File I/O - the universal I/O model

Universality of I/O

• One of the distinguishing features of the I/O model on UNIX systems is the concept of universality of I/O. This means that the same system calls (open(),read(), write(), close(), and so on) are used to perform I/O on all types of files, including devices. Thus, a program that employing these systems will work on any type of file.

$ ./copy test test.old  # Copy a regular file
$ ./copy a.txt /dev/tty  # Copy a regular file to this terminal
$ ./copy /dev/tty b.txt  # Copy input from this terminal to a regular file
$ ./copy /dev/pts/16 /dev/tty # Copy input from another terminal

• All system calls for performing I/O refer to open files using a file descriptor (a nonnegative integer). File descriptors are used to refer to all types of open files: pipes, FIFOs, sockets, terminals, devices and regular files. Each process has its own set of file descriptors.

Opening a file: open()

• A call to open() creates a new open file description, an entry in the system-wide table of open files. The open file description records the file offset and the file status flags (see below). A file descriptor is a non-negative integer that act as an abstract to handle file or I/O resources 0,1,2 are taken by STDIN, STDOUT, and STDERR.

#include <sys/stat.h>
#include <fcntl.h>
int open(const char *pathname, int flags, ... /* mode_t mode */);

// Returns file descriptor on success, or –1 on error

Explain

• The file to be opened is identified by the pathname argument. If pathname is a symbolic link, it is dereferenced.

• The flags argument is a bitmask that specifies the access mode for the file:

  Access mode	Description	Group
  O_RDONLY	Open for reading only	File access mode flags
  O_WRONLY	Open for writing only	File access mode flags
  O_RDWR	Open for reading and writing	File access mode flags
  O_CLOEXEC	Set the close-on-exec flag (since Linux 2.6.23)	File creation flags
  O_CREAT	Create file if it doesn’t already exist	File creation flags
  O_DIRECT	File I/O bypasses buffer cache	File creation flags
  O_DIRECTORY	Fail if pathname is not a directory	File creation flags
  O_EXCL	With O_CREAT: create file exclusively	File creation flags
  O_LARGEFILE	Used on 32-bit systems to open large files	File creation flags
  O_NOATIME	Don’t update file last access time on read() (since Linux 2.6.8)	File creation flags
  O_NOCTTY	Don’t let pathname become the controlling terminal	File creation flags
  O_NOFOLLOW	Don’t dereference symbolic links	File creation flags
  O_TRUNC	Truncate existing file to zero length	File creation flags
  O_APPEND	Writes are always appended to end of file	Open file status flags
  O_ASYNC	Generate a signal when I/O is possible	Open file status flags
  O_DSYNC	Provide synchronized I/O data integerity (since Linux 2.6.33)	Open file status flags
  O_NONBLOCK	Open in nonblocking mode	Open file status flags
  O_SYNC	Make file write synchronous	Open file status flags

  • File access mode flags: can be retrieved using the fcntl() F_GETFL operation
  • File creation mode flags: these flags control various aspects of the behavior of the open() call and options for subsequent I/O operations. They can’t be retrieved or changed.
  • Open file status flags: these flags can be retrieved and modified using the fcntl() F_GETFL and F_SETFL operations

Errors handling

• If an error occurs while trying to open the file, open() returns –1, and errno identifies the cause of the error

  NAME	Description
  EACCES	The file permissions don’t allow the calling process to open the file in the mode specified by flags. Alternatively, because of directory permissions, the file could not be accessed, or the file did not exist and could not be created.
  EISDIR	The specified file is a directory, and the caller attempted to open it for writing. This isn’t allowed.
  EMFILE	The process resource limit on the number of open file descriptors has been reached
  ENFILE	The system-wide limit on the number of open files has been reached.
  ENOENT	The specified file doesn’t exist, and O_CREAT was not specified, or O_CREAT was specified, and one of the directories in pathname doesn’t exist or is a symbolic link pointing to a nonexistent pathname (a dangling link).
  EROFS	The specified file is on a read-only file system and the caller tried to open it for writing.
  ETXTBSY	The specified file is an executable file (a program) that is currently executing. It is not permitted to modify (i.e., open for writing) the executable file associated with a running program. (We must first terminate the program in order to be able to modify the executable file.)

Example

Reading from a file: read()

#include <tlpi_hdr.h>

#define MAX_READ 20
int main(){
    char buffer[MAX_READ + 1];
    ssize_t numRead = read(STDIN_FILENO, buffer, MAX_READ);
    if (numRead == -1)
        errExit("read");
    printf("The input data was: %s\n", buffer);

    buffer[numRead] = '\0';
    printf("The input data was: %s\n", buffer);
}

• This system call doesn’t place a terminating null byte at the end of the string.