The Be Book - Classes And Methods

BPositionIO

Derived From:	`BDataIO`
Mix-in Classes:	–
Declared In:	`support/DataIO.h`
Library:	`libbe.so`
Allocation:	–

Class Overview

Constructor and Destructor

BPositionIO()

BPositionIO();

Does nothing. Constructors in derived classes should initialize the object to default values—for example, set the current position to the beginning of the data.

~BPositionIO()

virtual ~BPositionIO();

Does nothing.

Member Functions

Read(), ReadAt()

virtual ssize_t Read(void* buffer,
                     size_t numBytes);

virtual ssize_t ReadAt(off_t position,
                       void* buffer,
                       size_t numBytes) = 0;

Read() copies numBytes bytes of data from the object to the buffer. It returns the number of bytes actually read (which may be 0) or an error code if something goes wrong. Read() is implemented using Seek(), ReadAt(), and Position(), all of which are pure virtual functions.

ReadAt() is a pure virtual that must be implemented by derived classes to read numBytes bytes of data beginning at position in the data source, and to place them in the buffer. It should return the number of bytes actually read, or an error code if something goes wrong.

Seek(), Position()

virtual off_t Seek(off_t position,
                   int32 mode) = 0;

virtual off_t Position() const = 0;

Seek() is implemented by derived classes to modify the current position maintained by the object. The current position is an offset in bytes from the beginning of the object's data. How the position argument is interpreted will depend on the mode flag. Three possible modes should be supported:

Constant	Description
`SEEK_SET`.	The `position` passed is an offset from the beginning of allocated memory; in other words, the current position should be set to `position`.
`SEEK_CUR`.	The `position` argument is an offset from the current position; the current position should be incremented by `position`.
`SEEK_END`.	The `position` argument is an offset from the end of the data; the current position should be the sum of `position` plus the number of bytes in the data.

For the SEEK_SET mode, position should be a positive value. The other modes permit negative offsets.

Seek() should return the new position.

Position() should return the current position.

SetSize()

virtual status_t SetSize(off_t numBytes);

Returns B_ERROR to indicate that, in general, BPositionIO objects can't set the amount of memory in the repositories they represent. However, the BMallocIO class in this kit and BFile in the Storage Kit implement SetSize() functions that override this default.

Write(), WriteAt()

virtual ssize_t Write(const void* buffer,
                      size_t numBytes);

virtual ssize_t WriteAt(off_t position,
                        const void* buffer,
                        size_t numBytes) = 0;

Write() copies numBytes bytes of data from the object into the buffer. It returns the number of bytes actually written (which may be 0) or an error code if something goes wrong. Write() is implemented using Seek(), Position(), and WriteAt(), all of which are pure virtual functions.

WriteAt() is a pure virtual that must be implemented by derived classes to copy numBytes bytes of data from buffer and write them into the object, starting at byte position within the object. It should return the number of bytes actually read, or an error code if something goes wrong.