Files in C

A file is a repository of data that is stored in a permanent storage media, mainly in secondary memory. In order to use the files we should learn how to read information from a file and how to write information into a file.

A very important concept in C is the stream.The stream is a common logical interface to the various devices( files).A stream is linked to a file while using an open operation. A stream is disassociated from a file while using a close operation. The current location, also referred to as the current position, is the location in a file where the next file access will occur.There are two types of streams text and binary.

The following are some difference between text and binary files

Text file is human readable because everything is stored in terms of text. In binary file everything is written in terms of 0 and 1, therefore binary file is not human readable.
A newline(\n) character is converted into the carriage return-linefeed combination before being written to the disk. In binary file, these conversions will not take place.
In text file, a special character, whose ASCII value is 26, is inserted after the last character in the file to mark the end of file. There is no such special character present in the binary mode files to mark the end of file.
In text file, the text and characters are stored one character per byte. For example, the integer value 1245 will occupy 2 bytes in memory but it will occupy 5 bytes in text file. In binary file, the integer value 1245 will occupy 2 bytes in memory as well as in file.

The data format is usually line-oriented in text file. Here, each line is a separate command.

Binary file always needs a matching software to read or write it.(MP3 player Image Viewer)

Using files in C
There are four steps in using files

Declare a file pointer variable
Open a file using fopen() function
Process the file using suitable functions
Close the file using fclose() function

Declare a file pointer variable

To access any file, we need to declare a pointer to FILE structure and then associate it with the particular file. This pointer is referred to as file pointer.

Syntax for declaring file pointer

FILE * fp;

A pointer to FILE structure contains information, such as size, current file position, type of file etc., to perform operation on the file.Opening a file function

Open a file using fopen() function

The fopen() function takes two arguments, the name of the file and the mode in which the file is to be opened. Mode specify the purpose of opening the file i.e, whether for reading or writing.

Syntax for opening the file in C

fp = fopen(char *filename,char *mode);

When fopen() function opens a file in memory, it returns a pointer to this particular file.If fopen() function can't open the file then it will return NULL.

Example for opening a text file for reading in C

void main()

{

FILE *fp;

fp = fopen("file1.txt","r"); //statement 1

if(fp == NULL)

{

printf("\nCan't open file or file doesn't exist.");

exit(0);

}

In the above example, statement 1 will open an existing text file "file1.txt" in read mode and return a pointer to file. If file will not open, an appropriate message will be displayed.

File opening modes

Mode	Meaning
r	Open a text file for reading
w	Create a text file for writing
a	Append to a text file
rb	Open a binary file for reading
wb	Open a binary file for writing
ab	Append to a binary file
r+	Open a text file for read/write
w+	Create a text file for read/write
a+	Append or create a text file for read/write
r+b	Open a binary file for read/write
w+b	Create a binary file for read/write
a+b	Append a binary file for read/write

Note: The complete path of the file must be specified, if the file is not in the current directory. Most of the functions prototype for file handling is mentioned in stdio.h.

Close the file using fclose() function

When the reading or writing of a file is finished, the file should be closed properly using fclose() function. The fclose() function does the following tasks:

Flushes any unwritten data from memory.

Discards any unread buffered input.

Frees any automatically allocated buffer

Finally, close the file.

Syntax for closing the file in C

int fclose( FILE* );

Example for closing the file in C

void main()

{

FILE *fp;

fp = fopen("file1.txt","r");

if(fp == NULL)

{

printf("\nCan't open file or file doesn't exist.");

exit(0);

}

- - - - - - - - - -

fclose(fp);

}

Note: A stream’s buffer can be flushed without closing it by using the fflush() library function.The flushall() command flush all open streams.pointer

As long as your program is running, if you keep opening files without closing them, the most likely result is that you will run out of file descriptors/handles available for your process, and attempting to open more files will fail eventually. On Windows, this can also prevent other processes from opening or deleting the files you have open, since by default, files are opened in an exclusive sharing mode that prevents other processes from opening them.

Once your program exits, the operating system will clean up after you. It will close any files you left open when it terminates your process, and perform any other cleanup that is necessary (e.g. if a file was marked delete-on-close, it will delete the file then; note that that sort of thing is platform-specific).

However, another issue to be careful of is buffered data. Most file streams buffer data in memory before writing it out to disk. If you're using FILE* streams from the stdio library, then there are two possibilities:

Your program exited normally, either by calling the exit(3) function, or by returning from main (which implicitly calls exit(3)).
Your program exited abnormally; this can be via calling abort(3) or _Exit(3), dying from a signal/exception, etc.

