File handling-c programming language

1
This session Outline
Files Concepts
File Programs
Busy Bee Workshop – Session IXBusy Bee Workshop – Session IX

Console oriented Input/Output
Console oriented – use terminal (keyboard/screen)
scanf(“%d”,&i) – read data from keyboard
printf(“%d”,i) – print data to monitor
Suitable for small volumes of data
Data lost when program terminated
Busy Bee Workshop – FileBusy Bee Workshop – File

Real-life applications
Large data volumes
Need for flexible approach to store/retrieve data
Concept of files

Files
File – place on disk where group of related data is stored
E.g. your C programs, executables
High-level programming languages support file operations
Naming
Opening
Reading
Writing
Closing

Defining and opening file
To store data file in secondary memory (disk) must
specify to OS
Filename (e.g. sort.c, input.data)
Data structure (e.g. FILE)
Purpose (e.g. reading, writing, appending)

Filename
String of characters that make up a valid filename for OS
May contain two parts
Primary
Optional period with extension
Examples: a.out, prog.c, temp, text.out

General format for opening file
fp
contains all information about file
Communication link between system and program
Mode can be
r open file for reading only
w open file for writing only
a open file for appending (adding) data
FILE *fp; /*variable fp is pointer to type FILE*/
fp = fopen(“filename”, “mode”);
/*opens file with name filename , assigns identifier to fp */

Different modes
Writing mode
if file already exists then contents are deleted,
else new file with specified name created
Appending mode
if file already exists then file opened with contents safe
else new file created
Reading mode
if file already exists then opened with contents safe
else error occurs.
FILE *p1, *p2;
p1 = fopen(“data”,”r”);
p2= fopen(“results”, w”);

Additional modes
r+ open to beginning for both reading/writing
w+ same as w except both for reading and writing
a+ same as ‘a’ except both for reading and writing

Closing a file
File must be closed as soon as all operations on it completed
Ensures
 All outstanding information associated with file flushed out from
buffers
 All links to file broken
 Accidental misuse of file prevented
If want to change mode of file, then first close and open again

Closing a file
pointer can be reused after closing
Syntax: fclose(file_pointer);
Example:
FILE *p1, *p2;
p1 = fopen(“INPUT.txt”, “r”);
p2 =fopen(“OUTPUT.txt”, “w”);
……..
……..
fclose(p1);
fclose(p2);

Input/Output operations on filesC provides several different functions for reading/writing
getc() – read a character
putc() – write a character
fprintf() – write set of data values
fscanf() – read set of data values
getw() – read integer
putw() – write integer
read() – read data from binary file
write() – write data into binary file

getc() and putc()
handle one character at a time like getchar() and
putchar()
syntax: putc(c,fp1);
c : a character variable
fp1 : pointer to file opened with mode w
syntax: c = getc(fp2);
c : a character variable
fp2 : pointer to file opened with mode r
file pointer moves by one character position after every
getc() and putc()
getc() returns end-of-file marker EOF when file end
reached

Program to read/write using getc/putc
#include <stdio.h>
main()
{ FILE *fp1;
char c;
f1= fopen(“INPUT”, “w”); /* open file for writing */
while((c=getchar()) != EOF) /*get char from keyboard until CTL-Z*/
putc(c,f1); /*write a character to INPUT */
fclose(f1); /* close INPUT */
f1=fopen(“INPUT”, “r”); /* reopen file */
while((c=getc(f1))!=EOF) /*read character from file INPUT*/
printf(“%c”, c); /* print character to screen */
fclose(f1);
} /*end main */

fscanf() and fprintf()
similar to scanf() and printf()
in addition provide file-pointer
given the following
file-pointer f1 (points to file opened in write mode)
file-pointer f2 (points to file opened in read mode)
integer variable i
float variable f
Example:
fprintf(f1, “%d %fn”, i, f);
fprintf(stdout, “%f n”, f); /*note: stdout refers to screen */
fscanf(f2, “%d %f”, &i, &f);
fscanf returns EOF when end-of-file reached

getw() and putw()
handle one integer at a time
syntax: putw(i,fp1);
i : an integer variable
fp1 : pointer to file ipened with mode w
syntax: i = getw(fp2);
i : an integer variable
fp2 : pointer to file opened with mode r
file pointer moves by one integer position, data stored in
binary format native to local system
getw() returns end-of-file marker EOF when file end
reached

