Python: struct.calcsize, erroneous results or misunderstanding?-CodePudding

This basically is the problem I am facing:

Python 3.10.4 (tags/v3.10.4:9d38120, Mar 23 2022, 23:13:41) [MSC v.1929 64 bit (AMD64)] on win32
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>>> import struct
>>> struct.calcsize("h")
2
>>> struct.calcsize("l")
4
>>> struct.calcsize("hl")
8
>>>

I expect "hl" to occupy 6 bytes. 2 for the short and 4 for the long, and struct.calcsize confirms my premise for that, but adds them to get 8. What gives? Am I misunderstanding something here? Which has to be my first suspicion otherwise we're looking at nasty bug in struct.calcsize.

CodePudding user response：

This is alignment coming in. You can force non padding by adding = in front of your string:

struct.calcsize("hl")
8
struct.calcsize("=hl")
6

You can find that in the docs: No padding is added when using non-native size and alignment, e.g. with ‘<’, ‘>’, ‘=’, and ‘!’. in https://docs.python.org/3/library/struct.html Note depending on what you're doing, turning off the padding might have performance impact.