If your program exited normally, the C runtime will take care of flushing any buffered streams that were open. So, if you had buffered data written to a FILE* that wasn't flushed, it will be flushed on normal exit.

Conversely, if your program exited abnormally, any buffered data will not be flushed. The OS no idea there's some random data lying somewhere in memory that the program intended to write to disk but did not. So be careful about that.

Working with Text files

The following functions provides facility for reading and writing files

Reading	Writing
fgetc()	fputc()
fgets()	fputs()
fscanf()	fprintf()
fread()	fwrite()

Character Input and Output

Characters or a line (sequence of characters terminated by a newline) can be written or read from a file using following functions.

putc() / fputc() function

The library function putc() writes a single character to a specified stream.Its prototype is

int putc(int ch,FILE *fp);

Eg:

char c=’x’;

FILE *fp=fopen(“abc.txt”,”w”);

putc(c,fp); // fputc(c,fp) // char x is written to file abc.txt.

fputs() function

To write a line of characters to a stream, the library function fputs() is used.Its prototype is

char fputs(char *str,FILE *fp);

Eg:

char str[]=”cek”;

FILE *fp=fopen(“abc.txt”,”w”);

fputs(str,fp); // string cek is written into file abc.txt

getc() / fgetc() function

The function getc() and fgetc() are identical and can be used interchangeably. They input a single character from a specified stream.Its prototype is

int getc(FILE *fp);

Eg:

FILE *fp=fopen(“abc.txt”,”r”);

int c;

c=fgetc(fp); // reads a character

fgets() function

fgets() is a line oriented function for reading from a file.Its prototype is

char *fgets(char *str,int n, FILE *fp);

Eg:

FILE *fp=fopen(“abc.txt”,”r”);

char line[60];

char *c;

c=fgets(line,60,fp);

The other two file handling functions to be covered are fprintf() and fscanf().These functions operate exactly like printf() and scanf() except that they work with files.

Their prototypes are

int fprintf(FILE *fp, const char *control-string,…);

int fscanf(FILE *fp, const char *control-string,…);

Eg:

FILE *fp=fopen("abc.txt","w");
fprintf(fp,”%s”,”cek”);
FILE *fp=fopen("abc.txt","r");int n;
fscanf(fp,"%d",&n);

putw() and getw() function
The putw() function is used to write integers to the file.
Syntax of putw() function

putw(int number, FILE *fp);

The putw() function takes two arguments, first is an integer value to be written to the file and second is the file pointer where the number will be written.

eg:putw(n,fp)
The getw() function is used to read integer value form the file.
Syntax of getw() function
int getw(FILE *fp);
Eg:
while((num = getw(fp))!=EOF)

printf("\n%d",num);

Binary Files and Direct File I/O

The operations performed on binary files are similar to text files since both types of files can essentially be considered as stream of bytes. In fact the same functions are used to access files in C. When a file is opened, it must be designated as text or binary and usually this is the only indication of the type of file being processed.

Direct I/O is used only with binary-mode files. With direct output, blocks of data are written from memory to disk and in direct input block of data are read from disk to memory.For example, a single direct-output function can write an entire array to disk and a single direct input function call can read the entire array from disk back to memory.

The C file system includes two important functions for direct I/O: fread() and fwrite().Their prototypes are

size_t fread(void *buffer, size_t size,size_t num,FILE *fp)

size_t fwrite(void *buffer, size_t size,size_t num,FILE *fp)

The fread() function reads from the file associated with fp, num number of objects, each object size in bytes, into buffer pointed to by buffer.fwrite() function is the opposite of fread().It writes to files associated with fp,num number of objects, each object size in bytes, from the buffer pointed to by buffer.

Sample Programs

Write a c program snippet to open a file in write mode and check if the file opened successfully. if it fails , print an error message and terminate the program.

