CODEWITHSUNDEEP
c++binaryfiles
Celso Shigaki
Celso Shigaki

Reputation: 75

Writing bytes to file

I'm currently working on a DBF file manager and I'm having some problems...

One of the elements that compose the header is the last date that the file was updated. The problem is: the field format is YYMMDD and it MUST have 3 bytes.

How is it possible to write a date using that format using only 3 bytes? Also, another field represents the type of the file (if it have a memo or not).

The file type in my case is 03h and this field MUST use only 1 byte. I'm pretty confused.

Upvotes: 0

Views: 138

Answers (2)

webuster
webuster

Reputation: 2510

I would hold your data in 3 bytes as

  • first byte = year
  • second byte = month
  • third byte = day

There's plenty of room in each byte for the domain of each field (year, month, day). You could write them in an integer with bit-shifting operations, like this:

int year = 13;
int month = 7;
int day = 26;
int my_date = (year << 16) | (month << 8) | day;

Edit:

What I did in my_date: I basically concatenated the information you need (year, month, day) into a series of bits (8 bits per information field), as an integer. You know an int is 4 bytes. Consider for starters that my_date is 0, that is, all 32 bits are 0. The 4 bytes of it are as follows ("|" denotes concatenation; that's for ease of reading):

my_date = 0 | 0 | 0 | 0

When I write year << 16 I have

year << 16 = 0 | year | 0 | 0

In a similar fashion,

month << 8 = 0 | 0 | month | 0
day = 0 | 0 | 0 | day

When I apply the OR operator on all of them, my_date looks like this:

my_date = 0 | year | month | day

Accessing them:

year = (my_date & 0xFF0000) >> 16;
month = (my_date & 0xFF00) >> 8;
day = my_date & 0xFF;

Edit: how accessing works. We previously had 

my_date = 0 | year | month | day

If you do, for example, an AND with 0xFF00, which is 0 | 0 | FF | 0, you get

my_date & 0xFF00 = 0 | 0 | month | 0

Now all you need to do is shift your data back, so

(my_date & 0xFF00) >> 8 = 0 | 0 | 0 | month = month

Hope it's clearer now.

Upvotes: 1

Mustafa Chelik
Mustafa Chelik

Reputation: 2184

First byte for year: 2000 + YY. Can count from 2000 till 2255

Second byte for month: 1-12

Third byte for day: 1-31

Upvotes: 0

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