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struct.calcsize('cd') returns 16 instead of 9 #49395

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bgailer mannequin opened this issue Feb 3, 2009 · 5 comments
Closed

struct.calcsize('cd') returns 16 instead of 9 #49395

bgailer mannequin opened this issue Feb 3, 2009 · 5 comments
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@birkenfeld
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docs Documentation in the Doc dir type-bug An unexpected behavior, bug, or error

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@bgailer
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Mannequin

bgailer mannequin commented Feb 3, 2009

BPO 5145
Nosy @birkenfeld, @vstinner

Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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GitHub fields:

assignee = 'https://github.com/birkenfeld'
closed_at = <Date 2009-03-27.01:38:34.316>
created_at = <Date 2009-02-03.18:53:42.749>
labels = ['type-bug', 'invalid', 'docs']
title = "struct.calcsize('cd') returns 16 instead of 9"
updated_at = <Date 2009-03-27.01:38:34.256>
user = 'https://bugs.python.org/bgailer'

bugs.python.org fields:

activity = <Date 2009-03-27.01:38:34.256>
actor = 'vstinner'
assignee = 'georg.brandl'
closed = True
closed_date = <Date 2009-03-27.01:38:34.316>
closer = 'vstinner'
components = ['Documentation']
creation = <Date 2009-02-03.18:53:42.749>
creator = 'bgailer'
dependencies = []
files = []
hgrepos = []
issue_num = 5145
keywords = []
message_count = 5.0
messages = ['81085', '81089', '81193', '81197', '84249']
nosy_count = 3.0
nosy_names = ['georg.brandl', 'vstinner', 'bgailer']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue5145'
versions = ['Python 2.5']
@bgailer
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bgailer mannequin commented Feb 3, 2009

struct.calcsize('cd') returns 16 instead of 9

struct.calcsize('dc') returns 9 as expected

@bgailer bgailer mannequin added extension-modules C modules in the Modules dir type-bug An unexpected behavior, bug, or error labels Feb 3, 2009
@bgailer
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bgailer mannequin commented Feb 3, 2009

Someone pointed out that one must consider alignment. I had overlooked
that. It makes sense that a d is double-word aligned.

Perhaps the documentation should mention alignment.

@pitrou pitrou added docs Documentation in the Doc dir and removed extension-modules C modules in the Modules dir labels Feb 3, 2009
@birkenfeld
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The docs have this sentence:

"""
By default, C numbers are represented in the machine's native format and
byte
order, and properly aligned by skipping pad bytes if necessary
(according to the
rules used by the C compiler).
"""

What would you suggest be added?

@vstinner
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vstinner commented Feb 5, 2009

The default endiand and alignment is "native". Using <, >, = or !
endian, the alignement is standard.

Python 3.1a0 (py3k:69105M, Feb  3 2009, 15:04:35)
>>> struct.calcsize('cd')
12
>>> struct.calcsize('=cd')
9
>>> struct.calcsize('<cd')
9

@vstinner
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Sorry, but I don't see this issue as a bug because it's related to
memory alignment as explained in the documentation.

I choose to close this issue. If you think that the documentation
should be improved, please suggest an updated doc.
http://docs.python.org/library/struct.html#struct.calcsize

@ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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