14. fiile io

File I/O

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

File Operation Function fopen()

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

File Operation Function fopen()

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

File Operation Function fopen()

Example

– Open my_file.txt in the append mode (similar to write mode)

– If my_file.txt does not exist, it is created

– 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

File Operation Function fopen()

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

File Operation Function fopen()

Example

– Open my_file.txt in the read/write mode

– If my_file.txt does not exist, it is created

– If my_file.txt exists, the file content is erased

FILE *fp ;

fp = fopen(“my_file.txt”, “w+”);

10

File Operation Function fopen()

Example

– Open my_file.txt in the read/write mode

– If my_file.txt does not exist, it is created

– 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

File Operation Function fopen()

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 %d\n”, 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: %d\n", 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 ;

ifp = fopen("abc.txt", "r" ) ;

ofp = fopen("cal.txt", "w" ) ;

fscanf( ifp, "%d %d %d", &i, &j, &k ) ;

fprintf( ofp, "sum: %d\n", i + j + k ) ;

fclose( ifp ) ;

fclose( ofp ) ;

}

fprintf(), fscanf() Example

1 2 3

abc.txt

sum: 6

cal.txt

19

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", "a" ) ;

fscanf( ifp, "%d %d %d", &i, &j, &k ) ;

fprintf( ofp, "sum: %d\n", i + j + k ) ;

fclose( ifp ) ;

fclose( ofp ) ;

}

fprintf(), fscanf() Example

1 2 3

abc.txt

sum: 6

cal.txt

20

Formatted I/O Function fprintf() and fscanf()

#include <stdio.h>

void main() {

char c ;

FILE *ifp, *ofp ;

ifp = fopen("abc.txt", "r" ) ;

ofp = fopen("abc2.txt", "a" ) ;

while( feof(ifp) == 0 ) {

fscanf( ifp, "%c", &c ) ;

fprintf( ofp, "%c", c ) ;

}

fclose( ifp ) ;

fclose( ofp ) ;

}

fprintf(), fscanf() Example

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

Misc. File I/O Functions

fputc() , putc()

– Write a character to a file

– fputc() is identical to putc()

char c ;

FILE *fp = fopen("abc.txt", "r" ) ;

FILE *ofp = fopen("xyz.txt", "w" ) ;

while( (c = fgetc(fp)) != EOF )

fputc( c, ofp ) ;

int fputc ( int c, FILE *stream ) ;

23

Misc. File I/O Functions

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

Misc. File I/O Functions

FILE *fp;

int i;

char *data[3]={"to be\n","or not\n","to be\n"};

fp = fopen("abc.txt", "w");

for(i = 0; i<3; i++) fputs(data[i],fp);

fclose( fp );

25

Misc. File I/O Functions

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

Misc. File I/O Functions

fgets()

char s[5] ;

FILE* fp = fopen("abc.txt", "r" ) ;

fgets( s, 5, fp ) ;

123

1 2

1234567

27

Misc. File I/O Functions

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 ;

FILE *fp = fopen("abc.txt", "r" ) ;

while( !foef(fp) ) {

fscanf( fp, "%c", &c ) ;

printf("%c", c ) ;

}

while( !foef(fp) ) {

fscanf( fp, "%c", &c ) ;

printf("%c", c ) ;

}

abcd efghi

abc.txt

29

Accessing a File Randomly

rewind()

– Move file position indicator to the beginning of the file

void rewind( FILE* ) ; char c ;

FILE *fp = fopen("abc.txt", “r” ) ;

while( !foef(fp) ) {

fscanf( fp, "%c", &c ) ;

printf("%c", c ) ;

}

rewind( fp ) ;

while( !foef(fp) ) {

fscanf( fp, "%c", &c ) ;

printf("%c", c ) ;

}

abcd efghi

abc.txt

30

Accessing a File Randomly

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

Accessing a File Randomly

fseek()

fseek( file_ptr, offset, place);

0 (SEEK_SET) 1 (SEEK_CUR) 2 (SEEK_END)

0123456789

abcdefghjk

abc.txt

FILE *fp = fopen("abc.txt", “r” ) ;

fseek(fp, 3, SEEK_SET) ;

fseek(fp, 5, SEEK_CUR) ;

fseek(fp, -10, SEEK_END) ;

32

Accessing a File Randomly

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

Accessing a File Randomly

ftell()

char c ;

int pos ;

FILE *fp = fopen("abc.txt", “r” ) ;

while( !foef(fp) ) {

fscanf( fp, "%c", &c ) ;

printf("%d: %c\n", 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 test\n” ) ; fprintf( stdout, “This is a test\n” ) ;

scanf( “%d”, &k ) ; fscanf( stdin, “%d”, &k ) ;

35

stdin, stdout & stderr

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

stdin, stdout & stderr

#include <stdio.h>

void main() {

int k, j ;

fscanf( stdin, "%d %d", &k, &j ) ;

fprintf( stdout, "Sum: %d\n", k + j ) ;

fprintf( stderr, "OK\n") ;

}