Bug in dc?

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2 total messages Started by randall Fri, 02 Oct 1987 22:34

#86

Author: randall
Date: Fri, 02 Oct 1987 22:34

34 lines
1742 bytes

Running the short dc routine below to find the gcd of two numbers
appears to work for small numbers. The command line is:

%dc gcd

r & s are passing parameters, returned in lowest terms
initiate by: l7x
======================================================
[lx_1*sx]s1[ly_1*sy]s2[lyszlxsylzsx]s3[1sz]s4
[lxly%szlysxlzsy1lz>5lz0=6lz1=4]s5[lxsz]s6
[lrsxlssylx0>1ly0>2lxly>3l5xlrlz/srlslz/ss]s7
?sr?ssl7x
[r,s in lowest terms:]Pc
[r= ]Plrpc
[s= ]Plspc
q
======================================================

When r & s are about 100 digits or so in length, an error message appears:

%Segmentation fault (core dumped)

What is wrong? Is the stack being recursively called until it overflows?
I see nothing wrong in the subroutine above. Can anyone find out what is
the problem and let me know? (e-mail is preferable) Is this a bug in dc?

I am working with rational points on cubic surfaces, and need to have this
problem fixed, to investigate the action of the chord-tangent process on
creating new E(Q) pairs of the infinite birational pairs group.

reply to: Randall.Rathbun@thor.SanDiego.NCR.COM
  or      randall@thor.sandiego.NCR.COM

thanks sincerely   -   Randall

Re: Bug in dc? and another one?

#87

Author: Mike Wescott
Date: Wed, 07 Oct 1987 19:40

40 lines
1775 bytes

In article <1800@ncr-sd.SanDiego.NCR.COM> randall@ncr-sd.SanDiego.NCR.COM (0000-Randall Rathbun) writes:
> Running the short dc routine below to find the gcd of two numbers
> appears to work for small numbers.
> 
> ........
> 
> When r & s are about 100 digits or so in length, an error message appears:
> %Segmentation fault (core dumped)
> 
> What is wrong?

Looks like an overflow of readstk.  readstk has a hard coded limit of 100
in the SysVr3 source that I have access to.  It's a bug in that the exceeding
the limit is not gracefully handled.

While tracking down this problem I came across some strange looking code:

   In readc():
		if((readptr != &readstk[0]) && (*readptr != 0)){
			.
			.
			.
		}
		.
		.
		if(readptr != &readptr[0]){	<===== ???????????
			readptr--;
			.
			.
			.

Surely the author meant  "if(readptr != &readstk[0]) ...

In anycase the whole if stmt gets optimized to nothing.  Looks like the
relics of an attempt to put a more general file reading mechanism into dc.

	-Mike Wescott
-- 
	-Mike Wescott
	 wescott@ncrcae.Columbia.NCR.COM

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