The C standard library I/O functions allow
you to read and write data to both files and devices. There are no predefined
file structures in C, all data being treated as a sequence of bytes. These I/O
functions may be broken into two different categories : stream I/O and low-level
I/O.
The stream I/O functions treat a data file
as a stream of individual characters. The appropriate stream function can
provide buffered, formatted or unformatted input and output of data, ranging
from single characters to complicated structures. Buffering streamlines the I/O
process by providing temporary storage for data which takes away the burden
from the system of writing each item of data directly and instead allows the
buffer to fill before causing the data to be written.
The low-level I/O system on the other hand
does not perform any buffering or formatting of data --instead it makes direct
use of the system's I/O capabilities to transfer usually large blocks of
information.
Stream I/O
The C I/O system provides a consistent interface
to the programmer independent of the actual device being accessed. This
interface is termed a stream in C
and the actual device is termed a file.
A device may be a disk or tape drive, the screen, printer port, etc. but this
does not bother the programmer because the stream interface is designed to be
largely device independent. All I/O through the keyboard and screen that we
have seen so far is in fact done through special standard streams called stdin and stdout for input and output respectively. So in essence the console
functions that we have used so far such as printf(), etc. are special case
versions of the file functions we will now discuss.
There are two types of streams : text and
binary. These streams are basically the same in that all types of data can be
transferred through them however there is one important difference between them
as we will see.
Text Streams
A text stream is simply a sequence of
characters. However the characters in the stream are open to translation or
interpretation by the host environment. For example the newline character,
'\n', will normally be converted into a carriage return/linefeed pair and ^Z
will be interpreted as EOF. Thus the number of characters sent may not equal
the number of characters received.
Binary Streams
A binary stream is a sequence of data
comprised of bytes that will not be interfered with so that a one-to-one
relationship is maintained between data sent and data received.
Common File Functions
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fopen()
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open a stream
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fclose()
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close a stream
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putc()& fputc()
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write a character to a stream
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getc()&
fgetc()
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read a character from a stream
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fprintf()&
fscanf
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formatted I/O
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fgets()
& fputs()
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string handling
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fseek()
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position the file pointer at a particular
byte
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feof()
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tests if EOF
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Opening and Closing Files
A stream is associated with a specific file
by performing an open operation. Once a file is opened information may be
exchanged between it and your program. Each file that is opened has a unique
file control structure of type FILE ( which
is defined in <stdio.h> along with the prototypes for all I/O functions
and constants such as EOF (-1) ). A file
pointer is a pointer to this FILE structure which identifies a specific
file and defines various things about the file including its name, read/write
status, and current position. A file pointer variable is defined as follows
FILE
*fptr ;
The fopen() function opens a stream for use
and links a file with that stream returning a valid file pointer which is
positioned correctly within the file if all is correct. fopen() has the
following prototype
FILE *fopen( const char *filename,
const char *mode );
where filename is a pointer to a string of
characters which make up the name and path of the required file, and mode is a
pointer to a string which specifies how the file is to be opened. The following
table lists some values for mode.
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r
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opens a text file for reading (must
exist)
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w
|
opens a text file for writing
(overwritten or created)
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a
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append to a text file
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rb
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opens a binary file for reading
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wb
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opens a binary file for writing
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ab
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appends to a binary file
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r+
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opens a text file for read/write (must
exist)
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w+
|
opens a text file for read/write
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a+
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append a text file for read/write
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rb+
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opens a binary file for read/write
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wb+
|
opens a binary file for read/write
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ab+
|
append a binary file for read/write
|
If fopen( ) cannot open
"test.dat " it will a return a NULL pointer which should always be
tested for as follows.
FILE *fp ;
if (
( fp = fopen( "test.dat", "r" ) ) ==
NULL )
{
puts( "Cannot open
file") ;
exit( 1) ;
}
This will cause the program to be exited
immediately if the file cannot be opened.
The fclose() function is
used to disassociate a file from a stream and free the stream for use again.
fclose( fp ) ;
fclose() will automatically flush any data remaining in the data buffers to
the file.
Once a file pointer has been linked to a
file we can write characters to it using the fputc() function.
fputc( ch, fp
) ;
If successful the function returns the
character written otherwise EOF. Characters may be read from a file using the
fgetc() standard library function.
ch =
fgetc( fp ) ;
When EOF is reached in the file fgetc( ) returns
the EOF character which informs us to stop reading as there is nothing more
left in the file.
