Sender: nate AT cartsys DOT com
Message-ID: <363CFAE6.E042937D@cartsys.com>
Date: Sun, 01 Nov 1998 16:20:54 -0800
From: Nate Eldredge <nate AT cartsys DOT com>
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To: djgpp AT delorie DOT com
Subject: Re: HELP!!! I/O in DJGPP!
References: <ranla DOT 67 DOT 0010E5C5 AT post DOT tau DOT ac DOT il>
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Reply-To: djgpp AT delorie DOT com

Tal Lavi wrote:
> 
> HELP!!!
> 
> I can't seem to read a .bmp file correctly under DOS DJGPP(2.01).
> One of my problems is that there are just too much ways to read a file!
> But, let's start from the top:
> 
> I open the file like this: FILE f*=fopen("foo.bmp","rb");
> Is "rb" correct? Is that how binary files are opened for reading?

Yep.
 
> Now, I want to read some words, BUT NOT STORE THEM, just check for a valid
> value and move on:
> 
> if (getw(f)!=0) return(1);
> 
> Not only that the correct value is not read, but getw seem to actually read
> 4 bytes! and it also moves the file pointer in ftell by 4 instead of 2!
> Is that even possible?

Sure.  On DJGPP, an `int' is 32 bits.  The idea of a "word" being 16
bits is an 8086-ism; `getw', being more general, usually refers to the
processor's wordsize, which is typically the size of its registers (32
bits on a protected-mode 386), which is typically the size of `int'.  So
`getw' reads an `int', which is 4 bytes long.  

You've just learned why `getw' is not portable.  You should use `fread':

short s;
fread(s, sizeof(short), 1, f);

This will still only work on platforms with a 16-bit `short', but that
is much more widespread.

> getc, by the way, moves the file pointer by 1, not by 2, and it reads the
> correct value each and every time.

`getw' also returns the correct value; i.e. what it's supposed to.  You
just didn't expect it.  `getc' is supposed to read one `char', and it
does.  `getw' is supposed to read one `int', and it does.


> When I want to retrieve some values, and store them, I should use fscanf,
> right?

If they're in text, yes.  If binary, no.

> My question is: how can I read a DWord? What is the correct masking letter?

You can't read raw binary numbers with `?scanf'; use `fread' as above. 

> And another thing: how are the bytes arranged in the DWord? I mean: if a
> DWord contains the value 128, in which of those four bytes, will the value
> 128 be stored? ______

The 386 stores multi-byte integers least-significant byte first, so 128
will be in the low byte.  More generally,

unsigned long l = 0x12345678;
unsigned char a[] = { 0x78, 0x56, 0x34, 0x12 };

should have the same bits.

> I just can't believe no one knows the answers to my questions in "misc
> questions"!!!

Is that an earlier post?  I haven't seen it yet; perhaps you need to
repost it?
-- 

Nate Eldredge
nate AT cartsys DOT com