ts@cup.portal.com (Tim W Smith) (11/26/90)
For obscure reasons that I won't go into, I wanted a divide by zero in a program. I was compiling on SCO Unix System V/386 relase 2.2 (I think...) with whatever C compiler comes with this version of Unix. I tried the obvious: int i, j; i = 1; j = 0; i/=j; The compiler caught this. Grrr. Next I tried: i = 1; j = 1; i -= 1; j/=i; It still caught it. Double grrr! Next try: int i, j, *p; i = 1; j = 1; p = &i; j /= *p - 1; It still caught it! This one got past it: j = 5; for ( i = 0; i < 5; i++ ) j--; i /= j; After the previous try, I was a bit surprised that it didn't figure out this one! Tim Smith
lerman@stpstn.UUCP (Ken Lerman) (11/27/90)
In article <36233@cup.portal.com> ts@cup.portal.com (Tim W Smith) writes:
->For obscure reasons that I won't go into, I wanted a divide
->by zero in a program. I was compiling on SCO Unix System V/386
->relase 2.2 (I think...) with whatever C compiler comes with
->this version of Unix.
->
->I tried the obvious:
->
-> int i, j;
->
-> i = 1;
-> j = 0;
-> i/=j;
->
->The compiler caught this. Grrr. Next I tried:
->
-> i = 1;
-> j = 1;
-> i -= 1;
-> j/=i;
->
->It still caught it. Double grrr!
->
->Next try:
->
-> int i, j, *p;
->
-> i = 1;
-> j = 1;
-> p = &i;
-> j /= *p - 1;
->
->It still caught it!
->
->This one got past it:
->
-> j = 5;
-> for ( i = 0; i < 5; i++ )
-> j--;
-> i /= j;
->
->After the previous try, I was a bit surprised that it didn't
->figure out this one!
->
-> Tim Smith
I haven't tried it, but I'll bet that:
int divide(int a, int b){ return a/b; }
will cause your divide error when called by:
x = divide(1,0);
Of course, if your compiler is brilliant, you might have to put the
function divide in a separate file. :-)
Kengrogers@convex.com (Geoffrey Rogers) (11/28/90)
In article <36233@cup.portal.com> ts@cup.portal.com (Tim W Smith) writes: >I tried the obvious: > > int i, j; > > i = 1; > j = 0; > i/=j; > Most compilers today do constant folding and propogration on either a local or global (with the entire function) optimization. This is what you are getting bitting by. >This one got past it: > > j = 5; > for ( i = 0; i < 5; i++ ) > j--; > i /= j; > The best way I found to get around this problem is by doing: div(0,1) ... div(a,b) int a,b; { a = b/a; } This will work for almost all compilers (expect compilers that can do interprocedural optimizations, which there are not many). The only time I have done something like this is for testing. +------------------------------------+---------------------------------+ | Geoffrey C. Rogers | "Whose brain did you get?" | | grogers@convex.com | "Abbie Normal!" | | {sun,uunet,uiucdcs}!convex!grogers | | +------------------------------------+---------------------------------+
userAKDU@mts.ucs.UAlberta.CA (Al Dunbar) (11/29/90)
In article <36233@cup.portal.com>, ts@cup.portal.com (Tim W Smith) writes: >For obscure reasons that I won't go into, I wanted a divide >by zero in a program. I was compiling on SCO Unix System V/386 >relase 2.2 (I think...) with whatever C compiler comes with >this version of Unix. > >I tried the obvious: > > int i, j; > > i = 1; > j = 0; > i/=j; > >The compiler caught this. Grrr. Next I tried: I was curious about how my compiler would handle these attempts (QC 2.51), so I tried it, and it caught them all! Next I tried: scanf( "%d %d", i, j ); i/=j Would you believe it caught this too ("Warning: user may accidentally enter invalid input resulting in a runtime divide exception")? I didn't think so :-) -------------------+------------------------------------------- Al Dunbar | Edmonton, Alberta | "this mind left intentionally blank" CANADA | - Manuel Writer -------------------+------------------------------------------- #! r
chris@mimsy.umd.edu (Chris Torek) (11/29/90)
In article <5857@stpstn.UUCP> lerman@stpstn.UUCP (Ken Lerman) writes: >I haven't tried it, but I'll bet that: > int divide(int a, int b){ return a/b; } >will cause your divide error when called by: > x = divide(1,0); >Of course, if your compiler is brilliant, you might have to put the >function divide in a separate file. :-) Even that is unlikely to help for long. (You can expect better IBM PC compilers to do cross-module optimization within 5 years. [I actually expect it sooner than this. MIPS and Sun already do it.]) Here is something that *will* (must) work: const volatile int zero = 0; int exception = 1 / zero; A compiler cannot assume that a `const volatile int' has any particular value. For instance: extern const volatile int clock; /* mapped by linker to clock */ might tell you the current time in seconds since the machine was booted. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 405 2750) Domain: chris@cs.umd.edu Path: uunet!mimsy!chris
djones@megatest.UUCP (Dave Jones) (12/04/90)
From article <36233@cup.portal.com>, by ts@cup.portal.com (Tim W Smith): > For obscure reasons that I won't go into, I wanted a divide > by zero in a program. [But the compiler made it tough.] I too got into a mess like this the other day. I was writing some very low-level stuff and just wanted to use the assembly language output from the C compiler as a guide. The compiler "optimized" away some of my code. It didn't optimize away all of it, although it was all theoretically "dead". Luckily, checking it over, I figured out that something was amiss.