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Trailing-Edge - PDP-10 Archives - SRI_NIC_PERM_FS_1_19910112 - kcc-4/lib/stdio/ungetc.c
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/*
 *	UNGETC - unget a character to stream
 *
 *	Copyright (c) 1986 by Ian Macky, SRI International
 */

#include "stdio.h"

int ungetc(c, f)
char c;
register FILE *f;
{
    if ((f->siocheck != _SIOF_OPEN) || !(f->sioflgs & _SIOF_READ) || c == EOF)
	return EOF;			/* validate FILE block */
    if (f->sioflgs & _SIOF_PBC) {
	if (f->siocnt >= _SIO_NPBC)	/* room for another? */
	    return EOF;			/* no, so punt */
	f->siocnt++;			/* yes, bump count */
	return *f->siocp-- = c;		/* and store new char in buf */
    } else {
	f->sio2cp = f->siocp;		/* save old pointer */
	f->sio2cnt = f->siocnt;		/* save old count */
	f->siocp = f->siopbc + _SIO_NPBC - 2; /* point to before EOB */
	f->siocnt = 1;			/* start with 1 char */
	f->sioflgs |= _SIOF_PBC;	/* flag that we're in unget */
	return *(f->siocp + 1) = c;	/* store and return the char */
    }
}