14. fiile io

2
File Input/Output
 Input/Output device for programs
– Input: Keyboard
– Output: Monitor
 Is it possible to read or write data from/to files?
A B C
1 80 90 70
2 80 60 40
3 60 50 70
…
C program
A B C Avg.
1 80 90 70 80
2 80 60 40 60
3 60 50 70 60
…
read write
file1.txt file2.txt

3
File Input/Ooutput
 File Input/Output Process
Open the file
Read data from file
Write data to file
Close the file
fopen
fscanf, fprintf, fgets, fputs, …
fclose

4
File Operation Function fopen()
 fopen()
– Used to prepare external files to use in programs
• All files are associated with a data structure known as a stream
if fopen successes
– Return value: a pointer of FILE structure
• File stream is manipulated with FILE pointer returned by fopen
 fopen() Syntax
FILE *fopen( const char *filename, const char *mode );
Return by file pointer Filename in disk File open mode

5
 Example
– Open my_file.txt in the read mode
– Return NULL at failure
– Eg) If a file named “my_file.txt” does not exist
FILE *fp ;
fp = fopen(“my_file.txt”, “r”);

6
 Example
– Open my_file.txt in the write mode
– If my_file.txt does not exist, it is created
– If my_file.txt exists, the content of the file is erased
FILE *fp ;
fp = fopen(“my_file.txt”, “w”);

7
 Example
– Open my_file.txt in the append mode (similar to write mode)
– If my_file.txt exists, the file content is kept and new data is
appended at the end of file
FILE *fp ;
fp = fopen(“my_file.txt”, “a”);

8
 Example
– Open my_file.txt in the read/write mode
– Return NULL, if my_file.txt does not exist
– If my_file.txt exists, the file content is kept
– Read/write pointer is positioned at the beginning of the file
FILE *fp ;
fp = fopen(“my_file.txt”, “r+”);

9
 Example
– If my_file.txt exists, the file content is erased
FILE *fp ;
fp = fopen(“my_file.txt”, “w+”);

10
 Example
– If my_file.txt exists, the file content is kept
– Read/write pointer is positioned at the end of the file
FILE *fp ;
fp = fopen(“my_file.txt”, “a+”);

11
File pointer
 File pointer
– File I/O is processed through file pointer
 ‘FILE’ Type
 a type of structure defined with typedef in stdio.h contains
information of file
[Ex]
FILE *fp; /* declare ‘fp’ as
file pointer */

12
File Operation Function fclose()
 fclose()
– Close the file opened by fopen
 fclose() Syntax
int fclose( FILE *stream );
file pointer returned by fopen
Returns 0 : successful
EOF : error
(EOF is defined as –1 in stdio.h)

13
 fclose() Example
#include <stdio.h>
void main() {
FILE *fp ;
fp = fopen(“my_file.txt”, “r”);
...
fclose( fp ) ;
}

14
Formatted I/O Function fprintf()
 fprintf()
– printf() for file operation
– First argument is the file pointer which has one of write
modes (w, a, r+, w+, a+)
– The rest arguments are the same as printf()
– fprintf() writes data into abc.txt
FILE* fp = fopen( “abc.txt”, “w” ) ;
fprintf( fp, “%d %dn”, 1, 2 );

15
Formatted I/O Function fprintf()
 fscanf()
– scanf() for file operation
– First argument is file pointer which has one of read modes
(r, r+, w+, a+)
– The rest arguments are the same as printf()
– scanf() reads data from abc.txt
int i, j ;
FILE* fp = fopen( “abc.txt”, “r” ) ;
fscanf( fp, “%d %d”, &i, &j );

16
feof() Function
 feof()
– Check whether the file pointer is at the end of file or not
int feof( FILE *stream );
Opened file pointer
Returns none-zero : if it is EOF
0 : if it is not EOF

17
Formatted I/O Function fprintf() and fscanf()
#include <stdio.h>
void main() {
int i, j, k ;
FILE *ifp, *ofp ;
ifp = fopen("abc.txt", "r" ) ;
ofp = fopen("cal.txt", "w" ) ;
fscanf( ifp, "%d %d %d", &i, &j, &k ) ;
fprintf( ofp, "sum: %dn", i + j + k ) ;
fclose( ifp ) ;
fclose( ofp ) ;
}
 fprintf(), fscanf() Example
1 2 3
abc.txt
cal.txt is not exist

18
Formatted I/O Function fprintf() and
fscanf()
#include <stdio.h>
void main() {
int i, j, k ;
FILE *ifp, *ofp ;
ofp = fopen("cal.txt", "w" ) ;
fclose( ifp ) ;
fclose( ofp ) ;
}
1 2 3
abc.txt
sum: 6
cal.txt

