seq point/atomic var

[Roopa]

Hi,

Can anybody explain me the meanings of -

1. *Sequence Point* in a expression.
For ex: arr[i++] = i; /* Is undefined cause of non intervening seq point !*/
How does one go about computing the sequence point of long expressions.
Any example illustrating seq points in long expressions would be very
helpful.

2. *Atomic access* ( context of signal handler)

Thanks in advance
- Roopa

 

[Joona I Palaste]

Hi,

Can anybody explain me the meanings of -

1. *Sequence Point* in a expression.
For ex: arr[i++] = i; /* Is undefined cause of non intervening seq point !*/
How does one go about computing the sequence point of long expressions.
Any example illustrating seq points in long expressions would be very
helpful.

Sequence points include the following:
- The operators "&&", "||" and ","
- The conditional operator's "?" and ":" parts
- The end of a complete statement
Also there is a sequence point in calling a function, but I can never
remember exactly *where*.
For example, as you say, "arr[i++] = i;" is invalid, but for example
"i++, arr=i;" is completely valid, and so is
"arr[i++ ? 10 : 20] = i;". (Of course assuming the array arr is big
enough to have the appropriate index.)

 

[pete]

Hi,

Can anybody explain me the meanings of -

1. *Sequence Point* in a expression.
For ex: arr[i++] = i;
/* Is undefined cause of non intervening seq point !*/

You need a sequence point between accesses
when there are side effects.

The side effect of i++, is that i is incremented.
You don't know whether the right operand of the assignment, (i),
is supposed to be accessed prior to or after the side effect.

x = i + i; is fine because there are no side effects.

x = i++ + i; isn't fine

If the initial value of i was zero,
then it might seem like (i++ + i) should either equal zero or one,
depending on whether i++ or i is evaluated first,
but the case is that the C standard commitee just doesn't care
what happens if you're going to write code like that,
so it's undefined.

 

[Richard Bos]

Sequence points include the following:
- The operators "&&", "||" and ","
- The conditional operator's "?" and ":" parts

Only at the ?; it makes no sense to have a sequence point at the :,
since it is never possible to execute _both_ the expression before, and
that after the :.

- The end of a complete statement

Complete _expression_. E.g., also after each of the three expressions in
a single for statement.

Also there is a sequence point in calling a function, but I can never
remember exactly *where*.

After the evaluation of its arguments, and just before actually calling
the function.

Just before a library function returns.
After each printf()/scanf() output specification.
Surrounding the comparison function calls to qsort() and bsearch().
After a full declarator.

For more precise descriptions, and references to paragraph numbers in
the Standard, see its appendix C.

For example, as you say, "arr[i++] = i;" is invalid, but for example
"i++, arr=i;" is completely valid, and so is
"arr[i++ ? 10 : 20] = i;".

Not the latter. There's a sequence point between i++ and 10 or 20; but
there's not necessarily one between i++ and i, nor between 10, 20 and i.
(Interestingly, since there is a sequence point at the ?, there must be
a sequence point either between the i and the i++, or between the i and
then 10/20. The problem is, there is no way to tell which of those two
is true in your program.)

Richard