P14 File Handling in C

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File Handling in C


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Roadmap

File

File variable declaration

File open and Close

File read and write

Pre-opened files

Binary I/O


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Files

A file is a collection of related data. C

treats a file as a series of bytes. Many

files reside on disk; however, devices liketerminals, printers, and magnetic tapes

are also considered files.


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The C library contains a large numberofroutines for manipulating files. Thedeclarations for the structures and

functions used by the file functions arestored in the standard include file. Before doing anything with files,you must put the line:

#include


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How toopen a file?

Fir stly it searcheson the disk the file to be opened.

Then it loads the file from the disk into a place in

memory called buffer.

It sets up a character pointer that points to the first

characterof the buffer.


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File variable Declaration

The declaration for a file variable is:

FILE *file-variable; /* comment*/

For example:

#include

FILE *in_file; /* file containing the input data*/


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Fopen() and Fclose()

Before a file can be used, it must be opened usingthe function fopen. fopen returns a pointer to the filestructure for the file. The format for fopen is:

file-variable = fopen(name, mode);

where:

file-variable

is a file variable. A value of NULL is returned on error.

name

is the actual name of the file (data.txt, temp.dat, etc.).


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Mode

indicates if the file is to be read or written.

mode is "w" for writing and "r" for reading.The flag "b" can be added to indicate a

binary file. Omitting the binary flag

indicates an ASCII (text) file. Flags can be

combined. So "wb" is used to write abinary file.


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The fopen() function returns a file handle

that will be used in subsequent I/O

operations. If there is an I/O error, then

the value NULL is returned:


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Other modes "r" Searches file. If the file isopened successfully fopen( ) loads

it into memory and sets up a pointer which points to the firstcharacter in it. If the file cannot be opened fopen( ) returns NULL.Operations possible reading from a file

"w" Searches file. If the file exists, its contents are overwritten. If

the file doesnt exist, a new file is created. Returns NULL, if unabletoopen file. Operations possible writing to the file.

"a" Searches file. If the file isopened successfully fopen( ) loadsit into memory and sets up a pointer that points to the last characterin it. If the file doesnt exist, a new file is created. Returns NULL, ifunable toopen file. Operations possible - adding new contents atthe end of file.

"r+" Searches file. If isopened successfully fopen( ) loads it intomemory and sets up a pointer which points to the first character init. Returns NULL, if unable toopen the file. Operations possible -reading existing contents, writing new contents, modifyingexisting contents of the file.


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"w+" Searches file. If the file exists, its contents are

overwritten. If the file doesnt exist a new file is created.

Returns NULL, if unable toopen file. Operations possible -

writing new contents, reading them back and modifying

existing contents of the file.

"a+" Searches file. If the file isopened successfully fopen( )

loads it into memory and sets up a pointer which points to the

first character in it. If the file doesnt exist, a new file is created.

Returns NULL, if unable toopen file. Operations possible -

reading existing contents, appending new contents to end

of file. Cannot modify existing contents.


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Example 1

FILE *in_file; /* File to read */

in_file = fopen("input.txt", "r"); /* Open the

input file */if (in_file == NULL) { /* Test for error */

fprintf(stderr, "Error: Unable to input file

'input.txt'\n");exit (8);

}


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Closing a file

The function fclose will close the file. The format

of fclose is:

status= fclose(file-variable);

fclose(file-variable);

The variable statusis zero if the fclose was

successful or nonzero for an error. If you don'tcare about the status, the second form closes

the file and discards the return value.


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Pre-opened files

C provides three pre -opened files.


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Fgetc() & Fputc()

The function fgetc reads a single character from a file. If no more data exists in the file, the function will return theconstant EOF (EOF is defined in stdio.h ).

ch = fgetc(file);

Note that fgetc returns an integer, not a character. Thisreturn is necessary because the EOF flag must be anoncharacter value.