For Example :- Program to copy a file byte
by byte
#include <stdio.h>
void main()
{
FILE *fin, *fout ;
char dest[30], source[30], ch ;
puts( "Enter source file name" );
gets( source );
puts( "Enter destination file
name" );
gets( dest ) ;
if ( ( fin = fopen( source, "rb"
) ) == NULL ) // open as binary as we don’t
{// know what is in file
puts( "Cannot open input file
") ;
puts( source ) ;
exit( 1 ) ;
}
if ( ( fout = fopen( dest, "wb" )
) == NULL )
{
puts( "Cannot open output file
") ;
puts( dest ) ;
exit( 1 ) ;
}
while ( ( ch = fgetc( fin ) ) !=
EOF )
fputc( ch , fout ) ;
fclose( fin ) ;
fclose( fout ) ;
}
NB : When any stream I/O function
such as fgetc() is called the current position of the file pointer is
automatically moved on by the appropriate amount, 1 character/ byte in the case
of fgetc() ;
This is quite similar to working with
characters except that we use the functions fgets() and fputs()
whose prototypes are as follows :-
int fputs( const char *str, FILE *fp ) ;
char *fgets( char *str, int maxlen, FILE
*fp ) ;
For Example : Program to read lines of text
from the keyboard, write them to a file and then read them back again.
#include <stdio.h>
void main()
{
char file[80], string[80] ;
FILE *fp ;
printf( "Enter file Name : " );
gets( file );
if (( fp = fopen( file, "w"
)) == NULL )//open for writing
{
printf( "Cannot open file
%s", file ) ;
exit( 1 ) ;
}
while ( strlen ( gets( str ) ) > 0 )
{
fputs( str, fp ) ;
fputc( '\n', fp ) ; /* must append \n for readability -- not
stored by gets() */
}
fclose( fp ) ;
if (( fp = fopen( file, "r"
)) == NULL )//open for reading
{
printf( "Cannot open file
%s", file ) ;
exit( 1 ) ;
}
while (fgets( str, 79, fptr ) != EOF )// read at most 79 characters
puts( str ) ;
fclose( fp ) ;
}
Formatted I/O
For Example :- Program to read in a string
and an integer from the keyboard, write them to a disk file and then read and
display the file contents on screen.
#include <stdio.h>
#include <stdlib.h>
void main()
{
FILE *fp ;
char s[80] ;
int t ;
if ( ( fp = fopen( "test.dat",
"w" ) ) == NULL )
{
puts( "Cannot open file test.dat")
;
exit(1) ;
}
puts( "Enter a string and a
number") ;
scanf( "%s %d", s, &t );
fprintf( fp, "%s %d", s, t );
fclose( fp ) ;
if ( ( fp = fopen( "test.dat",
"r" ) ) == NULL )
{
puts) "Cannot open file") ;
exit(1) ;
}
fscanf( fp, "%s %d" , s , &t
) ;
printf( "%s, %d\n", s, t ) ;
fclose( fp ) ;
}
Note
: There are several I/O streams opened
automatically at the start of every C program.
stdin --- standard input ie. keyboard
stdout --- standard
output ie. screen
stderr --- again
the screen for use if stdout malfunctions
It is through these streams that the
console functions we normally use operate. For example in reality a normal
printf call such as
printf( "%s %d", s, t ) ;
is in fact interpreted as
fprintf( stdout, "%s %d", s, t )
;
fread() and fwrite()
These two functions are used to read and
write blocks of data of any type. Their prototypes are as follows where size_t
is equivalent to unsigned.
size_t fread( void *buffer, size_t num_bytes, size_t count,
FILE *fp ) ;
size_t fwrite( const void *buffer, size_t num_bytes, size_t count,
FILE *fp ) ;
where buffer
is a pointer to the region in memory from which the data is to be read or
written respectively, num_bytes is
the number of bytes in each item to be read or written, and count is the total number of items (
each num_bytes long ) to be read/written. The functions return the number of
items successfully read or written.
For Example :-
#include <stdio.h>
#include <stdlib.h>
struct tag {
float balance ;
char name[ 80 ] ;
} customer = { 123.232, "John" } ;
void main()
{
FILE *fp ;
double d = 12.34 ;
int i[4] = {10 , 20, 30, 40 } ;
if ( (fp = fopen ( "test.dat",
"wb+" ) ) == NULL )
{
puts( "Cannot open File" )
;
exit(1) ;
}
fwrite( &d, sizeof( double ), 1, fp ) ;
fwrite( i, sizeof( int ), 4, fp ) ;
fwrite( &customer, sizeof( struct tag
), 1, fp ) ;
rewind( fp ) ; /* repositions file pointer to start */
fread( &d, sizeof( double ), 1, fp ) ;
fread( i, sizeof( int ), 4, fp ) ;
fread( &customer, sizeof( struct tag ),
1, fp ) ;
fclose( fp ) ;
}
NB : Unlike all the other functions we have encountered so far fread and
fwrite read and write binary data in
the same format as it is stored in memory so if we try to edit one these files
it will appear completely garbled. Functions like fprintf, fgets, etc. read and
write displayable data. fprintf will write a double as a series of digits while
fwrite will transfer the contents of the 8 bytes of memory where the double is
stored directly.
