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From: allender@ux1.cso.uiuc.edu (Mark Allender)
Newsgroups: comp.lang.c
Subject: Little problem with sizeof on PC
Message-ID: <1991Apr23.022057.29511@ux1.cso.uiuc.edu>
Date: 23 Apr 91 02:20:57 GMT
Sender: usenet@ux1.cso.uiuc.edu (News)
Reply-To: allender@ux1.cso.uiuc.edu (Mark Allender)
Organization: TechoPunks Ltd. -- Party Goers to the Max
Lines: 50

I'm having a litle problem that I have a suspicision about, but want
to clarify.  Here's the situation....

I have a structure that is defined like:

struct header {
	int version[2];
	char unused[40];
	int stuff[8];
	char bogus;
	char mode;
	int time;
	char unused2[90];
	char filler[38];
	char filler2[15]
	float number;
};

The total size of the structure is 201 bytes (count it if you wish....).
Now, I want to read the beginning of a binary file into this structure,
so I do something like this:

	struct header Header;

	if ((readnum = read(fd, (char *)(&Header), sizeof(Header)).....

Things don't seem to get done correctly at this point.  A little investigation
shows that sizeof(Header) return 202, and not 201.  This is clearly not
what I want to do.

Now, I kind of figure that the problem has to do with the way structure
members are lined up in memory.  Am I correct in thinking that since there
are an odd number of bytes in the structure (201), that sizeof(Header)
returns an even number since things have to be word aligned (with a word
being 2 bytes)?  This would seem to make some sense.

In any case, what is the best way around this problem.  Could I do something
like
	if ((readnum = read(fd, (char *)(&Header), sizeof(Header) - 1))....

Seems like kind of a bad way to fix things....

Any help would be appreciated...

-Thanks in advance...

-Mark Allender
-University of Illinois at Urbana/Champaign
-Conversation Builder Project
-allender@cs.uiuc.edu