Re: fork bug [WAS: Re: "mount" bug]

Harry Kalogirou
2002-10-27T19:49:46Z

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> On 26 Oct 2002, Harry Kalogirou wrote:
>
> > "Cannot fork" is emmited by the shell, when the fork() system call
> > fails. The system call will fail :
> >
> > 1) If there are no more process slots available.
> > 2) There is not enough free memory.
> >
> > What exacly happend there, I realy can't tell. The only thing I can tell
> > is that something "very" bad happed there. I personaly havedn't seen
> > ELKS behave like that before.
>
> There appears to be a bug in the "get_pid()" function from fork.c. The
> code fragment:
> if (++last_pid == 32768)
> last_pid = 1;
> must eventually fail because last_pid is declared as type pid_t, which is
> typedef'ed as __s16, which is in turn typedef'ed as signed short int.
> Since signed 16-bit numbers roll over from 32767 to negative numbers, they
> can never equal 32768. A possible fix might be:
> if ( (++last_pid && 0x7fff) == 0 )
> last_pid = 1;
> or, perhaps smaller and faster:
> if !( (last_pid++) &= 0x7fff )
> last_pid++;
> The latter may have superfluous parentheses because I'm not sure of the
> precedence, and whether "variable++" is smaller and faster than
> "++variable" is probably compiler-dependent.
>

Commited...

Harry

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