Random Access I/O
The fseek() function is
used in C to perform random access I/O and has the following prototype.
int fseek ( FILE *fp, long num_bytes, int
origin ) ;
where origin
specifies one of the following positions as the origin in the operation
SEEK_SET --- beginning
of file
SEEK_CUR --- current
position
SEEK_END --- EOF
and where num_bytes is the offset in bytes to the required position in the
file. fseek() returns zero when successful, otherwise a non-zero value.
For Example if we had opened a file which
stored an array of integers and we wish to read the 50th value we might do the following
fseek ( fp, ( 49 * sizeof( int ) ),
SEEK_SET ) ;
fscanf( fp, "%d", &i ) ;
from anywhere in the program.
Low -- Level I/O
Low level file input and output in C does
not perform any formatting or buffering of data whatsoever, transferring blocks
of anonymous data instead by making use of the underlying operating system's
capabilities.
Low level I/O makes use of a file handle or
descriptor, which is just a non-negative integer, to uniquely identify a file
instead of using a pointer to the FILE structure as in the case of stream I/O.
As in the case of stream I/O a number of
standard files are opened automatically :-
standard input
--- 0
standard output --- 1
standard error --- 2
The following table lists some of the more
common low level I/O functions, whose prototypes are given in <io.h> and
some associated constants are contained in <fcntl.h> and
<sys\stat.h>.
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open()
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opens a disk file
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close()
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closes a disk file
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read()
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reads a buffer of data from disk
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write()
|
writes a buffer of data to disk
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The open function has the following
prototype and returns -1 if the open operation fails.
int open ( char *filename, int
oflag [, int pmode] ) ;
where filename is the name of the file to
be opened, oflag specifies the type of operations that are to be allowed on the
file, and pmode specifies how a file is to be created if it does not exist.
oflag may be any logical combination of the following constants which are
just bit flags combined using the
bitwise OR operator.
|
O_APPEND
|
appends to end of file
|
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O_BINARY
|
binary mode
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O_CREAT
|
creates a new file if it doesn't exist
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O_RDONLY
|
read only access
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O_RDWR
|
read write access
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O_TEXT
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text mode
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O_TRUNC
|
truncates file to zero length
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O_WRONLY
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write only access
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pmode is only used when O_CREAT is specified as part of oflag and may be
one of the following values
S_IWRITE
S_IREAD
S_IREAD | S_IWRITE
This will actually set the read / write
access permission of the file at the operating system level permanently unlike
oflag which specifies read / write access just while your program uses the
file.
The close() function has the following prototype
int close ( int handle ) ;
and closes the file associated with the
specific handle.
The read() and write()
functions have the following prototypes
int read( int handle, void *buffer,
unsigned int count ) ;
int write( int handle, void *buffer, unsigned
int count ) ;
where handle refers to a specific file
opened with open(), buffer is the storage location for the data ( of any type )
and count is the maximum number of bytes to be read in the case of read() or
the maximum number of bytes written in the case of write(). The function returns the
number of bytes actually read or written or -1 if an error occurred during the
operation.
Example : Program to read the first 1000
characters from a file and copy them to another.
#include
<io.h>
#include
<fcntl.h>
#include
<sys\stat.h>
void
main()
{
char
buff[1000] ;
int
handle ;
handle=open("
test.dat", O_BINARY|O_RDONLY, S_IREAD | S_IWRITE );
if (
handle == -1 ) return ;
if (
read( handle, buff, 1000 ) == 1000 )
puts( "Read successful");
else
{
puts( Read failed" ) ;
exit( 1 );
}
close(
handle ) ;
handle =
open("test.bak",
O_BINARY|O_CREAT|O_WRONLY|
O_TRUNC,
S_IREAD | S_IWRITE ) ;
if (
write( handle, buff, 1000 ) == 1000 )
puts( "Write successful") ;
else
{
puts( "Write Failed") ;
exit( 1 ) ;
}
close(
handle ) ;
}
Low level file I/O also provides a seek
function lseek with the following
prototype.
long _lseek( int handle, long offset, int origin );
_lseek uses the same origin etc. as fseek()
but unlike fseek() returns the offset, in bytes, of the new file position
from the beginning of the file or -1 if an error occurs.
