ILockBytes::ReadAt

Reads a specified number of bytes starting at a specified offset from the beginning of the byte array object.

HRESULT ReadAt(

ULARGE_INTEGER ulOffset,	//Specifies the starting point for reading data
void *pv,	//Points to the buffer into which the data is read
ULONG cb,	//Specifies the number of bytes to read
ULONG *pcbRead	//Pointer to location that contains actual number of bytes read
);

Parameters
ulOffset

[in]Specifies the starting point from the beginning of the byte array for reading data.

[in]Points to the buffer into which the byte array is read.

[in]Specifies the number of bytes of data to attempt to read from the byte array.

pcbRead

[out]Pointer to a location where this method writes the actual number of bytes read from the byte array. You can set this pointer to NULL to indicate that you are not interested in this value. In this case, this method does not provide the actual number of bytes read.

Return Values
S_OK

Indicates that the specified number of bytes were read, or the maximum number of bytes were read up to the end of the byte array.

E_FAIL

Data could not be read from the byte array.

STG_E_ACCESSDENIED

The caller does not have permission to access the byte array.

Comments
This method reads bytes from the byte array object. It reports the number of bytes that were actually read. This value may be less than the number of bytes requested if an error occurs or if the end of the byte array is reached during the read.

It is not an error to read less than the specified number of bytes if the operation encounters the end of the byte array. Note that this is the same end of file behavior as found in MS-DOS FAT file system files.