Re: Reading a C struct in java
Mark wrote:
On Tue, 29 Sep 2009 13:46:32 +0100, bugbear
<bugbear@trim_papermule.co.uk_trim> wrote:
Mark wrote:
This is already done. I just need to know how to get at the data and
make it useable in Java.
I am an experienced programmer in C/C++ but am new to Java BTW.
Just to be explicit, I guess the thing "in your head" is the C
idiom read(stream, &mystruct, sizeof(mystruct)
You definitely can't do this in Java, and (IMHO) you
shouldn't do it in C.
I know you can't do this in Java, hence my question. I'm beginning to
figure it out, but I am not yet familiar with all the Java classes ;-)
It's not a matter of memorizing a whole bucketload of Java
classes, it's about realizing what a Java class *is*: A Java class
is a collection of variables and methods and constructors. Period.
There are a few subtleties in the above that might not jump
out at you right away. One is the word "collection:" the code
and the data are just "collected," not "organized" or "ordered"
or "arranged" in any way. There is no notion of one variable
being "before" or "after" or "adjacent to" another; they're just
somewhere in the collection -- so you can forget about any notion
you might have had about "mapping" a bunch of raw bytes into a
Java object instance.
The same applies even to the individual data items themselves:
You know that a Java `int' is 32 bits wide, and that a Java `byte'
is eight bits wide. It does *not* follow that an `int' consists of
four `byte's -- that's true in C (often), but not in Java. As far
as Java is concerned, the `int' is an indivisible 32-bit atomic
value; it is *not* four individual `byte's, and most definitely not
an array of four `byte's.
Here's another thing: In C, an array is just a bunch of memory
slots arranged contiguously. In Java, an array is a full-fledged
object -- that is, Object -- with behaviors of its own. Use a bad
index on a C array and there's no telling what might happen; do so
in Java and you get an exception thrown into your teeth, reliably.
Similar remarks apply to Strings versus strings: A string is just
an array with a conventional structure, but a String is an object
with methods and data and so on. A String is *not* an array of
`char' -- try "Hello"[0] in Java, and I guarantee you will not
get an 'H'.
All the C/C++ compilers we use have pragmas to force known
alignment/padding etc.
That's unfortunate. It's probably tempted you to write bad
C code, *slow* C code, in an attempt to duck the issues. Your
compilers offer crutches; when you use them, you hobble like a
cripple. You will never walk, much less run, until you visit
Lourdes and discard your crutches there.
Byte order is not an issue in this layer,
since all parameters are encoded in strings.
See the remarks above about the difference between strings
and Strings. (Also, I doubt your assertion: Three of the four
arrays you showed us are only one char long, and hence can hold
only one C string value: "". It's possible, I guess, that your
protocol involves transmitting multiple known-to-be-constant and
hence information-empty bytes, but I doubt it. Most likely, you
are not dealing with strings, not entirely at any rate.)
--
Eric Sosman
esosman@ieee-dot-org.invalid