PointersTopics File and it’s operation Opening and Closing file Input/Output Operations on Files

File and It’s Operation File: A file is a place on the disk where a

group of related data is stored. File Operations:

1. Naming a file

2. Opening a file

3. Reading data from a file

4. Writing data to a file

5. Closing a file

Opening and Closing a file Opening a File: The general format for declaring

and opening a file:

FILE *fp;

fp = fopen(“filename”,”mode”);

Here filename is a valid name and mode can be one of the following:

r – open the file for reading only

w – open the file for writing only

a - open the file for appending data to it Closing a file: The general form:

fclose(file_pointer);

Input/Output Operations on Files putc(): It writes a character to the file

specified by the file pointer. The general format is-

putc(c,fp);

getc(): It reads a character from a file specified by the file pointer. The general format is-

c=getc(fp);

Writing to and Reading from a Filemain(){ FILE *fp;char c;fp=fopen(“Sample”,”w”);while((c=getchar())!=‘\n’)

putc(c,fp);fclose(fp);fp=fopen(“Sample”,”r”);While((c=getc(fp)!=EOF) printf(“%c”,c);getch();}

Input/Output Operations on Files putw(): It writes an integer to a file specified

by the file pointer. The general format is-

putw(integer, fp);

getw(): It reads an integer from a file specified by the file pointer. The general format is-

n=getw(fp);

Input/Output Operations on Files fprintf(): It writes a group of mixed data items

to the file specified by the file pointer. The general format is-

fprintf(fp,”control string”,list);

fscanf(): It reads a group of mixed data items from a file specified by the file pointer. The general format is-

fscanf(fp,”control string”,list);

Handling of Integer data filesmain(){FILE *fp;int x,k;fp=fopen(“Sample”,”w”);for(k=1;k<=10;k++){ scanf(“%d”,&x);putw(x,fp);}fclose(fp);fp=fopen(“Sample”,”r”);for(k=1;k<=10;k++){ x=getw(fp); printf(“%d ”,x}fclose(fp);}

Handling of files with mixed data typesmain(){FILE *fp;int x,k;float y;fp=fopen(“Sample”,”w”);for(k=1;k<=10;k++){ scanf(“%d %f”,&x,&y); fprintf(fp,”%d %f”,x,y); }fclose(fp);fp=fopen(“Sample”,”r”);for(k=1;k<=10;k++){ fscanf(fp,”%d %f”,&x,&y); printf(“\n%d %f ”,x,y}fclose(fp);}

Error Handling during I/O Operation Reasons of error during file operation:

1. Trying to read beyond the end-of-file mark.2. Device overflow3. Trying to use a file that has not been opened.4. Trying to perform an operation on a file, when the file is opened for another type of operation5. Opening a file with an invalid filename6. Attempting to write to a write-protected file

Continue… The feof() function can be used to test for an end of the file

condition. If fp is a pointer to file that has just been opened for reading, then the statement

if(feof(fp)) printf(“End of the file”);

would display the message “End of the file” on reaching the end of file condition.

The ferror() function reports the status of the file indicated. It returns nonzero integer if an error has been detected upto that point otherwise returns zero. The statement

if(ferror(fp) !=0) printf(“An error has occurred”);

would print the error message if the reading is not successful.

Random Access to Files When we are interested in accessing only a

particular part of a file and not in reading the other parts, then we need some other functions named fseek, ftell and rewind.

Ftell() takes a file pointer and returns a number of type long, that corresponding to the current position. The form-

n = ftell(fp);

n would give the relative offset (in bytes) of the current position.

Continue… rewind() takes a file pointer and reset the position

to the start of the file.

rewind(fp); Fseek() is used to move the file position to a

desired location within the file. The general format is

fseek(file_pointer, offset, position);

file_pointer is a pointer to the file concerned, offset is a number or variable of type long and position is an integer number. The offset specifies the number of positions (bytes) to be moved from the location specified by position.

Continue… Example of fseek():

fseek(fp,0L,0); go to the beginning

fseek(fp,0L,1); stay at current position

fseek(fp,0L,2); go to the end of the file

fseek(fp,m,0); move to (m+1)th byte in the file

fseek(fp,m,1); go forward by m bytes

fseek(fp,-m,1); go backward by m bytes from current position

fseek(fp,-m,2); to backward by bytes from the end

Use of fseek and ftell functionmain(){ FILE *fp; long n; char c; fp = fopen(“Random”,”w”); while((c=getchar())!=‘\n’) putc(c,fp); printf(“Number of char = %d\n”,ftell(fp)); fclose(fp); fp = fopen(“Random”,”r”); n = 0L;

Continue…while(feof(fp) = = 0) { fseek(fp, n, 0); printf(“position of %c is %ld\n”,getc(fp),ftell(fp); n = n+5L; } putchar(“\n”); fseek(fp,-1L,2); do{

putchar(getc(fp)); }while(!fseek(fp,-2L,1)); fclose(fp);}