive set of mechanisms for enabling var-
ious degrees of sharing are usually present within file systems.
The first form of such mechanisms is the classic UNIX permission bits.
To see permissions for a file foo.txt, just type:
prompt> ls -l foo.txt
-rw-r–r– 1 remzi wheel 0 Aug 24 16:29 foo.txt
We’ll just pay attention to the first part of this output, namely the
-rw-r–r–. The first character here just shows the type of the file: – for
a regular file (which foo.txt is), d for a directory, l for a symbolic link,
and so forth; this is (mostly) not related to permissions, so we’ll ignore it
for now.
We are interested in the permission bits, which are represented by the
next nine characters (rw-r–r–). These bits determine, for each regular
file, directory, and other entities, exactly who can access it and how.
The permissions consist of three groupings: what the owner of the file
can do to it, what someone in a group can do to the file, and finally, what
anyone (sometimes referred to as other) can do. The abilities the owner,
group member, or others can have include the ability to read the file, write
it, or execute it.
In the example above, the first three characters of the output of ls
show that the file is both readable and writable by the owner (rw-), and
only readable by members of the group wheel and also by anyone else
in the system (r– followed by r–).
The owner of the file can readily change these permissions, for exam-
ple by using the chmod command (to change the file mode). To remove
the ability for anyone except the owner to access the file, you could type:
prompt> chmod 600 foo.txt
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ASIDE: SUPERUSER FOR FILE SYSTEMS
Which user is allowed to do privileged operations to help administer the
file system? For example, if an inactive user’s files need to be deleted to
save space, who has the rights to do so?
On local file systems, the common default is for there to be some kind of
superuser (i.e., root) who can access all files regardless of privileges. In
a distributed file system such as AFS (which has access control lists), a
group called system:administrators contains users that are trusted
to do so. In both cases, these trusted users represent an inherent secu-
rity risk; if an attacker is able to somehow impersonate such a user, the
attacker can access all the information in the system, thus violating ex-
pected privacy and protection guarantees.
This command enables the readable bit (4) and writable bit (2) for the
owner (OR’ing them together yields the 6 above), but set the group and
other permission bits to 0 and 0, respectively, thus setting the permissions
to rw——-.
The execute bit is particularly interesting. For regular files, its presence
determines whether a program can be run or not. For example, if we have
a simple shell script called hello.csh, we may wish to run it by typing:
prompt> ./hello.csh
hello, from shell world.
However, if we don’t set the execute bit properly for this file, the fol-
lowing happens:
prompt> chmod 600 hello.csh
prompt> ./hello.csh
./hello.csh: Permission denied.
For directories, the execute bit behaves a bit differently. Specifically,
it enables a user (or group, or everyone) to do things like change di-
rectories (i.e., cd) into the given directory, and, in combination with the
writable bit, create files therein. The best way to learn more about this:
play around with it yourself! Don’t worry, you (probably) won’t mess
anything up too badly.
Beyond permissions bits, some file systems, such as the distributed
file system known as AFS (discussed in a later chapter), include more so-
phisticated controls. AFS, for example, does this in the form of an access
control list (ACL) per directory. Access control lists are a more general
and powerful way to represent exactly who can access a given resource.
In a file system, this enables a user to create a very specific list of who
can and cannot read a set of files, in contrast to the somewhat limited
owner/group/everyone model of permissions bits described above.
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For example, here are the access controls for a private directory in one
author’s AFS account, as shown by the fs listacl command:
prompt> fs listacl private
Access list for private is
Normal rights:
system:administrators rlidwka
remzi rlidwka
The listing shows that both the system administrators and the user
remzi can lookup, insert, delete, and administer files in this directory, as
well as read, write, and lock those files.
To allow someone (in this case, the other author) to access this direc-
tory, user remzi can just type the following command.
prompt> fs setacl private/ andrea rl
There goes remzi’s privacy! But now you have learned an even more
important lesson: there can be no secrets in a good marriage, even within
the file system3.
