C programming session 14

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Programming in C
Objectives
In this session, you will learn to:
Differentiate between high-level and low-level input/output
Work with low-level input/output functions
Use random access in files

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Differentiating Between High-Level and Low-Level Input/Output
In C, files and devices can be accessed by using two
groups of functions:
High-level I/O or stream-level I/O
Low-level I/O

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Difference Between High-level I/O and Low-level I/O
High-level or Stream-level I/O:
Is more flexible and convenient.
Hides complexity from the programmer.
Is slower.
Low-level I/O:
Provides direct access to files and devices.
Is complex (buffer management is to be done by the
programmer).
Is faster.
Uses a file descriptor to track the status of the file.

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Practice: 8.1
Which of the following statements is true?
In C, there are many interfaces between a program and
peripheral devices.
A file descriptor is a non-negative integer.
When you perform an operation on a file, the system uses the
name of the file to identify it.

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Practice: 8.1 (Contd.)
Solution:
A file descriptor is a non-negative integer.

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Low-level I/O functions are used for:
Accessing files and devices directly.
Reading binary files in large chunks.
Performing I/O operations quickly and efficiently.
Uses of Low-Level Input/Output

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The low-level I/O system in C provides functions that can be
used to access files and devices.
The basic low-level I/O functions are:
open()
close()
read()
write()
Working with Low-Level Input/Output Functions

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The open() function:
Is used to open an existing file or create a new file.
Returns a file descriptor for the file name passed to it.
Has the following syntax:
int open(char *filename, int flags, int perms
);
The open() Function

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The close() function:
Closes the file that was opened using the open() function.
Takes the file descriptor as a parameter to close the file.
Returns 0 on success and -1 in case of an error.
int close(int filedes);
The close() Function

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The read() function:
Reads data from a file.
Starts reading a file from the current file position.
int read (int filedes, char *buffer, int
size);
The read() function

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The write() function:
Enables a user to write contents to a file.
int write (int filedes, char *buffer, int
size);
The write() function

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Programming in C
Practice: 8.2
1. Which of the following statements is true?
a. At end-of-file, if a function is called repeatedly, it will give error.
b. In a read() function, the value of zero indicates end-of-file.
2. What will happen if you do not call the write() function in
a loop?

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Solution:
1. In a read() function, the value of zero indicates end-of-file.
2. If you do not call write() function in a loop then the entire
data would not be written.
Practice: 8.2 (Contd.)

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Error Handling:
Some of the low-level I/O functions return error flags when they
fail to perform a specified task.
You can find these types of errors using a variable, errno.
The following table lists some values of errno that are
common to the open(), close(), read(), and write()
functions.
Error Handling
errno values Description
EACCES Specifies that the program has failed to access one of the directories
in the file.
ENAMETOOLONG Indicates that the file name is too long.
ENOSPC Specifies that the disc is out of space and the file can not be created.
EIO Specifies that there was a hardware error.
EBADF Specifies that the file descriptor passed to read, write, or close the
file is invalid.

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There are certain errno values that are specific to the
open() function, which are shown with the help of the
following table.
EEXIST Specifies that if File already exists, and O_CREAT and O_EXCL are
set, then opening the file would conflict with the existing file and the
file will not open.
EISDIR Specifies that the file is actually a directory.
ENOENT Specifies that some of the file components do not exist.
EMFILE Specifies that too many files are open.
EROFS Specifies that the file is on a read only systembut either one of the
write permissions O_WRONLY, O_RDWR, or O_TRUNC is set.
Error Handling (Contd.)

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There are certain errno values that are specific to the
write() function, which are shown with the help of the
following table.
EFBIG Specifies that the file will become too large if the
data is written on it.
EINTR Specifies that the write operation is temporarily
interrupted.
Error Handling (Contd.)

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The read and write operations on files are usually sequential
in nature.
Random access permits non-sequential file access so that a
file can be read or written out of sequence.
Random access in low-level file routines is performed using
the lseek() function.
Using Random Access Seek in Files

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The lseek() function:
Returns the file position, as measured in bytes from the
beginning of the file.
long lseek (int filedes, long offset, int
origin);
The lseek() Function

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The following table lists the various values of the third
parameter (origin).
The lseek() Function (Contd.)
Values for origin Description
SEEK_SET or 0 Specifies that the offset is relative to the
beginning of the file. The offset can only be
positive.
SEEK_CUR or 1 Specifies that the offset is relative to the current
position. The offset can be positive or negative.
SEEK_END or 2 Specifies that the offset is relative to the end of
the file. The offset can be positive or negative.

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The following table lists some instructions for moving to the
end or beginning of a file.
The lseek() Function (Contd.)
Instruction Function used
To get to the end of a file lseek(filedes,0,2);
To return to the beginning of a file lseek(filedes,0,0);

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Summary
In this session, you learned that:
In C, files and devices can be accessed by using high-level I/O
or stream-level I/O and low-level I/O.
Low-level I/O functions are those, which provide direct access
to files and peripheral devices.
A file descriptor is a non-negative integer, which is returned by
the open() function and is used in read() and write()
functions. It tells about the permission to access the file.
Low-level input/output functions are used for the following
purposes:
Accessing files and devices directly
Reading the binary files in large chunks
Performing I/O operations quickly and efficiently
The open() function is used to open or create a file.

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Summary (Contd.)
The close() function is used for closing the file.
The read() function reads the existing file up to specified size
and stores it into a character array.
The write() function writes on the file taking input from a
character array up to specified size.
Most of the low-level I/O functions throw errors during file
handling.
errno is the variable that tells about the error occurred during
a low-level I/O operation.
Files and devices can also be accessed randomly.
The lseek() function is used to place the file position to the
desired place.
The lseek() function do not require to read or write the file
for positioning it.

C programming session 14

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