Re: memcpy() where assignment would do?
- From: Eric Sosman <Eric.Sosman@xxxxxxx>
- Date: Thu, 23 Aug 2007 11:49:14 -0400
kuyper wrote On 08/23/07 11:27,:
I've run across some rather peculiar code; here are the relevant lines
that left me confused :
unsigned char cr_file[384];
unsigned char num_char[0];
Note: this declaration actually works on our compiler, and it appears
to be equivalent to giving a length of 1. The developer inserted
compiler options into the make file to turn off the relevant warning
messages. Sadly, this is not the most confusing part of the code. This
is an example of the confusing part:
num_char[0] = 1;
memcpy(&cr_file[147], num_char, 1);
num_char is used only in this fashion; its value after the call to
memcpy() has no bearing on the behavior of the program. I may be
missing something, but it seems to me that this code is therefore
exactly equivalent to
cr_file[147] = 1;
In fact, I would expect that some compilers would generate identical
code for both ways of writing it.
Am I missing something? If not, could someone at least suggest a
plausible reason why the developer might write such bizarre code? I
can't ask the developer, he died recently, which is how I became
responsible for this code.
A guess: The bogus definition of num_char[0] may
actually allocate memory as would num_char[1], but has
some other bizarre effect as well. If it didn't do
something unusual the programmer would have written [1]
in the first place, instead of writing [0] and then
going to the extra work of figuring out how to turn the
error message off. The use of memcpy() instead of
`cr_file[147] = num_char[0]' or `cr_file[147] = 1' may
have something to do with whatever that weird effect is.
Guess #2: Does the code call "the" memcpy(), or some
out-of-the-blue substitute? Writing your own substitutes
for Standard library functions is a no-no, but we've
already seen that the author didn't feel held bound to
respect the Standard at all times ...
Guess #3: Somewhere in the dusty annals of the code's
ancestry you will find the word IOCCC -- or was it XYZZY?
--
Eric.Sosman@xxxxxxx
.
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