File Descriptors

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

These are the most common system calls relating to file descriptors. 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.


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:


ssize_t read(int fd, void* 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 (possibly a restartable 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 and restartable 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 indicate 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 indicate a temporary blip, and retrying the slow system call will likely succeed. These errors are EINTR and EAGAIN. The kernel uses these error codes to indicate an interruption or condition that the process may want to check.1 I/O libraries must sometimes mask these errors by retrying until the errors go away. For example, the stdio library’s fflush function retries its writes until restartable errors go away; and in pset 4, your io61_flush function must do the same.

Error codes like EIO and EINTR are defined in #include <cerrno>. 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 man 2 read.)


ssize_t write(int fd, const void* 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. Short writes are less common than short reads, but they can happen; for instance, the drive storing the file might not have space for all sz bytes, or a write attempt might be interrupted by a signal.2

The return value -1 indicates an error (possibly a restartable error) and means that no bytes were written. If any bytes were written, the return value will be greater than 0. The return value 0 is possible only if sz == 0.


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:

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.


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.3 The <cerrno> 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 for more.

  1. EINTR means a signal was delivered to the process before any reading or writing could occur, and EAGAIN means the system call would normally block, but the file descriptor is in non-blocking mode. ↩︎

  2. Different types of file have different behavior around short writes. When writing to a Linux pipe, for example, writes of 4096 or fewer bytes cannot be short. Such writes will happen either completely or not at all (i.e., the return value from write will either be the number of bytes requested or -1). (Ref; confirmed in the code↩︎

  3. The errno variable is actually thread-local—in a multithreaded program, each thread has its own errno↩︎