File I/O

Block I/O

• It means to read block of data from file or to write block of data into file.
• To perform such type of operations file must be opened in binary mode.
• To perform block i/o operations we can use functions such as fread, fwrite,…etc

fwrite()

• It is used to write a block of data from memory into file.
• Writes data from the specified address pointed to by ptr to the given stream.
• It repeates the operation specified no of times.
• On success the number of elements writen is returned.

fread()

• It is used to read block of data from file into memory.
• Reads data from the given stream into the specified address pointed to by ptr.
• It reads ntimes number of elements of spceified size.
• On success the number of elements read is returned.

  #include<stdio.h>
  main()
  {
    FILE *fp;
	int i,arr[10]={343,2355,3232,55,222,55,222,666,3232, 5545};
	fp=fopen("nos.dat","wb");
	if(fp==NULL)
		printf("can not open File ");
	else
	{ 
        printf("Data Stored");
		fwrite(arr,40,1,fp);
		fclose(fp);
	}
  }

Data stored..
  

  #include<stdio.h>
  main()
  {
    FILE *fp;
	int i,a[10],s=0;
	fp=fopen("nos.txt","rb");
	if(fp==NULL)
	printf("Fil does'nt exists......");
	else
	{
		fread(a,40,1,fp);	/* read 40 bytes from file */
		for(i=0;i<=9;i++)
				s=s+a[i];
		printf("Sum of all nos is %d",s);
		fclose(fp);
	}
  }

Sum of all nos is 15927
  

fseek()

• It is used to move file pointer to read or write data at a particular poistion
• Sets the file position of the stream to the given offset. The argument offset signifies the number of bytes to seek from the given whence position.
• The argument whence can be:
• SEEK_SET Seeks from the beginning of the file.
• SEEK_CUR Seeks from the current position.
• SEEK_END Seeks from the end of the file.
• On success zero is returned.
• On error a nonzero value is returned.

  #include<stdio.h>
  main()
  {
    FILE *fp;
	int a,b,c;
	fp=fopen("nos.txt","rb"); 	
	if(fp==NULL)
     	printf("File does'nt exists......");
	else
   	{
   		fseek(fp , 4 , SEEK_SET); 	 
   		fread(&a , 4 , 1 , fp);	
   		fseek(fp , 8 , SEEK_CUR); 	
   		fread(&b , 4 , 1 , fp);  	
   		fseek(fp , - 8 , SEEK_END);  
   		fread(&c , 4 , 1 , fp); 	
   		printf("The numbers are %d %d %d ",a,b,c);
   		fclose(fp);
   	}
  }

The numbers are 2355 222 3232