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noatom

fread question

7 posts in this topic

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?
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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.

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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 e?dd
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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?

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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.

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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.
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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().

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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()).

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