39.17 Making And Mounting A File System
We’ve now toured the basic interfaces to access files, directories, and
certain types of special types of links. But there is one more topic we
should discuss: how to assemble a full directory tree from many under-
lying file systems. This task is accomplished via first making file systems,
and then mounting them to make their contents accessible.
To make a file system, most file systems provide a tool, usually re-
ferred to as mkfs (pronounced “make fs”), that performs exactly this task.
The idea is as follows: give the tool, as input, a device (such as a disk par-
tition, e.g., /dev/sda1) and a file system type (e.g., ext3), and it simply
writes an empty file system, starting with a root directory, onto that disk
partition. And mkfs said, let there be a file system!
However, once such a file system is created, it needs to be made ac-
cessible within the uniform file-system tree. This task is achieved via the
mount program (which makes the underlying system call mount() to do
the real work). What mount does, quite simply is take an existing direc-
tory as a target mount point and essentially paste a new file system onto
the directory tree at that point.
An example here might be useful. Imagine we have an unmounted
ext3 file system, stored in device partition /dev/sda1, that has the fol-
lowing contents: a root directory which contains two sub-directories, a
and b, each of which in turn holds a single file named foo. Let’s say we
wish to mount this file system at the mount point /home/users. We
would type something like this:
3Married happily since 1996, if you were wondering. We know, you weren’t.
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TIP: BE WARY OF TOCTTOU
In 1974, McPhee noticed a problem in computer systems. Specifi-
cally, McPhee noted that “… if there exists a time interval between
a validity-check and the operation connected with that validity-check,
[and,] through multitasking, the validity-check variables can deliberately
be changed during this time interval, resulting in an invalid operation be-
ing performed by the control program.” We today call this the Time Of
Check To Time Of Use (TOCTTOU) problem, and alas, it still can occur.
A simple example, as described by Bishop and Dilger [BD96], shows how
a user can trick a more trusted service and thus cause trouble. Imagine,
for example, that a mail service runs as root (and thus has privilege to
access all files on a system). This service appends an incoming message
to a user’s inbox file as follows. First, it calls lstat() to get informa-
tion about the file, specifically ensuring that it is actually just a regular
file owned by the target user, and not a link to another file that the mail
server should not be updating. Then, after the check succeeds, the server
updates the file with the new message.
Unfortunately, the gap between the check and the update leads to a prob-
lem: the attacker (in this case, the user who is receiving the mail, and thus
has permissions to access the inbox) switches the inbox file (via a call
to rename()) to point to a sensitive file such as /etc/passwd (which
holds information about users and their passwords). If this switch hap-
pens at just the right time (between the check and the access), the server
will blithely update the sensitive file with the contents of the mail. The
attacker can now write to the sensitive file by sending an email, an esca-
lation in privilege; by updating /etc/passwd, the attacker can add an
account with root privileges and thus gain control of the system.
There are not any simple and great solutions to the TOCTTOU problem
[T+08]. One approach is to reduce the number of services that need root
privileges to run, which helps. The O NOFOLLOW flag makes it so that
open() will fail if the target is a symbolic link, thus avoiding attacks that
require said links. More radical approaches, such as using a transactional
file system [H+18], would solve the problem, there aren’t many transac-
tional file systems in wide deployment. Thus, the usual (lame) advice:
careful when you write code that runs with high privileges!
prompt> mount -t ext3 /dev/sda1 /home/users
If successful, the mount would thus make this new file system avail-
able. However, note how the new file system is now accessed. To look at
the contents of the root directory, we would use ls like this:
prompt> ls /home/users/
a b
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As you can see, the pathname /home/users/ now refers to the root
of the newly-mounted directory. Similarly, we could access directories a
and b with the pathnames /home/users/a and /home/users/b. Fi-
nally, the files named foo could be accessed via /home/users/a/foo
and /home/users/b/foo. And thus the beauty of mount: instead of
having a number of separate file systems, mount unifies all file systems
into one tree, making naming uniform and convenient.
