Advanced Programming File I/O for UNIX environments

Most file I/O requires only 5 functions: Open, read, write, Lseek, close

This chapter describes the I/O without buffering (read write calls a system call in the kernel)

File descriptor

For the kernel, all files are referenced by a file descriptor.

The file descriptor is a non-negative integer

When you open or create a new file, the kernel returns a file descriptor

The range of file descriptors is 0~open_max-1

Functions Open and Openat

Call the Open or Openat to open or create a file

int open (const char *path, int oflag, .../* mode_t mode */)

int Openat (int fd, const char *path, int oflag, .../* mode_t mode */

The last schedule parameter is used only when creating a new file.

The Oflag parameter can be used to illustrate several options for this function:

O_rdonly read-only Open

O_wronly only Write Open

O_RDWR Read/write Open

The file descriptor returned by open and Openat must be the smallest unused descriptor value

function creat

Call the creat function to create a new file

int creat (const char *path, mode_t mode)

The effect is equivalent to open (path, o_wronly | O_creat | O_trunc, Mode)

Creat opens the created file as a write-only method.

function close

Call this function to close an open file.

int close (int fd)

function Lseek

Each open file has a file offset. Used to measure the number of bytes computed at the beginning of a file.

Read, the write operation starts at the current offset and increases the read and write offset.

off_t lseek (int fd, off_t offset, int whence);

Whence parameters:

Seek_set a cheap offset byte from the beginning of the file

Seek_cur setting the current value from the file offset plus offset

Seek_end set offset to file length plus offset

Lseek returns a new offset after a successful call

function read

Read data from an open file

ssize_t Read (int fd, void *buf, size_t nbytes)

The call successfully returns the number of bytes read to the end of the file and returns 0.

Function write

Write data to an open file

ssize_t Write (int fd, void *buf, size_t nbytes)

The return value is usually the same as nbytes.

File sharing

The kernel uses 3 data structures to represent open files:

(1) Process table entry

1. File descriptors

2. Pointers to File table entries

(2) File Table entry

1. File status flags (such as read, write, non-blocking, etc.)

2. Current file offset

3. Pointers to V-node table entries

(3) V-node table entry

Each open file has a V-node structure.

Functions Pread and Pwrite

belongs to atomic operations.

Do not update the current file offset

function DUP and dup2

To copy an existing file descriptor

int dup (int fd)

int dup2 (int fd, int fd2)

A new file descriptor returned by the function shares the same file table entry as the parameter FD

Invoke DUP (FD) equivalent to Fcntl (FD, f_duped, 0)

Call Dup2 (FD,FD2) is equivalent to close (FD2); Fcntl (fd,f_duped, FD2);

function Sync, Fsync and Fdatasync

Writes data from the buffer to disk

function Fcntl

To change the properties of a file that has been opened

int fcntl (int fd, int cmd, .../* int arg */);

Parameter cmd:

Copy an existing descriptor F_DUPFD or f_dupfd_cloexec

Get/Set file descriptor F_getfd or F_SETFD

Get/Set file status flag F_GETFL or F_SETFL

Get/Set asynchronous I/O ownership f_getown or F_setown

Get/Set record lock F_getlk f_setlk f_setlkw

Advanced Programming File I/O for UNIX environments