Derived From: BPositionIO
Mix-in Classes:
Declared In:support/DataIO.h
Class Overview

Constructor and Destructor



The BMallocIO constructor creates an empty object and sets the default block size to 256 bytes. The constructor doesn't allocate any memory; memory is allocated when you first write to the object or when you call SetSize() to set the amount of memory.


virtual ~BMallocIO();

The BMallocIO destructor frees all memory that was allocated by the object.

Member Functions

Buffer(), BufferLength()

const void* Buffer() const;size_t BufferLength() const;

Buffer() returns a pointer to the memory that the BMallocIO object has allocated, or NULL if it hasn't yet had occasion to allocate any memory.

BufferLength() returns the number of data bytes in the buffer (not necessarily the full number of bytes that were allocated).


virtual ssize_t ReadAt(off_t position,
                       void* buffer,
                       size_t numBytes);

Reads up to numBytes bytes of data from the object and copies it into the buffer. Returns the actual number of bytes placed in the buffer. The data is read beginning at the position offset.

This function doesn't read beyond the end of the data. If there are fewer than numBytes of data available at the position offset, it reads only through the last data byte and returns a smaller number than numBytes. If position is out of range, it returns 0.

Seek(), Position()

virtual off_t Seek(off_t position,
                   int32 mode);
virtual off_t Position() const;

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:



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.


The position argument is an offset from the current position; the value of the argument is added to the current position.


The position argument is an offset from the end of the object's data (not necessarily from the end of allocated memory). Positive values seek beyond the end of the data; negative values seek backwards into the data.

Attempts to seek beyond the end of allocated memory are legal: When Write() is subsequently called, the object updates its conception of where the data ends to bring the current position within range. If necessary, enough memory will be allocated to accommodate any data added at the current position.

Both Seek() and Position() return the current position as an offset in bytes from the beginning of allocated memory.

SetBlockSize(), SetSize()

void SetBlockSize(size_t blockSize);virtual status_t SetSize(off_t numBytes);

SetBlockSize() sets the size of the memory blocks that the BMallocIO object deals with. The object allocates memory in multiples of the block size. The default is 256 bytes.

SetSize() sets the size of allocated memory to numBytes (modulo the block size). Shrinking the buffer should always be successful (B_OK); if the buffer can't be grown, B_NO_MEMORY is returned.


virtual ssize_t WriteAt(off_t position,
                        const void* buffer,
                        size_t numBytes);

Copies numBytes bytes of data from buffer into the object's data beginning at position.

A successful WriteAt() always returns numBytes - WriteAt() reallocates the buffer (in multiples of the block size) if it needs more room. If the reallocation fails, this function returns B_NO_MEMORY.

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