#include <stdio.h>

#include <stdlib.h>

int main() {

FILE *fp;

fp = fopen("example.txt", "w"); // Open file in write mode

if (fp == NULL) {

printf("Error: Could not open file for writing.\n");

exit(1); // Terminate the program

}

printf("File opened successfully for writing.\n");

// You can now write to the file here...

fclose(fp); // Close the file

return 0;

}

Read and display a file character by character using getc/fgetc function

#include <stdio.h>

#include <stdlib.h>

int main()

{

FILE *fp;

int ch;

fp=fopen("a.txt","r");

if(fp==NULL)

{

printf("Error opening file..");

exit(1);

}

while((ch=getc(fp))!=EOF) // can use ch=fgetc(fp) also

{

putchar(ch);

}

fclose(fp);

}
vowels consonants digits and special characters in a file ( university question)
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp;
int ch,vowels=0,consonants=0,digits=0,specc=0,c=0;
fp=fopen("a.txt","r");
if(fp==NULL)
{
printf("Error opening file..");
exit(1);
   }
while((ch=fgetc(fp))!=EOF)
{
   if(ch=='a' || ch=='e' || ch=='i' ||
           ch=='o' || ch=='u' || ch=='A' ||
           ch=='E' || ch=='I' || ch=='O' ||
           ch=='U')
        {
            ++vowels;
        }
        else if((ch>='a'&& ch<='z') || (ch>='A'&& ch<='Z'))
        {
            ++consonants;
        }
        else if(ch>='0' && ch<='9')
        {
            ++digits;
        }
        else if (ch ==' ' || ch =='\n')
        {
            ++specc;
        }
c++;
}
    printf("Vowels: %d",vowels);
    printf("\nConsonants: %d",consonants);
    printf("\nDigits: %d",digits);
    printf("\nSpecial characters: %d\n", c-specc-vowels-consonants-digits);
fclose(fp);
}
Reading line by line from a file using fgets

#include <stdio.h>

#include <stdlib.h>

int main()

{

FILE *fp;

char * ch;

char line[80];

fp=fopen("a.txt","r");

if(fp==NULL)

{

printf("Error opening file..");

exit(1);

}

while((fgets(line,80,fp)!=NULL)

{

printf("%s",line);

}

fclose(fp);

}
Reading word by word using fscanf
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp;
char t[100];
fp=fopen("a.txt","r");
if(fp==NULL)
{
printf("Error opening source file..");
exit(1);
   }
while(fscanf(fp,"%s",t)==1)
{
printf("%s\n",t);
}
fclose(fp);
}

Write data to the file( char by char using putc/fputc function)
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp;
int ch;
fp=fopen("a.txt","w");
if(fp==NULL)
{
printf("Error opening file..");
exit(1);
   }
do{
   ch=getchar();
   if (ch=='$') break;
   putc(ch,fp); //fputc(ch,fp);
}
while(1);
fclose(fp);
}
Write data to the file( as strings using fputs function)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
FILE *fp;
char t[80];
fp=fopen("a.txt","w");
if(fp==NULL)
{
printf("Error opening file..");
exit(1);
   }
printf("Enter strings...type end to stop\n");
do{
   fgets(t,80,stdin);
   printf("%s\n",t);
   if(strcmp(t,"end\n")==0 ) break;
   fputs(t,fp);
}
while(1);
fclose(fp);
}
C Program to delete a specific line from a text file
#include <stdio.h>

int main()

{ FILE *fp1, *fp2; //consider 40 character string to store filename

char filename[40];

char c; int del_line, temp = 1;

//asks user for file name

printf("Enter file name: ");

//receives file name from user and stores in 'filename'

scanf("%s", filename);

//open file in read mode

fp1 = fopen(filename, "r");

c = getc(fp1); //until the last character of file is obtained

while (c != EOF)

{ printf("%c", c); //print current character and read next character

c = getc(fp1);

}

//rewind

rewind(fp1);

printf(" \n Enter line number of the line to be deleted:");

//accept number from user.

scanf("%d", &del_line);

//open new file in write mode

fp2 = fopen("copy.c", "w");

c = getc(fp1);

while (c != EOF)

{ c = getc(fp1);

if (c == '\n') temp++; //except the line to be deleted

if (temp != del_line) { //copy all lines in file copy.c putc(c, fp2); } } //close both the files. fclose(fp1);

fclose(fp2);

//remove original file

remove(filename);

//rename the file copy.c to original name

rename("copy.c", filename);

printf("\n The contents of file after being modified are as follows:\n");

fp1 = fopen(filename, "r");

c = getc(fp1);

while (c != EOF)

{ printf("%c", c);

c = getc(fp1);

}

fclose(fp1); return 0;

}
Write data to the file( as strings using fprintf)
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp;
fp=fopen("a.txt","w");
if (fp==NULL)
{
    printf("error opening file..\n");
    exit(1);
   }
else
{
fprintf(fp,"%s","Welcome\n");
   fprintf(fp,"%s","to file handling in C\n");
   }
fclose(fp);
}