To see what is mounted on your system, and at which points, simply
run the mount program. You’ll see something like this:
/dev/sda1 on / type ext3 (rw)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
/dev/sda5 on /tmp type ext3 (rw)
/dev/sda7 on /var/vice/cache type ext3 (rw)
tmpfs on /dev/shm type tmpfs (rw)
AFS on /afs type afs (rw)
This crazy mix shows that a whole number of different file systems,
including ext3 (a standard disk-based file system), the proc file system (a
file system for accessing information about current processes), tmpfs (a
file system just for temporary files), and AFS (a distributed file system)
are all glued together onto this one machine’s file-system tree.
39.18 Summary
The file system interface in UNIX systems (and indeed, in any system)
is seemingly quite rudimentary, but there is a lot to understand if you
wish to master it. Nothing is better, of course, than simply using it (a lot).
So please do so! Of course, read more; as always, Stevens [SR05] is the
place to begin.
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ASIDE: KEY FILE SYSTEM TERMS
• A file is an array of bytes which can be created, read, written, and
deleted. It has a low-level name (i.e., a number) that refers to it
uniquely. The low-level name is often called an i-number.
• A directory is a collection of tuples, each of which contains a
human-readable name and low-level name to which it maps. Each
entry refers either to another directory or to a file. Each directory
also has a low-level name (i-number) itself. A directory always has
two special entries: the . entry, which refers to itself, and the ..
entry, which refers to its parent.
• A directory tree or directory hierarchy organizes all files and direc-
tories into a large tree, starting at the root.
• To access a file, a process must use a system call (usually, open())
to request permission from the operating system. If permission is
granted, the OS returns a file descriptor, which can then be used
for read or write access, as permissions and intent allow.
• Each file descriptor is a private, per-process entity, which refers to
an entry in the open file table. The entry therein tracks which file
this access refers to, the current offset of the file (i.e., which part
of the file the next read or write will access), and other relevant
information.
• Calls to read() and write() naturally update the current offset;
otherwise, processes can use lseek() to change its value, enabling
random access to different parts of the file.
• To force updates to persistent media, a process must use fsync()
or related calls. However, doing so correctly while maintaining
high performance is challenging [P+14], so think carefully when
doing so.
• To have multiple human-readable names in the file system refer to
the same underlying file, use hard links or symbolic links. Each
is useful in different circumstances, so consider their strengths and
weaknesses before usage. And remember, deleting a file is just per-
forming that one last unlink() of it from the directory hierarchy.
• Most file systems have mechanisms to enable and disable sharing.
A rudimentary form of such controls are provided by permissions
bits; more sophisticated access control lists allow for more precise
control over exactly who can access and manipulate information.
.
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References
[BD96] “Checking for Race Conditions in File Accesses” by Matt Bishop, Michael Dilger. Com-
puting Systems 9:2, 1996. A great description of the TOCTTOU problem and its presence in file
systems.
[CK+08] “The xv6 Operating System” by Russ Cox, Frans Kaashoek, Robert Morris, Nickolai
Zeldovich. From: https://github.com/mit-pdos/xv6-public. As mentioned before, a cool and
simple Unix implementation. We have been using an older version (2012-01-30-1-g1c41342) and hence
some examples in the book may not match the latest in the source.
[H+18] “TxFS: Leveraging File-System Crash Consistency to Provide ACID Transactions” by
Y. Hu, Z. Zhu, I. Neal, Y. Kwon, T. Cheng, V. Chidambaram, E. Witchel. USENIX ATC ’18, June
2018. The best paper at USENIX ATC ’18, and a good recent place to start to learn about transactional
file systems.
[K84] “Processes as Files” by Tom J. Killian. USENIX, June 1984. The paper that introduced the
/proc file system, where each process can be treated as a file within a pseudo file system. A clever idea
that you can still see in modern UNIX systems.
[L84] “Capability-Based Computer Systems” by Henry M. Levy. Digital Press, 1984. Available:
http://homes.cs.washington.edu/˜levy/capabook. An excellent overview of early capability-based
systems.
[MJLF84] “A Fast File System for UNIX” by Marshall K. McKusick, William N. Joy, Sam J.
