Here’s a brief introduction to file descriptors for CS 61.
For another presentation of this material, see CS:APP3e chapter 10, particularly through section 10.5. Section 10.4.2 may be particularly interesting for Problem Set 4!
A file descriptor is the Unix abstraction for an open input/output stream: a file, a network connection, a pipe (a communication channel between processes), a terminal, etc.
A Unix file descriptor thus fills a similar niche as a stdio FILE*
.
However, whereas a FILE*
(like stdin
or stdout
) is a pointer to
some object structure, a file descriptor is just an integer. For
example, 0, 1, and 2 are the file descriptor versions of stdin
,
stdout
, and stderr
, respectively.
(Integers are used because they’re easier for the operating system kernel to
verify than arbitrary pointers. Although the kernel has objects somewhat
similar to FILE*
s, it doesn’t give applications direct access to those
objects. Instead, an array called the file descriptor table stores an
array of such objects. The file descriptors that applications manipulate are
indexes into this table. It’s very easy to check that an integer is in
bounds.)
Logically, a file descriptor comprises a file reference, which represents
the underlying data (such as /home/kohler/grades.txt
), and a file
position, which is an offset into the file. There can be many file
descriptors simultaneously open for the same file reference, each with a
different position. For disk files, the position can be explicitly changed: a
process can rewind and re-read part of a file, for example, or skip around, as
we saw with strided I/O patterns. These files are called seekable. However,
not all types of file descriptor are seekable. Most communication channels
between processes aren’t, and neither are network channels.
File descriptor system calls
You will use the following system calls in Assignment 3. You may read
about them in detail using man
: for instance, man 2 open
, man 2 read
, man 2 lseek
. The “2
” means “tell me about the system call.”
Or you can check the book.
open
int open(const char* pathname, int flags, [mode_t mode])
Open the file pathname
according to mode
, which a set of flags containing
exactly one of O_RDONLY
(open for reading), O_WRONLY
(open for writing),
and O_RDWR
(open for both reading and writing), as well as other optional
flags. Returns a file descriptor for the open file, or -1 on error.
Other important flags include:
O_CREAT
: Create the file if it does not exist, using themode
argument to set the file’s initial permissions. (Typically themode
argument will be0660
orS_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP
, which allows the current user and group to read or write the file.O_CREAT | O_EXCL
: Create the file and fail if the file already exists.O_APPEND
: Open the file in append mode: everywrite
automatically jumps to the end of the file and makes the file longer.O_TRUNC
: Truncate the file to length 0.
read
ssize_t read(int fd, char* buf, size_t sz)
Read at most sz
bytes from file descriptor fd
into buffer buf
.
Returns the number of bytes read, if any. Returns 0 at end of file and
-1
on error.
Normally, read
returns sz
, but it can return less. For instance,
there might be just sz - 2
bytes left in the file, or there might only
be sz - 10
bytes available to read at the moment. A read
that
returns less than the requested number of bytes is called a short
read.
If sz > 0
, then the return value 0 is a reliable end-of-file
indicator. For instance, when reading a pipe, 0 means the other end of
the pipe has closed. Other short reads are not reliable end-of-file
indicators. For instance, when reading from the terminal, a read of 1024
bytes might return 1 byte because the user has only typed 1 byte so far;
the user might still type more bytes in the future.
The return value -1
indicates an error and means that no bytes were
read. If any bytes were read, the return value will be greater than 0.
However, not all errors are equally serious.
Permanent errors
The read
and write
system calls, as well as some other system calls, are
so-called “slow” system calls that can return different classes of error.
Some errors are serious, indicating problems with the underlying file. For
instance, the EIO
error indicates disk corruption, and ENOSPC
indicates
that the disk is full. These errors, which we’ll call permanent errors,
should be returned to the user.
Other, restartable errors are not serious; they are returned for special
reasons (we’ll discuss those reasons) and if possible should be masked
(hidden from the user). These errors are EINTR
and (sometimes) EAGAIN
.
Error codes like EIO
and EINTR
are defined in #include <errno.h>
. When a
system call returns an error, it generally returns -1; the error code is
returned in a special global variable called errno
.
Each system call manual page list all errors that can occur for that system
call. Read this page for read by looking at read(2)
. (That notation means
“the page for read in section 2 of the manual”; run [wo]man 2 read
.)
write
ssize_t write(int fd, const char* buf, size_t sz)
Write at most sz
bytes to file descriptor fd
from buffer buf
.
Returns the number of bytes written, if any. Returns -1
on error.
Normally, write
returns sz
, but as with read
, it might return
less: a short write. For instance, there might only be room for `sz
- 2` bytes on the disk, or the process reading from a pipe might be behind, or a signal might interrupt the write partway through.
The return value -1
indicates an error and means that no bytes were
written. If any bytes were written, the return value will be greater
than 0.
lseek
off_t lseek(int fd, off_t pos, int whence)
Change file descriptor fd
’s position and return the resulting position
relative to the beginning of the file. There are three important values for
whence
:
SEEK_SET
: Set the file position topos
.pos == 0
sets the position to the beginning of the file,pos == 1
sets it one byte in, and so forth.SEEK_CUR
: Change the file position relative to the current position.pos == 0
leaves the position unchanged,pos == 10
skips over the next 10 bytes, and so forth.SEEK_END
: Set the file position relative to the file size.pos == 0
sets the position to the end of the file,pos == -1
sets it to the last byte in the file, and so forth.
So lseek(fd, 0, SEEK_CUR)
returns the current position without changing it.
Returns -1
on error, which can happen, for example, if the file is not
seekable or the new file position is out of range for the file.
close
int close(int fd)
Close the file descriptor.
Understanding errors
The Unix error convention is that system calls return -1
on error. A
global variable, int errno
, is then set so the program can tell what
kind of error occurred. The <errno.h>
header file defines symbolic
names for specific error conditions. Each name starts with E
. For
example, the system calls above “return EBADF
if fd
is not an open
file descriptor.” This actually means that the system call returns the
value -1
(cast to the appropriate type), and the global errno
variable is set to the constant EBADF
.
The const char* strerror(int errnum)
library function returns a
textual string describing an error constant. For instance,
strerror(EINVAL)
returns "Invalid argument"
. This might be useful
for debugging.
A system call’s manual page will list the errors it might return.
Additional system calls
The following system calls might also be useful for problem set 4, depending on your implementation strategy. Read their manual pages, consult CS:APP3e or our handout code, or contact Piazza for more.
-
void* mmap(void* addr, size_t len, int prot, int flags, int fd, off_t offset)
Memory-map a portion of a file, returning the mapped address. ReturnsMAP_FAILED == (void*) -1
on error. Doesn’t work for all file types. -
int munmap(void* addr, size_t len)
Unmap a previously-mapped memory region. -
int madvise(void* addr, size_t len, int advice)
Provide prefetching advice for a portion of a memory-mapped region. -
int posix_fadvise(int fd, off_t pos, off_t len, int advice)
Provide prefetching advice for a portion of a file descriptor.