Re: Convert Java <long> to C <?>
- From: Simple Simon <ssimon@xxxxxxxxxxxxxx>
- Date: Fri, 31 Mar 2006 05:14:03 GMT
Apologies for top-posting, but...
Thank you Jordan!
random832@xxxxxxxxx wrote...
On 2006-03-31, Me <anti_spam_email2003@xxxxxxxxx> wrote:.
Simple Simon wrote:
Java longs are 8 bytes. I have a Java long that is coming in from
the network, and that represents milliseconds since Epoch (Jan 1 1970
00:00:00). I'm having trouble understanding how to get it into a
struct timeval object.
I get the ByteBuffer as an array of const unsigned char, 'buffer'.
Here's an example: 00 00 01 0A 29 1D 07 E4
This value maps somehow to Thu Mar 23 16:57:49 2006 and some
millisecs which I haven't sussed out yet.
I'm beating my simple brains out trying to figure out how to get that
into a struct timeval t so that t.tv_sec would spit out
Thu Mar 23 16:57:49 2006
if used as an argument to ctime().
Wha? timeval isn't in standard C and ctime takes a time_t *. All C says
is that time_t is an arithmetic type, it doesn't say the start time or
what unit of measurement it uses. Many C implementations use the number
of seconds since UNIX's epoch so you can get by with:
unsigned long long val = 0;
for (size_t i = 0; i < 8; ++i)
val = val<<8 | buffer[i];
time_t time = val/1000;
You're risking an overflow there, and certainly incorrect results if the
value is negative. Sign-extend, convert to signed long long, THEN divide
by 1000 to put in the time_t. Still can overflow, but then it'll only do
it on values that deserve it.
Suppose i have the unsigned long long value 0xFFFFFFFFFFFF0000, that is,
what is meant to be the signed value -65536. that divided by 1000 is
0x4189374BC6A7AE, which is a huge positive value. Converting it to
a 32-bit signed value will result in 0x4BC6A7AE, which is the time Thu
Apr 15 01:44:14 2010, instead of the time Wed Dec 31 18:58:55 1969 [both
times converted using Eastern time.]
And portable code to convert seconds-since-1970 to a time_t is certainly
_possible_. You can probably even get mktime to do most of the work, but
you'll have to know [or cleverly determine] what timezone you're working
in.
const char *tstr = ctime(&time);
But odds are, if you have access to sockets on your implementation, the
above should work.
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- From: Simple Simon
- Re: Convert Java <long> to C <?>
- From: Me
- Re: Convert Java <long> to C <?>
- From: Jordan Abel
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