Counting number of lines, characters and words in a file

#include <stdio.h>

#include <stdlib.h>

int main()

{

FILE *fp;

int ch;

int nl=0,nc=0,nw=0;

fp=fopen("a.txt","r");

if(fp==NULL)

{

printf("Error opening file..");

exit(1);

}

ch=getc(fp);

while(ch!=EOF)

{

if (ch=='\n') nl++;

if(ch==' ') nw++;

nc++;

ch=getc(fp);

}

fclose(fp);

printf("Number of lines=%d, Number of characters = %d,Number of words=%d\n",nl,nc,nw+nl);

}

Write a C program to read integers from a text file and count how many even and odd numbers are present. ( University question)

#include <stdio.h>

int main() {

FILE *file;

int num;

int evenCount = 0, oddCount = 0;

// Open the file in read mode

file = fopen("numbers.txt", "r");

// Check if file was opened successfully

if (file == NULL) {

printf("Error opening file.\n");

return 1;

}

// Read integers from the file until EOF

while (fscanf(file, "%d", &num) != EOF) {

if (num % 2 == 0)

evenCount++;

else

oddCount++;

}

// Close the file

fclose(file);

// Display the results

printf("Total even numbers: %d\n", evenCount);

printf("Total odd numbers : %d\n", oddCount);

return 0;

}

Write a C program to create a text file (input text through keyboard).Display the contents and size of the file created. ( University question)

#include <stdio.h>

#include <stdlib.h>

int main() {

FILE *fp;

char filename[100], ch;

long size = 0;

printf("Enter the filename to create: ");

scanf("%s", filename);

getchar(); // To consume the newline after filename input

// Open file in write mode

fp = fopen(filename, "w");

if (fp == NULL) {

printf("Error creating file!\n");

return 1;

}

printf("Enter text (press Enter followed by Ctrl+D to stop):\n");

// Input text from keyboard and write to file

while ((ch = getchar()) != EOF) {

fputc(ch, fp);

}

fclose(fp);

// Reopen file in read mode to display contents and find size

fp = fopen(filename, "r");

if (fp == NULL) {

printf("Error reading file!\n");

return 1;

}

printf("\nContents of the file '%s':\n\n", filename);

while ((ch = fgetc(fp)) != EOF) {

putchar(ch);

size++;

}

printf("\n\nSize of the file: %ld bytes\n", size);

fclose(fp);

return 0;

}

Cheking whether two files are identical or different

#include <stdio.h>

#include <stdlib.h>

int main()

{

FILE *fp1,*fp2;

int ca,cb;

char fname1[50],fname2[50];

printf("Enter the first file name...\n");

scanf("%s",fname1);

printf("Enter the second file name...\n");

scanf("%s",fname2);

fp1=fopen(fname1,"r");

fp2=fopen(fname2,"r");

if(fp1==NULL)

{

printf("Error opening file1..");

exit(1);

}

else if(fp2==NULL)

{

printf("Error opening file2..");

exit(1);

}

else

{

ca=getc(fp1);

cb=getc(fp2);

while(ca!=EOF && cb!=EOF && ca==cb)

{

ca=getc(fp1);

cb=getc(fp2);

}

fclose(fp1);

fclose(fp2);

if(ca==cb)

printf("Files are identical\n");

else if (ca!=cb)

printf("Files are different \n");

}

Copy one file to another character by character using getc and putc function( you can also use fgetc and fputc)

#include <stdio.h>

#include <stdlib.h>

int main()

{

FILE *fp1,*fp2;

int ch;

char fname1[50],fname2[50];

printf("Enter the source file name...\n");

scanf("%s",fname1);

printf("Enter the destination file name...\n");

scanf("%s",fname2);

fp1=fopen(fname1,"r");

fp2=fopen(fname2,"w");

if(fp1==NULL)

{

printf("Error opening source file..");

exit(1);

}

else if(fp2==NULL)

{

printf("Error opening destination file..");

exit(1);

}

else

{

while((ch=fgetc(fp1))!=EOF)
   {
     fputc(ch,fp2);
   }

}

fclose(fp1);

fclose(fp2);

printf("Files succesfully copied\n");