For Example : Program to determine the size
in bytes of a file.
#include <stdio.h>
#include <io.h>
#include <fcntl.h>
#include <sys\stat.h>
void main()
{
int handle ;
long length ;
char name[80] ;
printf( “Enter file name : ” ) ;
gets( name ) ;
handle=open( name,O_BINARY| O_RDONLY,
S_IREAD | S_IWRITE );
lseek(
handle, 0L, SEEK_SET ) ;
length = lseek( handle, 0L, SEEK_END ) ;
close( handle ) ;
printf( “The length of %s is %ld bytes \n”,
name, length ) ;
}
Exercises
1. Write a program that determines the following statistics pertaining
to a text file.
i.
Total number of characters
ii. Number of alphabetic characters
iii. Number of words
iv. Number of non alphabetic characters
v. Tabulates the usage of each letter of the alphabet.
2. Write a program that computes the value of Sin( x ) for x in the
range 0 to 2p in steps of 0.01 radians and stores
them in a binary file. This look-up table is commonly used to improve program
performance in practical programming rather than calculating values on the
spot. Using the standard library random number generator to generate the angles
compare the time it takes to ‘calculate’ Sin(x) for 100 values of x using the
look-up table and calculating them straight. You might find the standard
library time functions useful to compare times accurately.
3.
Programming Assignment : Simple DataBase.
Write a simple database program which
stores and manages information of the type contained in the following structure
by making use of a dynamically allocated array / list of structures of this
type as described below.
typedef struct details {
int rec_id ;
char name[20] ;
char address[80] ;
long UCCid ;
} DETAILS ;
typedef struct list {
DETAILS **data_items ;
int numrecords ;
int selrecord ;
} LIST ;
The list structure defined above contains
three data items. <numrecords> is the total number of records in the list
at present, <selrecord> is the current record selected, and
<data_items> is a pointer to a pointer to type DETAILS, i.e. a doubly
indirected pointer to the actual data.
The data is arranged as illustrated below
in the case of a list with two records.
|
|
The list structure tells us that there are
two records in the list, the current being the first in the list. The pointer
mylist->data_items, of type DETAILS **, has been allocated memory to store
two addresses, of type DETAILS *, i.e. the addresses for each individual record.
Each of these individual pointers, i.e. *(mylist->data_items + i), has been
allocated sufficient memory to store an individual record.
Your program should set up a data structure
of the type described above and allow the user to perform the following tasks.
1. Add a record to the
database.
2. Search for a record by
field in the database.
3. Order the database by
field.
4. Retrieve a record from the
database.
5. Extract a record from the
database, deleting it completely.
6. Save the database
appropriately to a file.
7. Load an existing database
from a file.
Your program should contain the following
functions / features.
void initlist( LIST *list ) ;
This function should set <selrecord>
= <numrecords> = 0 and <data_items> = NULL.
void add( LIST *list, DETAILS *new );
This function should add the record pointed
to by <new> onto the end of the list pointed to by <list>. This
means that <selrecord> and <numrecords> will have to be modified
appropriately and the pointer
<list->data_items> must be resized
to hold the address of one extra record ( using realloc( ) for
example ), and memory must be allocated for the actual record i.e. for *( list-> data_items +
list->numrecords - 1 ).
DETAILS *probe( LIST *list, int i ) ;
This function returns a pointer to the
current record and automatically moves you onto the next record. If the current
selection is 0, i.e. no record exists in the list, the function should return
NULL. If the current selection is otherwise invalid <selrecord> should be
reset to the first record and continue as normal. If <i> is equal to zero
the list is to be reset and continue as normal, otherwise ignore <i>.
void extract( LIST *list );
This function removes the current
selection completely from the list. It removes nothing if the current record is
invalid.
void swap( LIST *list, int i, int j ) ;
Swaps records i and j in the list. Note
you should only swap the actual addresses of the individual records.
void orderlist( LIST *list, int field ) ;
This function should ideally order the
complete list in terms of the field given, e.g. in terms of name, UCCid, etc.
However it will suffice to do this in terms of name only say.
DETAILS *search( LIST *list, char *item, int
field ) ;
Search the list from the current position
on, for the next occurrence of the searchitem, <item>, in a particular
field of the list ( with the same proviso as above ). The function returns the
null pointer if the item is not found, or a pointer to the particular record if
it is found. The record becomes the current selection.
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