Advanced UNIX progamming
Fall 2002
Instructor: Ashok Srinivasan
Lecture 7
Acknowledgements: The syllabus and power point presentations are modified versions of those by
T. Baker and X. Yuan
Announcements
• Reading assignment
– APUE Chapter 3
• Pages 47-56, 56-62
– APUE Chapter 5
• Section 5.4 is particularly important
– APUE Chapter 7
• Section 7.3 is important
• You should know the material from 7.1, 7.2, 7.4 – 7.9
from previous courses and classes
– APUE Chapter 8
• Sections 8.1-8.3, 8.5-8.6, 8.9-8.10
• You should also understand the idea behind race
conditions
Review
• UNIX file system
– File system abstraction
– Directories
– File descriptors
Week 3 Topics
• UNIX file system
– File system abstraction
– Directories
– File descriptors
• Unix API Programming Examples and
Techniques
– Example with direct IO
• open, close, fdopen, lseek, unlink
– Variable argument list
• HW1 hints
Week 3 Topics ... continued
• File I/O
– File descriptors
• open, creat, close, dup, dup2
– I/O redirection
• Process management
– fork, exit, wait, waitpid, execv
• Pipes
– Named and unnamed pipes
– Implementing pipe in a shell
UNIX file system
• File system abstraction
• Directories
• File descriptors
File Descriptors ... review
File Descriptors ... continued
• The POSIX standard defines the following
– File descriptor: A per-process, unique,
nonnegative integer used to identify an open file
for the purposes of file access
– Open file description: A record of how a process
or group of processes are currently accessing a
file
• Each file descriptor refers to exactly one open file
description, but an open file description may be referred
to by more than one file descriptor
• A file offset, file status, and file access modes are
attributes of an open file description
– File access modes: Specification of whether the
file can be read and written
File Descriptors ... continued
– File offset: The byte position in the file where the
next I/O operation through that open file
description begins
• Each open file description associated with a regular file,
block special file, or directory has a file offset
• There is no file offset specified for a pipe or FIFO
(described later)
– File status: Includes the following information
• append mode or not
• blocking/nonblocking
• Etc
File Descriptors ... continued
– FIFO special file: A type of file with the property
that data written to such a file is read on a first-infirst-out basis
– Pipe: An object accessed by one of the pair of file
descriptors created by the pipe() function
• Once created, the file descriptors can be used to
manipulate the pipe, and it behaves identically to a FIFO
special file when accessed this way
• It has no name in the file hierarchy
File Descriptors ... continued
• Important points
– A file descriptor does not describe a file
• It is just a number that is ephemerally associated with a
particular open file description
– An open file description describes a past "open"
operation on a file; its does not describe the file
– The description of the file is in the inode
• There may be several different open file descriptors (or
none) referring at it any given time
Unix API Programming Examples
and Techniques
• Examples with direct IO
– open, close, fdopen, lseek, unlink
• Variable argument list
• Note:
– Read the POSIX standard and sse man
pages to get information on system calls
– Look into the system header files
• Example: /usr/include/sys/types.h
Direct I/O
• Using open()
– The usual C-language stream-oriented I/O
operations, like printf(), use buffers and process
data character-by-character
– They are implemented using the lower-level direct
I/O operations read() and write()
– In situations where we do not want to view the
data as characters, or where we want greater
efficiency, it is better to use the direct I/O
operations
Using man
• Look at the man page for write()
• If there is more than one page on a given name, man will
give you the one that is first in the chapter order of the
Unix manual.
• Shell commands are in Section 1, I/O and OS interface
calls are in Section 2 and Section 3 respectively
– Specification of section number varies
• On Red Hat Linux, type man 2 write or man -S 2 write to
see the page on write from Section 2 of the Unix manual
• On Solaris, you can type man -s 2 write
man page for open
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
• Solaris 2.6 includes the following synopsis
– int open(const char *path, int oflag, /* mode_t mode */ ...);
• Red Hat Linux 6.2
– int open(const char *pathname, int flags, mode_t mode);
• 1996 POSIX standard synopsis is as follows
– int open(const char *path, int oflag, ...);
• The latest official POSIX/Unix synopsis
– No <sys/types.h>
What does the ... mean here? Will a compiler allow this in an actual
program?
Variable Argument Lists
• The ... indicates a variable number of
arguments
– Similar to that in printf()
– For more on variable argument lists, look
at the file /usr/include/stdarg.h
– Functions with variable argument lists can
be dangerous
• It is difficult to check types, and the use of a
correct number of arguments
Example Programs
• example1.c illustrates a common
programming error
– Failure to provide the correct number of
arguments to a vararg function
• example2.c illustrates opening a file
File I/O
• File descriptors
– open, creat, close, dup, dup2
• I/O redirection
File descriptors
• Implication of the file descriptor/open file
descriptions/inode table organization in UNIX
– open and creat
• search for the first empty slot in the process file
descriptor table
• allocate an open file description in the file table, which
has a pointer to the inode table
• See example3.c
– dup and dup2
• Duplicate the file descriptor in the process file descriptor
table
• See example3b.c
• Where is the current file position stored?
I/O redirection
• All UNIX processes have three predefined
files open
– stdin -- STDIN_FILENO (0)
– stdout -- STDOUT_FILENO (1)
– stderr -- STDERR_FILENO (2)
• We can redirect the I/O by manipulating these
file descriptors
• See example4.c and example5.c