}

}
Copy one file to another after replacing lower case letters with corresponding uppercase letters.(university question)
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp1,*fp2;
int ch;
char fname1[50],fname2[50];
printf("Enter the source file name...\n");
scanf("%s",fname1);
printf("Enter the destination file name...\n");
scanf("%s",fname2);
fp1=fopen(fname1,"r");
fp2=fopen(fname2,"w");
if(fp1==NULL)
{
printf("Error opening source file..");
exit(1);
   }
else if(fp2==NULL)
   {
    printf("Error opening destination file..");
    exit(1);
   }
else
{
    while((ch=fgetc(fp1))!=EOF)
   {
      if(islower(ch)) ch=toupper(ch);

fputc(ch,fp2);
}
fclose(fp1);
fclose(fp2);
printf("Files succesfully copied\n");
}
}

Write a C program to replace vowels in a text file with character ‘x’.(university question)

#include <stdio.h>

#include <stdlib.h>

int isvowel(char ch)

{

ch=tolower(ch);

switch(ch)

{

case 'a':

case 'e':

case 'i':

case 'o':

case 'u':

return 1;

}

return 0;

}

int main()

{

FILE *fp1,*fp2;

int ch;

char fname1[50],fname2[50];

fp1=fopen("vow.dat","r");

fp2=fopen("x.dat","w");

if(fp1==NULL)

{

printf("Error opening source file..");

exit(1);

}

else if(fp2==NULL)

{

printf("Error opening destination file..");

exit(1);

}

else

{

while((ch=fgetc(fp1))!=EOF)

{

if(isvowel(ch)) ch='x';

fputc(ch,fp2);

}

fclose(fp1);

fclose(fp2);

//remove the original file

remove("vow.dat");

//rename the file temp file x.dat to original name

rename("x.dat", "vow.dat");

printf("Files successfully copied\n");

}

Copy one file to another line by line using fgets and fputs function

#include <stdio.h>

#include <stdlib.h>

int main()

{

FILE *fp1,*fp2;

int ch;

char fname1[50],fname2[50],t[80];

printf("Enter the source file name...\n");

scanf("%s",fname1);

printf("Enter the destination file name...\n");

scanf("%s",fname2);

fp1=fopen(fname1,"r");

fp2=fopen(fname2,"w");

if(fp1==NULL)

{

printf("Error opening source file..");

exit(1);

}

else if(fp2==NULL)

{

printf("Error opening destination file..");

exit(1);

}

else

{

while((fgets(t,sizeof(t),fp1)!=NULL))

{

fputs(t,fp2);

}

fclose(fp1);

fclose(fp2);

printf("Files succesfully copied\n");

}

}
Merging the contents of two files into a third file(university question)
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp1,*fp2,*fp3;
char t[100];
fp1=fopen("first.txt","r");
fp2=fopen("second.txt","r");
fp3=fopen("third.txt","w");
if(fp1==NULL||fp2==NULL)
{
printf("Error opening source file..");
exit(1);
   }
else if(fp3==NULL)
   {
    printf("Error opening destination file..");
    exit(1);
   }
else
{
while((fgets(t,sizeof(t),fp1)!=NULL))
{
fputs(t,fp3);
}
while((fgets(t,sizeof(t),fp2)!=NULL))
{
fputs(t,fp3);
}
fclose(fp1);
fclose(fp2);
fclose(fp3);
printf("Files succesfully merged\n");
}
}

Reading numbers from a file and separating even and odd numbers into two different files

#include <stdio.h>

#include <stdlib.h>

int main()

{

FILE *fp,*fpe,*fpo;

int n;

fp=fopen("num.dat","r");

fpe=fopen("enum.dat","w");

fpo=fopen("onum.dat","w");

if (fp==NULL||fpe==NULL||fpo==NULL)

{

printf("error opening file..\n");

exit(1);

}

else

{

while(fscanf(fp,"%d",&n)==1)

{

if ( n%2==0)

fprintf(fpe,"%d\n",n);

else

fprintf(fpo,"%d\n",n);

}

fclose(fp);

fclose(fpe);

fclose(fpo);

}

}
Note: this program can also be written using putw() getw() function

Write Palindrome words from a file to a new file

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int palindrome(char str[50])
{
int i,j;
   for(i=0,j=strlen(str)-1;i<j;i++,j--)
    if(str[i]!=str[j]) return 0;
return 1;
}
int main()
{
     FILE *fp1,*fp2;
char word[50];
fp1=fopen("words.txt","r");
fp2=fopen("palwords.txt","w");
   while((fscanf(fp1,"%s",word))!=EOF)
   {
     if(palindrome(word)) fprintf(fp2,"%s\n",word);
   }

fclose(fp1);
   fclose(fp2);

