The functions implement the underlying functionality of the
read(2),
write(2),
readv(2), and
writev(2) system calls. They are also used throughout the kernel as high-level access routines for file I/O.
The
dofileread() function attempts to read
nbytes of data from the object referenced by file entry
fp into the buffer pointed to by
buf. The
dofilewrite() function attempts to write
nbytes of data to the object referenced by file entry
fp from the buffer pointed to by
buf.
The
dofilereadv() and
dofilewritev() functions perform the same operations, but scatter the data with the
iovcnt buffers specified by the members of the
iov array.
The offset of the file operations is explicitly specified by
*offset. The new file offset after the file operation is returned in
*offset. If the FOF_UPDATE_OFFSET flag is specified in the
flags argument, the file offset in the file entry
fp is updated to reflect the new file offset, otherwise it remains unchanged after the operation.
The file descriptor
fd is largely unused except for use by the ktrace framework for reporting to userlevel the process's file descriptor.
Upon successful completion the number of bytes which were transferred is returned in
*retval.