Thread: Binary files

Hi,

This is the code:
--------------------

Code:

#include <stdio.h>

struct student{
	int id,age;
	char name[30];
};


int main()
{
	int i;
	struct student record;
	FILE *fb;
	
	fb = fopen("data1.txt","wb");
	for(i=1; i<=2; i++)
	{
		printf("Enter the student id,name,age");
   		scanf("%d %s %d",&record.id,record.name,&record.age);
		fwrite(&record,sizeof(struct student),1,fb);
	}
	fclose(fb);

	fb=fopen("data1.txt","rb");
	
	//for(i=1; i<=sizeof(fb)+1; i++)	
                 while(!feof(fb))
	{
		fread(&record,sizeof(struct student),1,fb);
		printf("ID %d\tName %s\tage %d\n",record.id,record.name,record.age);
	}
	
	fclose(fb);
                return 0;
		
}

The out put of the file duplicates the last record:

Enter the student id,name,age123 A 21
Enter the student id,name,age456 B 22
ID 123 Name A age 21
ID 456 Name B age 22
ID 456 Name B age 22

Also, what is the wrong of the red line in the code if I replaced while(!feof(fb)) with it?

Code:

         while(!feof(fb))
	{
		fread(&record,sizeof(struct student),1,fb);
		printf("ID %d\tName %s\tage %d\n",record.id,record.name,record.age);
	}

feof() isn't true until you try reading from the file. So the above loop checks, finds you're not at the end of the file yet. You read a record but -- UH OH! -- you're at the end of the file. Now feof() would return true, but before it gets a chance to you're printing out data that you shouldn't.

The best way to control file I/O loops is generally with the return value of whatever function you're using to read from the file. MD's example does just this and doesn't have the bug yours does.

BTW, the sizeof operator doesn't work on file sizes. But you don't need it if you use the return value of fread() to control the loop.

Originally Posted by Moony

for(i=1; i<=sizeof(fb)+1; i++)

Again, sizeof doesn't give you the file size. It gives you the size of fb, which is a pointer, and the size of a pointer is probably 4 on your system. sizeof(fb) will always give you the same number on your system no matter how big the file is that's associated with the stream fb points to.

If you know you're writing 6 records, why not just loop 6 times again? Or use the while(fread()) method. When you try reading the 7th record fread() will return 0 and break the loop for you.