struct.(un)pack and ASCIIZ strrings

[Terry Reedy]

"Sergey Dorofeev" <sergey at fidoman.ru> wrote in message 
news:d926sj$1jc8$1 at gavrilo.mtu.ru...
>I can use string.unpack if string in struct uses fixed amount of bytes.

I presume you mean struct.unpack(format, string).  The string len must be 
known when you call, but need not be fixed across multiple calls with 
different strings.

> But is there some extension to struct modue, which allows to unpack
> zero-terminated string, size of which is unknown?
> E.g. such struct: long, long, some bytes (string), zero, short, 
> short,short.



Size is easy to determine.  Given the above and string s (untested code):
prelen = struct.calcsize('2l')
strlen = s.find('\0', prelen) - prelen
format = '2l %ds h c 3h' % strlen # c swallows null byte

Note that C structs can have only one variable-sized field and only at the 
end.  With that restriction, one could slice and unpack the fixed stuff and 
then directly slice out the end string.  (Again, untested)

format = 2l 3h' # for instance
prelen = struct.calcsize(format)
tup = struct.unpack(format, s[:prelen])
varstr = s[prelen, -1] # -1 chops off null byte

Terry J. Reedy