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POSIX: Files
Introduction to Operating Systems: Discussion 1
Read Solaris System Interface Guide:
Ch. 5.1 Basic File I/O
POSIX - Portable Operating System Interface
 POSIX
is a popular standard for Unix-like
operating systems
 POSIX is actually a collection of standards that
cover system calls, libraries, applications and
more…
 POSIX 1003.1 defines the C language interface to a
Unix-like kernel
 A kernel
 is
is the part of an operating system which
always resident in memory
 handles low level resource management tasks
POSIX and Unix
 Most
current Unix-like operating systems (OS) are
POSIX compliant (or nearly so):
Linux, BSD, Solaris, AIX, IRIX
 While an understanding of each operating system’s
design is necessary to fully utilize its API, most
functions work almost identically on any compliant
operating system
 We will develop for the Solaris operating system
 The
code we write should be portable to any POSIX OS
The Basic File I/O API
 While
various programming languages provide specific
means of accessing files, every POSIX compliant OS must
implement the following basic file I/O functions
 creat()
 open()
Read more about these functions in the Solaris
 close()
System Interface Guide at
http://docs.sun.com/app/docs/doc/806-4750
 read()
 write()
 ioctl()
Finding Files in Unix
 To
use the Basic File I/O API, we must be able to
uniquely identify the file to be opened. We use
 A relative
path, describing a path through the file system
tree starting with the current working directory
OR
 An absolute path, describing a path starting at the root
directory of the file system
 When ever a path is mentioned in the discussion that
follows, it can be either relative or absolute
Opening a file
 When
Checks to make sure the opener
has permission to access the file
 Adds an entry in the process' open
file table for the given file



This entry contains information about
where the file is stored on disk, it's
size, where the file pointer is currently
located, etc.
Returns the offset in the open file
table associated with that file

Open File Table
a file is opened, the OS:
In the picture to the right, the open
call returns 3 for "My_file"
0
stdin
1
stdout
2
stderr
3
My_file
4
5
6
7
The open() call
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *path, int flags,
mode_t mode);
 char *path: contains the path name of the file to be opened


This is a C-string, not a C++ string object
int flags: specifies the method of access

O_RDONLY, O_RDWR, O_WRONLY

The flags are defined as constants in fcntl.h
The open() call
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *path, int flags,
mode_t mode);
 mode_t mode: this optional parameter is used to set the access
permissions upon file creation



the mode_t struct is defined in types.h
Mode constants are defined in stat.h
The return value is the index in the open file table of the newly
opened file. All subsequent interaction with that file (read, write,
close) is specified using this integer
read() and write()
#include <fcntl.h>
ssize_t read(int fd, void *buffer, size_t n);
ssize_t write(int fd, const void *buffer, size_t n);


int fd: file descriptor that has been obtained from open()
void *buffer:




size_t n: the maximum number of bytes that are to be read/written
from/to the file
ssize_t is a data type (typedef long) defined in types.h


In read, a pointer to the address in memory where the data read from the file
will be stored
In write, a pointer to the address in memory where the data written to the file is
currently stored
This return value is the actual number of bytes read, or written
size_t is a data type (typedef unsigned long) defined in types.h
The close() call
 Please
make it a habit to close all files that you program
has used as soon as you are done using them
#include <fcntl.h>
int close(int filedes);
 Note
that filedes is the offset in the open file table of the
file you want closed

It is the integer returned by the open system call when the file
was first opened
 Remember,
closing resources timely can improve system
performance and prevent deadlocks from happening
A simple example:
#include <fcntl.h> /*controls file attributes*/
#include <unistd.h>
main()
{ int fd; /* a file descriptor */
ssize_t nread;
/* number of bytes read */
char buf[1024];
/* data buffer */
/* open the file "data" for reading */
fd = open("data", O_RDONLY);
/* read in the data */
nread = read(fd, buf, 1024);
/* close the file */
close(fd);}