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# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
include <config.h>
include <syserr.h>
include <fio.h>
# FREADP -- Read from a file, directly accessing the file data in the FIO
# buffer rather than copying the data from the FIO buffer to the user buffer.
# This technique can be used for very efficient file access, but is not as
# general as an ordinary read. In particular the requested data segment
# must lie entirely within the FIO buffer, and the referenced data must be
# used before a file fault causes the buffer contents to be replaced. The
# file size should be known in advance and any attempt to read outside the
# file boundaries is interpreted as an error.
pointer procedure freadp (fd, offset, nchars)
int fd # file to be accessed
long offset # file offset in chars
int nchars # nchars to "read"
pointer bp, fiop
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 >= itop[fd]) {
if (bp == NULL) {
call fmkbfs (fd)
next
}
if (ffault (fd, offset, nchars, FF_READ) == EOF)
call filerr (FNAME(fiodes[fd]), SYS_FREADP)
fiop = iop[fd]
}
iop[fd] = fiop + nchars
if (iop[fd] > itop[fd])
call filerr (FNAME(fiodes[fd]), SYS_FREADP)
return (fiop)
}
end
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