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# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
include <config.h>
include <syserr.h>
include <fio.h>
# UNREAD -- Push a binary block of data back into the input stream. Pushback
# is last in first out, i.e., the pushed back data will be read before input
# resumes at the point at which it was interrupted. Multiple blocks of data
# may be pushed back into the input until the push back buffer overflows.
# Overflow is often an indication of recursion in whatever routine is doing
# the pushback.
procedure unread (fd, buf, nchars)
int fd # file
char buf[ARB] # data block to be pushed back
int nchars # nchars to push back
int or()
pointer pb_iop, pb_sp
errchk syserr, syserrs, fmkpbbuf
include <fio.com>
begin
fp = fiodes (fd)
if (fd <= 0 || fp == NULL)
call syserr (SYS_FILENOTOPEN)
if (FPBBUF(fp) == NULL)
call fmkpbbuf (fd)
# Push the old pb_iop, iop, itop and bufptr on the stack for later
# restoration of the interrupted input stream by filbuf. Note bufptr
# must be set to <= iop to avoid a buffer fault.
pb_sp = FPBSP(fp) - 1
pb_iop = FPBIOP(fp)
Memi[pb_sp] = pb_iop
pb_sp = pb_sp - 1
Memi[pb_sp] = bufptr[fd]
pb_sp = pb_sp - 1
Memi[pb_sp] = itop[fd]
pb_sp = pb_sp - 1
Memi[pb_sp] = iop[fd]
# Check that room remains for the data.
if (((pb_sp - 1) * SZ_INT + 1) - pb_iop < nchars)
call syserrs (SYS_FPBOVFL, FNAME(fp))
# Move the data block into the buffer. Set the iop to point to the
# first char of the block. Note: data grows upwards while the stack
# grows downward.
bufptr[fd] = pb_iop
iop[fd] = pb_iop
call amovc (buf, Memc[pb_iop], nchars)
pb_iop = pb_iop + nchars
itop[fd] = pb_iop
FPBSP(fp) = pb_sp
FPBIOP(fp) = pb_iop
fflags[fd] = or (fflags[fd], FF_PUSHBACK)
end
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