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When setjmp is called how can the if statement evaluate to true or false
when setjmp only returns 0 or non-zero?


struct pcb {
void *(*start_routine) (void *);
void *arg;
jmp_buf state;
int stack[1024];
};


struct pcb *pcb_pointer;
pcb_pointer = (struct pcb *) malloc(sizeof(struct pcb));


if(setjmp(pcb_pointer->state)) {
current->start_routine(current->arg);
printf("Thread returned\n");
exit(0);
}

In article <d1sjvi$5db$>, JS <dsa.@asdf.com> wrote:
>When setjmp is called how can the if statement evaluate to true or false
>when setjmp only returns 0 or non-zero?

Have a coffee, go outside, and sit under a tree (or, if the weather
fails to be cooperative, at least get away from the computer), and
contemplate the difference between true/false and nonzero/zero, and you
will be enlightened.


dave

--
Dave Vandervies
>The Kremlin has a mother these days?
After a fflush(stdin), it might end up having three mothers.
--Joona I Palaste and Gordon Burditt in comp.lang.c

>When setjmp is called how can the if statement evaluate to true or false
>when setjmp only returns 0 or non-zero?

There is no true or false in C89. if statements execute the "then" clause
(even though there's no "then" keyword) if the condition evaluates to
non-zero.

While C99 has true and false, this has nothing to do with if statements.

Gordon L. Burditt

"Dave Vandervies" <> skrev i en meddelelse
news:d1sl51$u98$...
> In article <d1sjvi$5db$>, JS <dsa.@asdf.com> wrote:
> >When setjmp is called how can the if statement evaluate to true or false
> >when setjmp only returns 0 or non-zero?
>
> Have a coffee, go outside, and sit under a tree (or, if the weather
> fails to be cooperative, at least get away from the computer), and
> contemplate the difference between true/false and nonzero/zero, and you
> will be enlightened.


Well I have only Java experience and not yet found anything about this
definition in K&R.

"Gordon Burditt" <> skrev i en meddelelse
news:4241d80d$0$88029$...
> >When setjmp is called how can the if statement evaluate to true or false
> >when setjmp only returns 0 or non-zero?
>
> There is no true or false in C89. if statements execute the "then" clause
> (even though there's no "then" keyword) if the condition evaluates to
> non-zero.
>
> While C99 has true and false, this has nothing to do with if statements.
>
> Gordon L. Burditt

Where can I find C89 and C99??

JS wrote:
>>>When setjmp is called how can the if statement evaluate to true or false
>>>when setjmp only returns 0 or non-zero?
>>
>>Have a coffee, go outside, and sit under a tree (or, if the weather
>>fails to be cooperative, at least get away from the computer), and
>>contemplate the difference between true/false and nonzero/zero, and you
>>will be enlightened.
>
>
>
> Well I have only Java experience and not yet found anything about this
> definition in K&R.

I have found that 0 (zero) and !0 (not zero) work well for false and
true checking.

Mark Odell <> writes:

> I have found that 0 (zero) and !0 (not zero) work well for false and
> true checking.

I have found that 1 works well as !0.
--
Ben Pfaff
email:
web: http://benpfaff.org

Ben Pfaff wrote:
> Mark Odell <> writes:
>
>
>>I have found that 0 (zero) and !0 (not zero) work well for false and
>>true checking.
>
>
> I have found that 1 works well as !0.

Yeah but that breaks my "no numbers except zero" rule. Besides,
sometimes 1 looks like l (ell).

I found this example:

#include <setjmp.h>
#include <stdio.h>

jmp_buf ex;

static int foo (int a, int b)
{
if (!b)
longjmp (ex, 1); /* THROW */
else
return a/b;
}

int main (void)
{
int x = 0, y = 1, z = 0;
if (setjmp (ex) == 0) /* TRY : longjmp branches back to here */
{
x = foo(y, z);
}
else /* CATCH */
{
printf ("Exception: attempt to divide by zero\n");
}
}

When foo is called after setjmp has been called, longjmp is called. Then
control jumps to setjmp but this time setjmp does not return 0 and
therefore: printf ("Exception: attempt to divide by zero\n"); is executed.

But I don't understand why setjmp don't return 0 after the longjmp call. Is
it because the second parameter to longjmp is used as the return value for
setjmp?

Or does the second paramter replace 0 in: if (setjmp (ex) == 0)?

JS wrote:
> When setjmp is called how can the if statement evaluate to true or false
> when setjmp only returns 0 or non-zero?

Since false is 0 and true is non-zero, I fail to see your problem.