Python struct giving incorrect length

问题

Couple of issues with python struct. Please let me know what is correct.

1. Document mentions length of l/L as 4 but when checked with calcsize it gives 8.

   >>> struct.calcsize('l')
   8
   

2. struct module calcsize is giving wrong size. If individual element size is calculated, it's sum is 90 but when calculated together with calcsize it gives 92.

   >>> struct.calcsize('8s2sIII30s32s6s')
   92
   
   >>> struct.calcsize('8s')
   8
   
   >>> struct.calcsize('2s')
   2
   
   >>> struct.calcsize('III')
   12
   
   >>> struct.calcsize('30s')
   30
   
   >>> struct.calcsize('32s')
   32
   
   >>> struct.calcsize('6s')
   6
   

回答1:

Elaborating answer posted by jonrsharpe in comments.

1. The ‘Standard size’ column refers to the size of the packed value in bytes when using standard size; that is, when the format string starts with one of '<', '>', '!' or '='. When using native size, the size of the packed value is platform-dependent.

   >>> struct.calcsize('l')
   8
   
   >>> struct.calcsize('=l')
   4
   

2. Because of padding. Use = to not use padding.

   >>> struct.calcsize('=8s2sIII30s32s6s')
   90
   

来源：https://stackoverflow.com/questions/46048166/python-struct-giving-incorrect-length