John Machin wrote:
Robert Kern wrote:
Ethan Furman wrote:
Greetings,
I'm looking at the struct module for binary packing of ints and
floats. The documentation refers to C datatypes. It's been many
years since I looked at C, but I seem to remember that the data type
sizes were not fixed -- for example, an int might be two byes on one
machine, and four bytes on the next. Can any C programmers verify
this? If it is true, does that mean that struct.pack('h', 8001)
might give me different results depending on the machine it's
running on?
Right. I believe (but could be wrong) that "char" is defined to be
one byte, but that "short", "int", "long", and "long long" are
defined as "at least as big as the previous type".
In practice, though, on nearly every machine that Python runs on,
"char" is one byte, "short" is two bytes, and "int" is four bytes.
"longs" and "long longs" tend to vary substantially, though; never
assume sizes for them.
Single-precision floats are always four bytes and double-precision
floats are always eight bytes. "long doubles" vary; they could be
twelve bytes or sixteen.
If you want to deal with fixed sizes, use struct.calcsize() to test
the sizes of each of the integer types, assign them to width-specific
aliases, and always use these aliases.
This is all true if you want to operate in "native" mode; however in
"standard" mode the sizes are fixed -- otherwise there'd be no easy
way of reading/writing the fixed-size fields in many common file formats.
As the manual says:
"""
Native size and alignment are determined using the C compiler's sizeof
expression. This is always combined with native byte order.
Standard size and alignment are as follows: no alignment is required
for any type (so you have to use pad bytes); short is 2 bytes; int and
long are 4 bytes; long long (__int64 on Windows) is 8 bytes; float and
double are 32-bit and 64-bit IEEE floating point numbers, respectively.
"""
If, as I suspect, Ethan's purpose is be able to read/write files in a
long-established PC file format, he will need to '<' for littleendian
order, and an appropriate selection from bBhHiI and d will do what he
needs.
HTH,
John
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Next question: when using struct.pack to store an integer, I get a
deprecation warning if the int is too big... I would rather have an
error. Is there a setting somewhere that controls this?
>>> struct.pack('<i', 4294967295)
c:\python25\interact.py:1: DeprecationWarning: struct integer overflow
masking is deprecated
'\xff\xff\xff\xff'
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