A similar function, fputc , exists for writing a singlecharacter.

fputc(character, file);


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Example2

#include

const char FILE_NAME[] = "input.txt";

#include

int main()

{

int count = 0; /* numberof charactersseen */FILE *in_file; /* input file */

/* characteror EOF flag from input */

int ch;

in_file = fopen(FILE_NAME, "r");

if (in_file == NULL) {printf("Cannot open %s\n", FILE_NAME);

exit(8);

}


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while (1) {

ch = fgetc(in_file);

if (ch == EOF)

break;

++count;

}

printf("Numberof characters in %s is %d\n",

FILE_NAME, count);

fclose(in_file);return (0);

}


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Fgets() and fputs()

The functions fgets and fputs workon one line at a time. The formatof the fgets call is:

string_ptr= fgets(string, size, file);

fputs issimilar to fgets except that it writes a string instead ofreading it. The format of the fputs function is:

string_ptr= fputs(string, file);

The parameters to fputs are similar to the ones for fgets. fputsneeds no size because it gets the size of the line to write fromthe length of the string. (It keeps writing until it hits a null, '\0'.)


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where:

string_ptr

is equal tostring if the read wassuccessful, or NULL ifend -of-file or an error is detected.

string

is a character array in which the function places thestring.

size

is the size of the character array. fgets reads until it getsa line (complete with ending \n) or it readssize -1characters. It then ends the string with a null (\0).


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Problems can occur if the size specified is

too big. C pr ovides a convenient way of

making sure that the size parameter is just

right through the use of the sizeof

operator.

The sizeofoperator returns the size of its

argument in bytes.


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For example:

charstring[100];

. . .fgets(string, sizeof(string), in_file);


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Fprintf()

The function fprintf converts data and writes

it to a file. The general form of the fprintf

function is:

count= fprintf(file, format,parameter-1,

parameter-2, ...);

fprintfhas twosister functions: printf and

sprintf. sprintf issimilar to fprintf, except

that the first argument is a string.


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where:

count

is the numberof characterssent or -1 if anerroroccurred.

format

describeshow the arguments are to beprinted.

parameter-1, parameter-2, ...

are parameters to be converted and sent.


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example3#include

#include

int main()

{

char name[100]; /* name of the file to use */

FILE *in_file; /* file for input */

printf("Name? ");Scanf(%s,&name);

in_file = fopen(name, "r");

if (in_file == NULL) {

fprintf(stderr, "Could not open file\n");

exit(8);

}

printf("File found\n");fclose(in_file);

return (0);

}


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Example4

int main()

{

int i;

FILE *fout;

int str[257];

for(i=0;i


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example4

main()

{

int i = 10 ;

char ch = 'A' ;

float a = 3.14 ;

charstr[20] ;

printf ( "\n%d %c %f", i, ch, a ) ;

sprintf ( str, "%d %c %f", i, ch, a ) ;

printf ( "\n%s", str ) ;

}

In this program the printf( ) printsout the valuesofi, ch anda on the screen, whereassprintf( ) stores these valuesin the character array str.


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Fscanf()

scanfhassimilarsister functions: fscanf and

sscanf. The format for fscanf is:

number= fscanf(file, format, &parameter-1,

...);


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where:

number

is the numberof parameterssuccessfully converted. Ifthere was input but nothing could be converted, a zero

is returned. If no data is present, EOF is returned.file

is a file opened for reading.

format

describes the data to be read.parameter-1

is the first parameter to be read.


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Binary I/O

Binary I/O is accomplished through two

routines: fread and fwrite. The syntax for

fread is:read_size = fread(data_ptr, 1, size, file);

fwrite has a calling sequence similar tofread:

write_size = fwrite(data_ptr, 1, size, file);


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where:

read_size

is the size of the data that was read. If this value is lessthan size, then an end-of-file or error was encountered.

data_ptris the pointer to the data to be read. This pointer mustbe cast to a character point (char *) if the data is anytype other than a character.

size

is the numberof bytes to be read.file

is the input file.


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P14 File Handling in C

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