This document describes the BMallocIO interface and some basics of how it is implemented. +The document has the following sections:
+ + + +The BMallocIO class represent a buffer of dynamically allocated memory. The buffer is +automatically allocated by multiplies of a blocksize you can specify, so it will always be +big enough to contain the data. The best source of information for the BMallocIO interface +can be found +here in the Be Book. +
+ +The following use cases cover the BMallocIO functionality:
+ +Construction 1: The BMallocIO constructor set the blocksize to 256.
Destruction: The BMallocIO destructor frees the allocated memory.
Reading 1: When ReadAt() is called, the BMallocIO returns the number of bytes read from the specified +position. ReadAt() takes three arguments: the position where to begin the read operation, the buffer where to put the read data, +and the number of bytes to read. This function does not read outside of the buffer. +If the specified position is invalid (i.e. outside bounds) this function returns 0. If the read operation +begins at a valid position, but the sum of position and bytes to read is bigger than the size of the buffer, BMallocIO +returns just the available data.
Reading 2. BMallocIO inherits the Read() function from BPositionIO. This function read the specified amount +of data from the current position, and put it into the specified buffer, then it moves the I/O index forward of the number of read bytes. +This function behaves like the above.
Writing 1: When WriteAt() is called, BMallocIO returns the number of bytes written to the specified position. +WriteAt() takes three arguments: the position where to begin the write operation, the buffer from which to read the data to write, and the +number of bytes to write. +If the write position is beyond the buffer length, BMallocIO enlarges the buffer to accomodate the data. If enlarging fails, the function +returns B_NO_MEMORY. +
Writing 2. BMallocIO inherits the Write() function from BPositionIO. This function write the specified amount +of data to the current position of the BMallocIO object, reading from the specified buffer, then it moves the I/O index forward +of the number of read bytes. +This function behaves like the above.
Size Changes: The SetSize() member function enlarges or shrink the amount of data which can be read/write. +Shrinking the buffer is always possible, and the function returns B_OK. Zero frees the memory, while negative values are +not allowed, and the function returns B_ERROR. +
Seeking. Seek() sets the position in the data buffer where the Read() and Write() functions (inherited from +BPositionIO) begin reading and writing. How the position argument is understood depends on the mode flag. There are three possible modes: +
+ SEEK_SET. The position passed is an offset from the beginning of allocated memory; in other + words, the current position is set to position. For this mode, position should be a positive + value, but the BMallocIO object doesn't do any checking, so it is possible to seek to a negative position + (but any consequent Read() or Write() calls will fail). +
+ SEEK_CUR. The position argument is an offset from the current position; the value of the + argument is added to the current position.
+ SEEK_END. The position argument is an offset from the end of the buffer for a BMallocIO + object. Positive values seek beyond the end of the buffer or data; negative + values seek backwards into the data.
Position: The Position() call always return the current position.
Setting the BlockSize: The SetBlockSize() call let you specify +the blocksize which BMallocIO uses to allocate memory.
Getting the Buffer: The Buffer() call returns the buffer used internally +by BMallocIO.
Getting the Buffer Lenght: The BufferLength() call returns the length +of the buffer.
The implementation of the BMessageFilter is simple. It consist in implementing memory read/write on a buffer +with an index.
+ + + \ No newline at end of file