Re: streambuf in binary mode
smith4894@excite.com wrote:
I'm working on writing my own streambuf classes (to use in my custom
ostream/isteam classes that will handle reading/writing data to a
mmap'd file).
I know that at least STLport will use memory mapping if it can.
When reading from the mmap file, I essentially have a char buffer in my
streambuf class, that I'm registering with setp(). on an overflow()
call, I simply copy the contents of the buffer into the mmap'd file via
memcpy().
I don't see a reason to buffer this in between, write to the mmap'd file
immediately ... except of course you need additional translations like
CR<->CR/LF translations or have a nontrivial codecvt facet.
If I want to use this to write binary data via the streambuf classes
ie, are there any special considerations I need to be aware of in my
streambuf classes? Do I need to set any special flags to indicate that
the data is in binary mode perhaps? Any special precautions I need to
take, in overflow(int_type) for example?
My setup seems to work with binary data *MOST* of the time, however
there are rare occasions when there is inconsistency with the data i'm
writing and reading...
Any advice, comments would be much appreciated
1. C++ IOStreams are made for text-based IO. Shoehorning them into using
some packed, in-memory like representation (often called "binary", but that
is just misleading as all IO is binary) is possible to some extent but
generally not advisable.
2. Beware things like type representations. Depending on the system, the
high byte of an integer might come first or last in memory (endianess).
Further, a 'long' might be 32 or 64 bit (sizes). Also, some integers are
represented as twos complement, others as sign/magnitude (representation).
The different representations are particularly visible in floating point
types.
3. Beware type conversions. A 'char' might be signed or unsigned, converting
it to an unsigned integer could therefore give surprising results. Use
static_cast<unsigned char>() first.
4. Use an existing serialisation/persistence framework. If you really need
this for serious work, build on other people's experience and work rather
than inventing your own wheel.
Uli
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