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# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
include <config.h>
include <syserr.h>
include <fio.h>
# UNGETC -- Push a character back into the input stream. Pushback is last
# in first out, i.e., the last character pushed will be the first character
# returned in the next getc, getline, or read. Multiple characters and
# strings 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.
#
# Single character pushback is fairly expensive, but the i/o system is
# very efficient for even character at a time input and is optimized with
# that in mind. The overhead for pushing back an entire string is about
# the same as for a single character, so recursive macro expansion may be
# implemented quite efficiently with this pushback technique.
procedure ungetc (fd, ch)
int fd # file
char ch # char to be pushed back
pointer pb_sp, pb_iop
int or()
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. Note bufptr
# must be set to a value less than or equal to iop to avoid a buffer
# fault.
pb_iop = FPBIOP(fp)
pb_sp = FPBSP(fp) - 1
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]
# Deposit the char in the pbbuf and set up i/o pointers. When iop
# reaches itop filbuf will pop the old input pointers off the pbstack.
# Note: pushed back data grows upward, while the pb stack grows
# downward.
Memc[pb_iop] = ch
bufptr[fd] = pb_iop
iop[fd] = pb_iop
pb_iop = pb_iop + 1
itop[fd] = pb_iop
# Check for overflow.
if (pb_iop >= (pb_sp - 1) * SZ_INT + 1)
call syserrs (SYS_FPBOVFL, FNAME(fp))
FPBSP(fp) = pb_sp
FPBIOP(fp) = pb_iop
fflags[fd] = or (fflags[fd], FF_PUSHBACK)
end
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