Leffler, Robert S. Fabry. ACM TOCS, 2:3, August 1984. We’ll talk about the Fast File System (FFS)
explicitly later on. Here, we refer to it because of all the other random fun things it introduced, like long
file names and symbolic links. Sometimes, when you are building a system to improve one thing, you
improve a lot of other things along the way.
[P+13] “Towards Efficient, Portable Application-Level Consistency” by Thanumalayan S. Pil-
lai, Vijay Chidambaram, Joo-Young Hwang, Andrea C. Arpaci-Dusseau, and Remzi H. Arpaci-
Dusseau. HotDep ’13, November 2013. Our own work that shows how readily applications can
make mistakes in committing data to disk; in particular, assumptions about the file system creep into
applications and thus make the applications work correctly only if they are running on a specific file
system.
[P+14] “All File Systems Are Not Created Equal: On the Complexity of Crafting Crash-Consistent
Applications” by Thanumalayan S. Pillai, Vijay Chidambaram, Ramnatthan Alagappan, Samer
Al-Kiswany, Andrea C. Arpaci-Dusseau, and Remzi H. Arpaci-Dusseau. OSDI ’14, Broom-
field, Colorado, October 2014. The full conference paper on this topic – with many more details and
interesting tidbits than the first workshop paper above.
[SK09] “Principles of Computer System Design” by Jerome H. Saltzer and M. Frans Kaashoek.
Morgan-Kaufmann, 2009. This tour de force of systems is a must-read for anybody interested in the
field. It’s how they teach systems at MIT. Read it once, and then read it a few more times to let it all
soak in.
[SR05] “Advanced Programming in the UNIX Environment” by W. Richard Stevens and Stephen
A. Rago. Addison-Wesley, 2005. We have probably referenced this book a few hundred thousand
times. It is that useful to you, if you care to become an awesome systems programmer.
[T+08] “Portably Solving File TOCTTOU Races with Hardness Amplification” by D. Tsafrir, T.
Hertz, D. Wagner, D. Da Silva. FAST ’08, San Jose, California, 2008. Not the paper that introduced
TOCTTOU, but a recent-ish and well-done description of the problem and a way to solve the problem
in a portable manner.
c© 2008–20, ARPACI-DUSSEAU
THREE
EASY
PIECES
28 INTERLUDE: FILES AND DIRECTORIES
Homework (Code)
In this homework, we’ll just familiarize ourselves with how the APIs
described in the chapter work. To do so, you’ll just write a few different
programs, mostly based on various UNIX utilities.
Questions
1. Stat: Write your own version of the command line program stat,
which simply calls the stat() system call on a given file or di-
rectory. Print out file size, number of blocks allocated, reference
(link) count, and so forth. What is the link count of a directory, as
the number of entries in the directory changes? Useful interfaces:
stat(), naturally.
2. List Files: Write a program that lists files in the given directory.
When called without any arguments, the program should just print
the file names. When invoked with the -l flag, the program should
print out information about each file, such as the owner, group, per-
missions, and other information obtained from the stat() system
call. The program should take one additional argument, which is
the directory to read, e.g., myls -l directory. If no directory is
given, the program should just use the current working directory.
Useful interfaces: stat(), opendir(), readdir(), getcwd().
3. Tail: Write a program that prints out the last few lines of a file. The
program should be efficient, in that it seeks to near the end of the
file, reads in a block of data, and then goes backwards until it finds
the requested number of lines; at this point, it should print out those
lines from beginning to the end of the file. To invoke the program,
one should type: mytail -n file, where n is the number of lines
at the end of the file to print. Useful interfaces: stat(), lseek(),
open(), read(), close().
4. Recursive Search: Write a program that prints out the names of
each file and directory in the file system tree, starting at a given
point in the tree. For example, when run without arguments, the
program should start with the current working directory and print
its contents, as well as the contents of any sub-directories, etc., until
the entire tree, root at the CWD, is printed. If given a single argu-
ment (of a directory name), use that as the root of the tree instead.
Refine your recursive search with more fun options, similar to the
powerful find command line tool. Useful interfaces: figure it out.
OPERATING
SYSTEMS
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