[Python-bugs-list] [ python-Bugs-404328 ] struct.calcsize returns wrong size

[nobody]

Bugs #404328, was updated on 2001-02-26 08:21
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Category: Python Library
Group: Not a Bug
Status: Closed
Priority: 5
Submitted By: Nobody/Anonymous
Assigned to: Nobody/Anonymous
Summary: struct.calcsize returns wrong size

Initial Comment:
struct.calcsize returns the wrong size because it takes
into account the alignment of the type.
I needed to read a 56 packed structure and then write
it back.
Python insists that it needs 62 bytes for the fmt
'chl6s20schdhdh'

Using Python-2.1a2 source code I can get the correct
answer by
commenting out in Modules/structmodule.c
        if (e == NULL)
            return -1;
        itemsize = e->size;
-->  //size = align(size, c, e);
        x = num * itemsize; 
        size += x;

Basically packing in a string doesn't require any
alignment. The alignment is neccesary once unpacking
takes place. However if the unpacking is done to a
local c variable then the alignment should not be a
problem e.g.

long Unpack_long(char * offset)
{
long value;
	memcpy(&value,offset,sizeof(long);
	return value;
}

Regards Rene de Zwart

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Comment By: Tim Peters
Date: 2001-02-28 11:51

Message:
Logged In: YES 
user_id=31435

If he doesn't want native alignment, there are 4 format 
codes documented that use "standard" (i.e., no) alignment 
instead.

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Comment By: Fred L. Drake, Jr.
Date: 2001-02-28 11:41

Message:
Logged In: YES 
user_id=3066

This is documented behavior of the struct module, not a bug.  The purpose of the module is to allow packing and unpacking of C structures; I don't know of any that exhibits the alignment behavior that you describe -- C "short" values are aligned to sizeof(short) boundaries.

If your C compiler is exhibiting different behavior, look to see if you're using compiler options that affect the packing of structures -- this is not "normal" behavior.

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