C program using getw, putw,fscanf, fprintf
#include <stdio.h>
main()
{ int i,sum1=0;
FILE *f1;
/* open files */
f1 = fopen("int_data.bin","w");
/* write integers to files in binary
and text format*/
for(i=10;i<15;i++) putw(i,f1);
fclose(f1);
f1 = fopen("int_data.bin","r");
while((i=getw(f1))!=EOF)
{ sum1+=i;
printf("binary file: i=%dn",i);
} /* end while getw */
printf("binary sum=%d,sum1);
fclose(f1);
}
#include <stdio.h>
main()
{ int i, sum2=0;
FILE *f2;
/* open files */
f2 = fopen("int_data.txt","w");
/* write integers to files in binary
and text format*/
for(i=10;i<15;i++) printf(f2,"%dn",i);
fclose(f2);
f2 = fopen("int_data.txt","r");
while(fscanf(f2,"%d",&i)!=EOF)
{ sum2+=i; printf("text file: i=
%dn",i);
} /*end while fscanf*/
printf("text sum=%dn",sum2);
fclose(f2);
}

On execution of previous Programs
binary file: i=10
binary file: i=11
binary file: i=12
binary file: i=13
binary file: i=14
binary sum=60,
text file: i=10
text file: i=11
text file: i=12
text file: i=13
text file: i=14
text sum=60

Errors that occur during I/O
Typical errors that occur
trying to read beyond end-of-file
trying to use a file that has not been opened
perform operation on file not permitted by ‘fopen’ mode
open file with invalid filename
write to write-protected file

Error handling
given file-pointer, check if EOF reached, errors while
handling file, problems opening file etc.
check if EOF reached: feof()
feof() takes file-pointer as input, returns nonzero if all
data read and zero otherwise
if(feof(fp))
printf(“End of datan”);
ferror() takes file-pointer as input, returns nonzero
integer if error detected else returns zero
if(ferror(fp) !=0)
printf(“An error has occurredn”);

Error while opening file
if file cannot be opened then fopen returns a NULL
pointer
Good practice to check if pointer is NULL before
proceeding
fp = fopen(“input.dat”, “r”);
if (fp == NULL)
printf(“File could not be opened n ”);

Random access to files
how to jump to a given position (byte number) in a file
without reading all the previous data?
fseek (file-pointer, offset, position);
position: 0 (beginning), 1 (current), 2 (end)
offset: number of locations to move from position
Example: fseek(fp,-m, 1); /* move back by m bytes from
current
position */
fseek(fp,m,0); /* move to (m+1)th byte in file */
fseek(fp, -10, 2); /* what is this? */
ftell(fp) returns current byte position in file
rewind(fp) resets position to start of file

Command line arguments
can give input to C program from command line
E.g. > prog.c 10 name1 name2
….
how to use these arguments?
main ( int argc, char *argv[] )
argc – gives a count of number of arguments (including
program name)
char *argv[] defines an array of pointers to character (or
array of strings)
argv[0] – program name
argv[1] to argv[argc -1] give the other arguments as strings

Example args.c
args.out 2 join leave 6
6
leave
join
2
args.out
#include <stdio.h>
main(int argc,char *argv[])
{
while(argc>0) /* print out all arguments in reverse order*/
{
printf("%sn",argv[argc-1]);
argc--;
}
}

File handling-c programming language

File handling-c programming language

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File handling-c programming language