Thread: reading then displaying some data from a random access file

Code:

void show(void)
{
	FILE       *fptr;
	long	   i,j=0;
	
	struct info rec={0,"",""};

	if ( (fptr=fopen("file.dat", "rb+"))== NULL )
	{
		printf("File could not be opened.");
	}
	else
	{
		
		for(k=0; feof(fptr)==0; k++)
		{
	
			fseek(fptr, k * (sizeof(struct info)), SEEK_SET);
			fread(&rec, sizeof(struct info), 1, fptr);
			

			if (rec.ID!=0)
			{
				printf("Your ID:%u\nYour name:%s\nYour job:%s\n", rec.ID,  rec.name,  rec.work);
			}

		}
	
		fclose(fptr);
	}
}

There is a function that adds structures to the file, there is also one that deletes ones, etc...
now I have a problem with displaying all the records (ie. structures) there, so how do I do so?
There is something wrong about the code and I can`t get it!



EDIT: I think SEEK_SET could be replaced with SEEK_CUR, like this:

Code:

		for(; feof(fptr)==0; )
		{
	
			fseek(fptr, (sizeof(struct info)), SEEK_CUR);
			fread(&rec, sizeof(struct info), 1, fptr);

but this doesn`t work also, and I can`t get why!

Originally Posted by kool_hamster

now I have a problem with displaying all the records (ie. structures) there, so how do I do so?
There is something wrong about the code and I can`t get it!

EDIT: I think SEEK_SET could be replaced with SEEK_CUR, like this:

There is no need to use fseek at all.

Code:

			fseek(fptr, k * (sizeof(struct info)), SEEK_SET);
			fread(&rec, sizeof(struct info), 1, fptr);

So the first time thru, k is zero so we start at the beginning of the file. Does the first record print okay?

The next pass, fptr is already sizeof(struct info) bytes into the file, because that's what was just read with it. Presuming the structs are packed one after the other, you shouldn't need to move it "manually" with fseek.

You need to check the return results of fseek and fread.

Does your struct contain only arrays of chars (good) or char pointers (very bad)?

You need to check the return results of fseek and fread.

Does your struct contain only arrays of chars (good) or char pointers (very bad)?

char pointers, but the whole program worked fine with them. :s

I`m thinking of a solution, it may work, but it`s not like "standard" or "neat", so is there a known way around this problem?

EDIT:
But I don`t understand, if it`s typically working with records, why would it matter if what is inside the record is varying in size?

> char pointers, but the whole program worked fine with them. :s
Yeah, right.

You write out a pointer, quit the program and re-start it.
Is that pointer still valid?
You read the pointer back in from the disk, and it's pointing to la-la-land.

It might work in a simple static test, but that's just dumb luck.
But as soon as it gets more complicated, and you start pointing to either stack arrays or dynamically allocated memory, then you're on losing ground.


Put another way, load your data file into a hex editor - do you see your strings?
If not, you're SOL.