}

Reading an array and writing to a file using fwrite and reading the file using fread

#include <stdio.h>

#include <stdlib.h>

#define SIZE 10

int main()

{

int i,a[SIZE],b[SIZE];

FILE *fp;

printf("Enter 10 elements in array a...\n");

for(i=0;i<10;i++)

scanf("%d",&a[i]);

fp=fopen("num.dat","w");

if(fp==NULL)

{

printf("error opening file..\n");

exit(1);

}

fwrite(a,sizeof(int),SIZE,fp);

fclose(fp);

/*opening the file and reading to array b*/

fp=fopen("num.dat","r");

if(fp==NULL)

{

printf("error opening file..\n");

exit(1);

}

fread(b,sizeof(int),SIZE,fp);

printf("array b is...\n");

for(i=0;i<10;i++)

printf("%d\n",b[i]);

fclose(fp);

}
Program for writing struct to file using fwrite
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct person

{

int id;

char fname[20];

char lname[20];

};

int main ()

{

FILE *outfile;

// open file for writing

outfile = fopen ("person.dat", "w");

if (outfile == NULL)

{

fprintf(stderr, "\nError opend file\n");

exit (1);

}

struct person input1 = {1, "rohit", "sharma"};

struct person input2 = {2, "mahendra", "dhoni"};

// write struct to file

fwrite (&input1, sizeof(struct person), 1, outfile);

fwrite (&input2, sizeof(struct person), 1, outfile);

if(fwrite != 0)

printf("contents to file written successfully !\n");

else

printf("error writing file !\n");

return 0;

}
Write a C program to create a file and store information about a person, in terms of his name, age and salary.(university question)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct person
{
    char name[50];
    int age;
    int salary;
};
int main ()
{
    FILE *outfile;
    // open file for writing
    outfile = fopen ("person.dat", "w");
    if (outfile == NULL)
    {
        fprintf(stderr, "\nError opend file\n");
        exit (1);
    }
    struct person input1 = {"rohit",45,20000};
    struct person input2 = {"mahendra",25,15000};
       // write struct to file
    fwrite (&input1, sizeof(struct person), 1, outfile);
    fwrite (&input2, sizeof(struct person), 1, outfile);
     if(fwrite != 0)
        printf("contents to file written successfully !\n");
    else
        printf("error writing file !\n");
    return 0;

}

Program for reading struct using fread

This program will read the file person.dat file in the previous program.

#include <stdio.h>

#include <stdlib.h>

// struct person with 3 fields

struct person

{

int id;

char fname[20];

char lname[20];

};

int main ()

{

FILE *infile;

struct person input;

// Open person.dat for reading

infile = fopen ("person.dat", "r");

if (infile == NULL)

{

fprintf(stderr, "\nError opening file\n");

exit (1);

}

// read file contents till end of file

while(fread(&input, sizeof(struct person), 1, infile))

printf ("id = %d name = %s %s\n", input.id,

input.fname, input.lname);

return 0;

}

A stock file contains item_code, item_name, quantity and reorder_level.Write a C program to create a stock file of N items. Write a function to list items with quantity less than reorder_level.

( University Question)

#include <stdio.h>

#include <stdlib.h>

// Define the structure for item

struct Item {

int item_code;

char item_name[50];

int quantity;

int reorder_level;

};

// Function to display items that need reordering

void displayReorderItems(const char *filename) {

FILE *fp;

struct Item it;

fp = fopen(filename, "r"); // Open the file in read mode

if (fp == NULL) {

printf("Error opening file!\n");

return;

}

printf("\nItems that need reordering:\n");

printf("----------------------------------------\n");

printf("Code\tName\t\tQty\tReorder Level\n");

while (fscanf(fp, "%d %s %d %d", &it.item_code, it.item_name, &it.quantity, &it.reorder_level) != EOF) {

// Display the item if quantity is less than reorder level

if (it.quantity < it.reorder_level) {

printf("%d\t%-15s\t%d\t%d\n", it.item_code, it.item_name, it.quantity, it.reorder_level);

}

fclose(fp); // Close the file

}

int main() {

FILE *fp;

struct Item it;

int n, i;

char filename[] = "stock.txt"; // Text file for storing stock data

fp = fopen(filename, "w"); // Open the file in write mode

if (fp == NULL) {

printf("Error creating file!\n");

return 1;

