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

Find at most related files.
including files from this version of Unix. 
#include <stdio.h>

FILE *
_findiop()
{
	extern FILE *_lastbuf;
	register FILE *iop;

	for(iop = _iob; iop->_flag & (_IOREAD|_IOWRT|_IORW); iop++)
		if (iop >= _lastbuf)
			return(NULL);

	return(iop);
}