19
fscanf()
#include <stdio.h>
void main() {
int i, j, k ;
FILE *ifp, *ofp ;
ofp = fopen("cal.txt", "a" ) ;
fclose( ifp ) ;
fclose( ofp ) ;
}
1 2 3
abc.txt
sum: 6
cal.txt

20
fscanf()
#include <stdio.h>
void main() {
char c ;
FILE *ifp, *ofp ;
ofp = fopen("abc2.txt", "a" ) ;
while( feof(ifp) == 0 ) {
fscanf( ifp, "%c", &c ) ;
fprintf( ofp, "%c", c ) ;
}
fclose( ifp ) ;
fclose( ofp ) ;
}
This is a
file.
1 2 3
abc.txt
This is a
file.
1 2 3
abc2.txt

21
Misc. File I/O Functions
 fgetc() , getc()
– Read a character from a file
• If there is no more character to read, return EOF
– fgetc() is equivalent to getc()
char c ;
FILE *fp = fopen("abc.txt", "r" ) ;
while( (c = fgetc(fp)) != EOF )
printf("%c", c ) ;
int fgetc ( FILE *stream ) ;

22
 fputc() , putc()
– Write a character to a file
– fputc() is identical to putc()
char c ;
FILE *ofp = fopen("xyz.txt", "w" ) ;
while( (c = fgetc(fp)) != EOF )
fputc( c, ofp ) ;
int fputc ( int c, FILE *stream ) ;

23
 fputs() : Writing a string
– puts() for file I/O
– Print a string into file
– At success, it returns a non-negative value
– At failure, it returns EOF
int fputs(char *str, FILE *fp);

24
FILE *fp;
int i;
char *data[3]={"to ben","or notn","to ben"};
fp = fopen("abc.txt", "w");
for(i = 0; i<3; i++) fputs(data[i],fp);
fclose( fp );

25
 fgets() : Reading a String
– gets() for file I/O
– Read (num -1) characters from fp and store to str
– When fp meets ‘n’ or EOF, it stops reading and store NULL
into str
– At success, it returns the address of str. Otherwise, it
returns NULL
char *fgets(char *str, int num, FILE *fp);

26
 fgets()
char s[5] ;
FILE* fp = fopen("abc.txt", "r" ) ;
fgets( s, 5, fp ) ;
123
1 2
1234567

27
 Difference between gets and fgets
– gets: read before ‘n’. Not store ‘n’
– fgets: read before ‘n’. Store also ‘n’

28
Accessing a File Randomly
 How to read the file again?
char c ;
while( !foef(fp) ) {
fscanf( fp, "%c", &c ) ;
printf("%c", c ) ;
}
printf("%c", c ) ;
}
abcd
efghi
abc.txt

29
 rewind()
– Move file position indicator
to the beginning of the file
void rewind( FILE* ) ;
char c ;
FILE *fp = fopen("abc.txt", “r” ) ;
printf("%c", c ) ;
}
rewind( fp ) ;
printf("%c", c ) ;
}
abcd
efghi
abc.txt

30
 fseek()
– Move file position indicator to offset from place
– Value of place
0 (starting point of the file)
1 (current position)
2 (end of the file)
fseek( file_ptr, offset, place);

31
 fseek()
fseek( file_ptr, offset, place);
0 (SEEK_SET)
1 (SEEK_CUR)
2 (SEEK_END)
0123456789
abcdefghjk
abc.txt
fseek(fp, 3, SEEK_SET) ;
fseek(fp, 5, SEEK_CUR) ;
fseek(fp, -10, SEEK_END) ;

32
 ftell()
– Return the current value of file position indicator
– Returned value is the number of bytes from the beginning of
the file
– Value of indicator is increased by one when it read a
character
int ftell( FILE* );

33
 ftell()
char c ;
int pos ;
printf("%d: %cn", ftell(fp), c ) ;
}
abcd
efghi
abc.txt

34
stdin, stdout & stderr
 Print output with fprintf
– stdout is a file pointer for monitor
 Read from keyboard with fscanf
– Stdin is a file pointer for keyboard
printf( “This is a testn” ) ; fprintf( stdout, “This is a testn” ) ;
scanf( “%d”, &k ) ; fscanf( stdin, “%d”, &k ) ;

35
Standard C files in stdio.h
Written in C Name Remark
stdin standard input file connected to the keyboard
stdout standard output file connected to the screen
stderr standard error file connected to the screen
 Three kinds of file pointer in stdio.h

36
#include <stdio.h>
void main() {
int k, j ;
fscanf( stdin, "%d %d", &k, &j ) ;
fprintf( stdout, "Sum: %dn", k + j ) ;
fprintf( stderr, "OKn") ;
}

14. fiile io

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