}

printf("Enter number of items: ");

scanf("%d", &n);

for (i = 0; i < n; i++) {

printf("\nEnter details of item %d\n", i + 1);

printf("Item Code: ");

scanf("%d", &it.item_code);

printf("Item Name: ");

scanf(" %[^\n]", it.item_name); // Read string with spaces

printf("Quantity: ");

scanf("%d", &it.quantity);

printf("Reorder Level: ");

scanf("%d", &it.reorder_level);

// Write item details to the file

fprintf(fp, "%d %s %d %d\n", it.item_code, it.item_name, it.quantity, it.reorder_level);

}

fclose(fp); // Close the file after writing data

// Call function to display items that need reordering

displayReorderItems(filename);

return 0;

}

In a small firm, employee numbers are given in serial numerical order, that is 1, 2, 3, etc. − Create a file of employee data with following information: employee number, name, sex, gross salary. − If more employees join, append their data to the file. − If an employee with serial number 25 (say) leaves, delete the record by making gross salary 0. − If some employee’s gross salary increases, retrieve the record and update the salary. Write a program to implement the above operations.

Here's a full C program using structures and file handling to manage a basic employee database with the following operations:

Create/Add Employees
Append New Employee Data
Delete Employee by Setting Salary to 0
Update Gross Salary
Display All Records

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#define FILENAME "employees.dat"

struct Employee {

int emp_no;

char name[50];

char sex;

float gross_salary;

};

void addEmployees(int count) {

FILE *fp = fopen(FILENAME, "ab"); // append binary

struct Employee emp;

if (!fp) {

printf("Error opening file!\n");

return;

}

for (int i = 0; i < count; i++) {

printf("\nEnter details for employee %d:\n", i + 1);

printf("Employee No: ");

scanf("%d", &emp.emp_no);

printf("Name: ");

scanf(" %[^\n]", emp.name);

printf("Sex (M/F): ");

scanf(" %c", &emp.sex);

printf("Gross Salary: ");

scanf("%f", &emp.gross_salary);

fwrite(&emp, sizeof(emp), 1, fp);

}

fclose(fp);

}

void deleteEmployee(int emp_no) {

FILE *fp = fopen(FILENAME, "rb+");

struct Employee emp;

if (!fp) {

printf("Error opening file!\n");

return;

}

while (fread(&emp, sizeof(emp), 1, fp)) {

if (emp.emp_no == emp_no) {

emp.gross_salary = 0;

fseek(fp, -sizeof(emp), SEEK_CUR);

fwrite(&emp, sizeof(emp), 1, fp);

printf("Employee %d marked as deleted (salary set to 0).\n", emp_no);

fclose(fp);

return;

}

printf("Employee not found.\n");

fclose(fp);

}

void updateSalary(int emp_no, float new_salary) {

FILE *fp = fopen(FILENAME, "rb+");

struct Employee emp;

if (!fp) {

printf("Error opening file!\n");

return;

}

while (fread(&emp, sizeof(emp), 1, fp)) {

if (emp.emp_no == emp_no) {

emp.gross_salary = new_salary;

fseek(fp, -sizeof(emp), SEEK_CUR);

fwrite(&emp, sizeof(emp), 1, fp);

printf("Salary updated for employee %d.\n", emp_no);

fclose(fp);

return;

}

printf("Employee not found.\n");

fclose(fp);

}

void displayEmployees() {

FILE *fp = fopen(FILENAME, "rb");

struct Employee emp;

if (!fp) {

printf("Error opening file!\n");

return;

}

printf("\nEmployee Records:\n");

printf("%-10s %-20s %-5s %-10s\n", "EmpNo", "Name", "Sex", "Salary");

while (fread(&emp, sizeof(emp), 1, fp)) {

if (emp.gross_salary > 0)

printf("%-10d %-20s %-5c %-10.2f\n", emp.emp_no, emp.name, emp.sex, emp.gross_salary);

}

fclose(fp);

