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File Management in C

Faculty : Gaurav Garg Asst. Prof CSE, AITM Palwal

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Often it is not enough to just display the data on the screen. This is because if the data is large, only a limited amount of it can be stored in memory and only a limited amount of it can be displayed on the screen. It would be inappropriate to store this data in memory for one more reason. Memory is volatile and its contents would be lost .

So to store all data in systematic way so that when we need that data that can be available easily, this data is stored in the form of files.

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Console oriented Input/Output

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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

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Real-life applications

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Large data volumes

E.g. physical experiments (CERN collider), human genome, population records etc.

Need for flexible approach to store/retrieve data

Concept of files

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Files

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File – place on disc where group of related data is storedE.g. your C programs, executables

High-level programming languages support file operationsNamingOpeningReadingWritingClosing

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Defining and opening file

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To store data file in secondary memory (disc) must specify to OS

Filename (e.g. sort.c, input.data)

Data structure (e.g. FILE)

Purpose (e.g. reading, writing, appending)

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Filename

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String of characters that make up a valid filename for OS

May contain two partsPrimaryOptional period with extension

Examples: a.out, prog.c, temp, text.out

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General format for opening file

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fp contains all information about fileCommunication link between system and program

Mode can ber open file for reading onlyw open file for writing onlya 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 */

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Different modes

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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

safeelse new file created

Reading modeif file already exists then opened with contents safeelse error occurs.

FILE *p1, *p2;p1 = fopen(“data”,”r”);p2= fopen(“results”, w”);

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Additional modes

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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

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Opening a File

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Before we can read (or write) information from (to) a file on a disk we must open the file. To open the file we have called the function fopen( ).

fopen( ) performs three important tasks when you open the file in “r” mode:

1.Firstly it searches on the disk the file to be opened.

2.Then it loads the file from the disk into a place in memory called buffer.

3.It sets up a character pointer that points to the first character of the buffer.

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Closing a file

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File must be closed as soon as all operations on it completed

Ensures All outstanding information associated with file flushed out from

buffersAll links to file brokenAccidental misuse of file prevented

If want to change mode of file, then first close and open again

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Closing a file

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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);

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/*WAP for Display contents of a file on screen. */

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# include "stdio.h" main( ) { FILE *fp ; char ch ; fp = fopen ( "PR1.C", "r" ) ; while ( 1 ) { ch = fgetc ( fp ) ; if ( ch == EOF ) break ; printf ( "%c", ch ) ; } fclose ( fp ) ; } www.advanced.edu.in

Input/Output operations on files

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C provides several different functions for reading/writing

getc() – read a characterputc() – write a characterfprintf() – write set of data values fscanf() – read set of data valuesgetw() – read integer putw() – write integer

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getc() and putc()

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handle one character at a time like getchar() and putchar()

syntax: putc(c,fp1);c : a character variablefp1 : pointer to file opened with mode w

syntax: c = getc(fp2);c : a character variablefp2 : 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

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Trouble in Opening a File

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There is a possibility that when we try to open a file using the function fopen( ), the file may not be opened. While opening the file in “r” mode, this may happen because the file being opened may not be present on the disk at all. And you obviously cannot read a file that doesn’t exist. Similarly, while opening the file for writing, fopen( ) may fail due to a number of reasons, like, disk space may be insufficient to open a new file, or the disk may be write protected or the disk is damaged and so on.

it is important for any program that accesses disk files to check whether a file has been opened successfully before trying to read or write to the file.

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#include "stdio.h" main( ) { FILE *fp ;

fp = fopen ( "PR1.C", "r" ) ; if ( fp == NULL ) { puts ( "cannot open file" ) ; exit( ) ; } }

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Program to read/write using getc/putc

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#include <stdio.h>main(){ FILE *f1;

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 */

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Record I/O in Files

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So far we have dealt with reading and writing only characters and strings. What if we want to read or write numbers from/to file? Furthermore, what if we desire to read/write a combination of characters, strings and numbers? For this first we would organize this dissimilar data together in a structure and then use fprintf( ) and fscanf( ) library functions to read/write data from/to file. Following program illustrates the use of structures for writing records of employees.

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/* Writes records to a file using structure */

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#include "stdio.h" main( ) { FILE *fp ; char another = 'Y' ; struct emp { char name[40] ; int age ; float bs ; } ; struct emp e ;

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fp = fopen ( "EMPLOYEE.DAT", "w" ) ; if ( fp == NULL ) { puts ( "Cannot open file" ) ; exit( ) ; } while ( another == 'Y' ) { printf ( "\nEnter name, age and basic salary: " ) ; scanf ( "%s %d %f", e.name, &e.age, &e.bs ) ; fprintf ( fp, "%s %d %f\n", e.name, e.age, e.bs ) ; printf ( "Add another record (Y/N) " ) ; fflush ( stdin ) ; another = getche( ) ; } fclose ( fp ) ;

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In this program we are just reading the data into a structure variable using scanf( ), and then dumping it into a disk file using fprintf( ). The user can input as many records as he desires. The procedure ends when the user supplies ‘N’ for the question ‘Add another record (Y/N)’.

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OUTPUT

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Enter name, age and basic salary: Sunil 34 1250.50

Add another record (Y/N) Y Enter name, age and basic salary: Sameer 21

1300.50 Add another record (Y/N) Y Enter name, age and basic salary: Rahul 34

1400.55 Add another record (Y/N) N

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fscanf() and fprintf()

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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 ifloat variable f

Example:fprintf(f1, “%d %f\n”, 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

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getw() and putw()

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handle one integer at a timesyntax: putw(i,fp1);

i : an integer variablefp1 : pointer to file ipened with mode w

syntax: i = getw(fp2);i : an integer variablefp2 : 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

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Errors that occur during I/O

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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

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Error handling

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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 otherwiseif(feof(fp))printf(“End of data\n”);

ferror() takes file-pointer as input, returns nonzero integer if error detected else returns zeroif(ferror(fp) !=0)printf(“An error has occurred\n”);

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Error while opening file

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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 ”);

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Dynamic memory allocation

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Malloc() and calloc():int *ptr; ptr = malloc(10 * sizeof (*ptr)); /* without a cast */ ptr = (int *)malloc(10 * sizeof (*ptr));

Differences between malloc() and calloc()There are 2 differences between these functions.

First, malloc() takes a single argument (the amount of memory to allocate in bytes), while calloc() needs 2 arguments (the number of variables to allocate in memory, and the size in bytes of a single variable). Secondly, malloc() does not initialize the memory allocated, while calloc() initializes the allocated memory to ZERO.

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