File Descriptors

Here’s a brief introduction to file descriptors for CS 61.

For another presentation of this material, see CS:APP2e chapter 10, particularly through section 10.5. Section 10.4.2 may be particularly interesting for Assignment 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.

(Why use integers? Because of process isolation! The kernel must verify every object passed to it by a user program. Otherwise, a process might be able to construct a malformed object that, when used by the kernel, could screw up isolation. This is another version of the page_alloc restriction in WeensyOS and OS02, where the kernel had to prevent processes from allocating memory in the kernel’s address space. And integers are much easier to verify than arbitrary pointers. The kernel gives each process its own file descriptor table, a simple array that maps integers to valid file descriptors. It’s very easy to check that an integer is in the array bounds.)

Logically, a file descriptor comprises a file object, which represents the underlying data (such as /home/kohler/grades.txt), and a position, which is an offset into the file. There can be many file descriptors simultaneously open for the same file object, each with a different position. We saw this with the l17/r10-stridestdiomulti.c program (see Lecture 18 just after the break). 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 4. You may read about them in detail by typing man on your appliance: 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.

ssize_t read(int fd, char *buf, size_t sz) Read bytes from file descriptor fd into buffer buf. Read at most sz bytes. Returns the number of bytes read. This is normally equal to sz. It might be less, however. For instance, there might be just sz - 2 bytes left in the file, or (if the file descriptor is connected to a pipe) there might be sz - 10 bytes available to read at the moment. Returns 0 at end of file, and -1 on error.

ssize_t write(int fd, const char *buf, size_t sz) Write bytes to file descriptor fd from buffer buf. Write at most sz bytes. Returns the number of bytes written. This is normally equal to sz, but it might be less. For instance, if the disk is full, and there was only room for sz - 2 bytes. Returns -1 on error.

off_t lseek(int fd, off_t pos, int whence) Change file descriptor fd’s position. Normally whence == SEEK_SET. Then the file’s position is set to pos; so pos == 0 sets the position to the beginning of the file, pos == 1 sets it one byte in, and so forth. You may also set whence == SEEK_CUR, which changes the position relative to the current position, or whence == SEEK_END, which sets the position relative to the file’s size (i.e., lseek(fd, -1, SEEK_END) sets the position to the file’s last byte). Returns the new position, measured in bytes past the beginning of the file. 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. 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 Assignment 4, depending on your implementation strategy. Read their manual pages, consult CS:APP2e 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. Returns MAP_FAILED == (void *) -1 on error. Doesn’t work for all file types. (CS:APP2e §9.8.4, l18/memreader.c)

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.