V7/usr/src/libc/stdio/ungetc.c

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including files from this version of Unix. 
#include <stdio.h>

ungetc(c, iop)
register FILE *iop;
{
	if (c == EOF)
		return(-1);
	if ((iop->_flag&_IOREAD)==0 || iop->_ptr <= iop->_base)
		if (iop->_ptr == iop->_base && iop->_cnt==0)
			*iop->_ptr++;
		else
			return(-1);
	iop->_cnt++;
	*--iop->_ptr = c;
	return(0);
}