}

int main() {

int choice, emp_no, count;

float new_salary;

do {

printf("\nEmployee Management Menu:\n");

printf("1. Add Employees\n");

printf("2. Delete Employee (Set Salary to 0)\n");

printf("3. Update Salary\n");

printf("4. Display Employees\n");

printf("5. Exit\n");

printf("Enter your choice: ");

scanf("%d", &choice);

switch (choice) {

case 1:

printf("How many employees to add? ");

scanf("%d", &count);

addEmployees(count);

break;

case 2:

printf("Enter employee number to delete: ");

scanf("%d", &emp_no);

deleteEmployee(emp_no);

break;

case 3:

printf("Enter employee number to update: ");

scanf("%d", &emp_no);

printf("Enter new gross salary: ");

scanf("%f", &new_salary);

updateSalary(emp_no, new_salary);

break;

case 4:

displayEmployees();

break;

case 5:

printf("Exiting...\n");

break;

default:

printf("Invalid choice!\n");

}

} while (choice != 5);

return 0;

}

Random Access To File

There is no need to read each record sequentially, if we want to access a particular record. C supports these functions for random access file processing.

fseek()

ftell()

rewind()

fseek()
This function is used for seeking the pointer position in the file at the specified byte.
Syntax:

fseek(FILE *fp,long offset,int position)

Where
fp-file pointer ---- It is the pointer which points to the file.
offset -displacement ---- It is positive or negative.This is the number of bytes which are skipped backward (if negative) or forward( if positive) from the current position.This is attached with L because this is a long integer.
Pointer position:
This sets the pointer position in the file.

Value Pointer position

0 Beginning of file.

1 Current position

2 End of file

Ex:

1) fseek( p,10L,0)
0 means pointer position is on beginning of the file,from this statement pointer position is skipped 10 bytes from the beginning of the file.
2)fseek( p,5L,1)
1 means current position of the pointer position.From this statement pointer position is skipped 5 bytes forward from the current position.
3)fseek(p,-5L,1)

From this statement pointer position is skipped 5 bytes backward from the current position.

ftell()

This function returns the value of the current pointer position in the file.The value is count from the beginning of the file.

Syntax: long ftell(FILE *fptr);
Where fptr is a file pointer.

rewind()

This function is used to move the file pointer to the beginning of the given file.

Syntax:

void rewind(FILE *fptr);

Where fptr is a file pointer.

Example program for fseek():

Write a program to read last ‘n’ characters of the file using appropriate file functions(Here we need fseek() and fgetc()).

#include<stdio.h>

#include<conio.h>

void main()

{

FILE *fp;

char ch;

clrscr();

fp=fopen("file1.c", "r");

if(fp==NULL)

printf("file cannot be opened");

else

{

printf("Enter value of n to read last ‘n’ characters");

scanf("%d",&n);

fseek(fp,-n,2);

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

{

printf("%c\t",ch);

}

fclose(fp);

}

Two persons want to access a file "sample.txt". First person want to read the data from the file. The second person want to read and write the data from and to the file simultaneously. Can you help them to do so by writing the corresponding programming codes?

first person

#include <stdio.h>

int main() {

FILE *fp;

char ch;

fp = fopen("sample.txt", "r"); // Open in read-only mode

if (fp == NULL) {

printf("Error: Cannot open file for reading.\n");

return 1;

}

printf("Contents of the file:\n");

while ((ch = fgetc(fp)) != EOF) {

putchar(ch);

}

fclose(fp);

return 0;

}

second person

#include <stdio.h>

int main() {

FILE *fp;

char ch;

fp = fopen("sample.txt", "r+"); // Open in read + write mode

if (fp == NULL) {

printf("Error: Cannot open file for read/write.\n");

return 1;

}

printf("Reading current contents:\n");

while ((ch = fgetc(fp)) != EOF) {

putchar(ch);

}

// Move file pointer to the end to write

fseek(fp, 0, SEEK_END);

fprintf(fp, "\nThis is a new line added by the second person.\n");

printf("\nNew line written to the file.\n");

fclose(fp);

return 0;

}

Programs to try

1.Create a file and perform the following

i) Write data to the file

ii) Read the data in a given file & display the file content on console

iii) append new data and display on console

2.Open a text input file and count number of characters, words and lines in it; and store the results

in an output file.

3.Copy one file to another.

4.Merge the content of two files and copy to the other.

5.Read numbers from a file and separate even and odd numbers into two different files.

6.Create a structure employee with fields empid,name and salary.Write the structure into a file.

Read and display employee details from the file ( use fread and fwrite)

7.Write an integer array into a file.Read and display the array elements in reverse order.

8.Read last n characters from a file.

9.Find the palindrome words from a file and write it into a new file.

10.Write a C program to create a text file (input text through keyboard).Display the contents and size of the file created.

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