sizeof struct

[John Mundt]

In article <2951 at cbnewsc.ATT.COM> hartman at cbnewsc.att.com (mark.a.hartman) writes:
>In article <29722 at pbhya.PacBell.COM> afh at PacBell.COM (Alan Hobesh) writes:
>>
>>
>>The following c code prints the size of the definded structure to be 44,
>>when compiled on an AT&T 3B20 running UNIX V5.2.1.
>>However, when the code is downloaded to a PC and compiled using Turbo C,
>>the size of the structure is reported to be 42.
>>
>>Why is there a difference and which is the correct size?
>
>The AT&T 3B20 compiler rounds structures up to an even word length,
>in your case from 42 bytes to 44.  If you need code that is portable
>between the two machines, try adding something like
>
>	char	fill[2];
>
>to the end of the structure to fill it out.  Then, "sizeof struct"
>will be the same on both machines.


That is no guarantee that the two structures will be portable between
both machines.  Granted, both will be the same length, but the offset
to different members of the structure can vary.

I did not see the original structure, so if it was something like

struct foo {
	char fee[42];
};

there would be no problem, but also no need to have a structure.


However, if it were something as simple as

struct foo {
	int x;
	char y[38];
};

y could be offset 2 or 4 bytes from the beginning of the structure.
Further, if it were

struct foo {
	char y[37];
	int x;
};

then x could conceivably be at an offset of 37, 38, 40, or 48
depending on the system, the size of the word, and where different
types must be located.  Moving structures from disk to different
machines is not always so straight forward.  Much easier to convert
to ascii delimited strings and back again.

-- 
---------------------
John Mundt   Teachers' Aide, Inc.  P.O. Box 1666  Highland Park, IL
john at chinet.chi.il.us
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