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# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
include <config.h>
include <syserr.h>
include <fio.h>
# FWRITEP -- Write to a file, directly accessing the file data in the FIO
# buffer rather than copying the data from the user buffer to the FIO buffer.
# This technique can be used for very efficient file access, but is not as
# general as an ordinary write. In particular the requested data segment
# must lie entirely within the FIO buffer, and the data must be written into
# the FIO buffer before a file fault causes the buffer contents to be flushed
# to disk. The file size should be known in advance and any attempt to write
# outside the file boundaries is interpreted as an error.
#
# NOTE -- This routine returns a pointer into the FIO buffer. No data is
# transferred in the call itself. The data is not actually written to the
# output file until the FIO buffer is faulted out. If the output file is
# readwrite and offset,nchars does not span the entire buffer, file data
# will be read into the buffer when it is first faulted in. Hence, FWRITEP
# may be used for updating the contents of a file.
pointer procedure fwritep (fd, offset, nchars)
int fd # file to be accessed
long offset # file offset in chars
int nchars # nchars to "write"
int junk
pointer fiop, bp
int ffault()
errchk filerr, ffault, fmkbfs
include <fio.com>
begin
# Move file buffer onto file block containing the file offset.
# Verify that the buffer contains nchars of file data in contiguous
# storage. If the file buffer already contains the referenced
# data segment no fault is necessary and this is quite fast.
# The iop is left pointing to the first char following the
# referenced data block.
repeat {
bp = bufptr[fd]
fiop = offset - boffset[fd] + bp # lseek
if (fiop < bp || fiop >= otop[fd]) {
if (bp == NULL) {
call fmkbfs (fd)
next
}
junk = ffault (fd, offset, nchars, FF_WRITE)
fiop = iop[fd]
otop[fd] = buftop[fd]
}
iop[fd] = fiop + nchars
if (iop[fd] > otop[fd])
call filerr (FNAME(fiodes[fd]), SYS_FWRITEP)
return (fiop)
}
end
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