Jump to content

  • Log In with Google      Sign In   
  • Create Account


fread question


Old topic!
Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.

  • You cannot reply to this topic
7 replies to this topic

#1 noatom   Members   -  Reputation: 782

Like
0Likes
Like

Posted 31 December 2012 - 01:03 PM

Suppose I have a file like this:
1
2
3
4

and I can use fread the first time like this:
fread(&sourcefile,sizeof(int),1,mFile);

so mFile will contain 1 now.

But if I call fread again:
fread(&sourcefile,sizeof(int),1,mFile);

and again:
fread(&sourcefile,sizeof(int),1,mFile);

exactly like that,will mFile contain:
1
2
3

What I don't understand is that in a book fread is called with exactly the same arguments many times,in the ideea that every time new data is added.So does it check to see what data is in mFile and read the source file starting from where it left in mFile?

Sponsor:

#2 Cornstalks   Crossbones+   -  Reputation: 6974

Like
2Likes
Like

Posted 31 December 2012 - 01:19 PM

Are the numbers in your file stored as text? If so, you're reading it wrong (you'd want something more like fscanf(mFile, "%d", &sourcefile);).

 

Now, assuming you're reading a binary file and you're reading things correctly, the answer to your question is "yes." Behind the scenes, there is a "get pointer" that is associated with mFile, and when you do an fread(), it reads from the current position of the get pointer, and then increments the get pointer for the next read. You can use ftell(mFile) to tell you the position of this internal pointer, and if you call it each time you call fread(), you will see it advances. Note that the value returned by ftell() is only meaningful if you've opened the file as a binary file; if you're reading it in text mode, the result of ftell() is not meaningful to you.


[ I was ninja'd 71 times before I stopped counting a long time ago ] [ f.k.a. MikeTacular ] [ My Blog ] [ SWFer: Gaplessly looped MP3s in your Flash games ]

#3 e‍dd   Members   -  Reputation: 2105

Like
1Likes
Like

Posted 31 December 2012 - 01:21 PM

Suppose I have a file like this:
1
2
3
4
Are you saying it contains the bytes 0x30, 0x0D, 0x0A, 0x31, 0x0D, 0x0A, 0x32, 0x0D, 0x0A, 0x33 (assuming little endian and typical windows text formatting)?
Or are you saying it contains something like 0x01, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x03, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00?

The distinction is important. But I'll assume the latter as that's what the rest of your question implies.


and I can use fread [...3 times...]

exactly like that,will mFile contain:
1
2
3
Reading a file does not change its content. Think of a FILE as containing a pointer or cursor in it somewhere that is moved along every time you read something from the file.

The first fread() call will read an int (on success) and advance the cursor sizeof(int) bytes, so that the cursor is now at 0x02,00,00,00 (again assuming 'binary' formatting).

The fseek() function allows you to change the position of this cursor. ftell() tells you its position.

Not sure if this answers your question directly, but I hope it helps.

EDIT: ninja'd :(

Edited by e‍dd, 31 December 2012 - 01:26 PM.


#4 e‍dd   Members   -  Reputation: 2105

Like
0Likes
Like

Posted 31 December 2012 - 01:25 PM

Note that the value returned by ftell() is only meaningful if you've opened the file as a binary file; if you're reading it in text mode, the result of ftell() is not meaningful to you.

Is this really true?



#5 Cornstalks   Crossbones+   -  Reputation: 6974

Like
0Likes
Like

Posted 31 December 2012 - 01:27 PM

Note that the value returned by ftell() is only meaningful if you've opened the file as a binary file; if you're reading it in text mode, the result of ftell() is not meaningful to you.

Is this really true?

Yup.


[ I was ninja'd 71 times before I stopped counting a long time ago ] [ f.k.a. MikeTacular ] [ My Blog ] [ SWFer: Gaplessly looped MP3s in your Flash games ]

#6 noatom   Members   -  Reputation: 782

Like
0Likes
Like

Posted 31 December 2012 - 01:41 PM

thanks guys.Yes the file is in binary.I was not sure how the function managed to read further in the file,I thought it started from the beginning every time,now because you explained it,I do.

#7 e‍dd   Members   -  Reputation: 2105

Like
0Likes
Like

Posted 01 January 2013 - 12:15 PM

Yup.

That's the kind of thing I was expecting. We'd probably be debating semantics, but I wouldn't say it's 'meaningless' as you can still use ftell() to 'save' a position to which you can return later with fseek().



#8 Cornstalks   Crossbones+   -  Reputation: 6974

Like
0Likes
Like

Posted 01 January 2013 - 02:38 PM

Yup.

That's the kind of thing I was expecting. We'd probably be debating semantics, but I wouldn't say it's 'meaningless' as you can still use ftell() to 'save' a position to which you can return later with fseek().

Yeah, sorry, my wording could've been better. What I meant by "not meaningful to you" is that to you, the value isn't meaningful (but to the implementation it is, of course, as it must work properly with fseek()).


[ I was ninja'd 71 times before I stopped counting a long time ago ] [ f.k.a. MikeTacular ] [ My Blog ] [ SWFer: Gaplessly looped MP3s in your Flash games ]




Old topic!
Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.



PARTNERS