What is the function of "VAR" in "foreach (VAR = [VAR1, VAR2, ...] in ARRAY) STMT"?

What is the function of "VAR" in "foreach (VAR = [VAR1, VAR2, ...] in ARRAY) STMT"?

[Nan Xiao]

Hi all,

From the "foreach" section in SystemTap
doc(https://sourceware.org/systemtap/langref/Statement_types.html#SECTION00076000000000000000),
I can see there is a foreach usage:

        foreach (VAR = [VAR1, VAR2, ...] in ARRAY) STMT

But execute the following script generate errors:

# cat test.stp
#!/usr/bin/stap

global reads

probe vfs.read
{
        reads[pid(), execname()]++
}

probe timer.s(3)
{
        foreach (var = [pid, name] in reads)
                printf("%d\n", reads[var]);
        exit()
}

# ./test.stp
semantic error: inconsistent arity (2 vs 1): identifier 'reads' at
./test.stp:13:18
        source:                 printf("%d\n", reads[var]);
                                               ^

semantic error: arity 2 first inferred here: identifier 'reads' at :7:2
        source:         reads[pid(), execname()]++
                        ^

Pass 2: analysis failed.  [man error::pass2]


What is the function of "VAR" in "foreach (VAR = [VAR1, VAR2, ...] in
ARRAY) STMT"? How to use it?
Thanks in advance!

Best Regards
Nan Xiao

Re: What is the function of "VAR" in "foreach (VAR = [VAR1, VAR2, ...] in ARRAY) STMT"?

[Frank Ch. Eigler]

xiaonan830818 wrote:

> [...]
> I can see there is a foreach usage:
>         foreach (VALUE = [VAR1, VAR2, ...] in ARRAY) STMT

See [man stap].  This syntax is for automatically fetching
the ARRAY[VAR1,VAR2,...] value into a VALUE variable, so:

> But execute the following script generate errors:
> [...]
> probe timer.s(3)
> {
>         foreach (var = [pid, name] in reads)
>                 printf("%d\n", reads[var]);

... would be correctly written as

>                 printf("%d\n", var)


- FChE

Re: What is the function of "VAR" in "foreach (VAR = [VAR1, VAR2, ...] in ARRAY) STMT"?

[Nan Xiao]

Hi Frank,

Yeah, it works! Thanks very much for your help!
Best Regards
Nan Xiao


On Fri, Nov 27, 2015 at 3:56 AM, Frank Ch. Eigler <fche@redhat.com> wrote:
>
> xiaonan830818 wrote:
>
>> [...]
>> I can see there is a foreach usage:
>>         foreach (VALUE = [VAR1, VAR2, ...] in ARRAY) STMT
>
> See [man stap].  This syntax is for automatically fetching
> the ARRAY[VAR1,VAR2,...] value into a VALUE variable, so:
>
>> But execute the following script generate errors:
>> [...]
>> probe timer.s(3)
>> {
>>         foreach (var = [pid, name] in reads)
>>                 printf("%d\n", reads[var]);
>
> ... would be correctly written as
>
>>                 printf("%d\n", var)
>
>
> - FChE

Re: What is the function of "VAR" in "foreach (VAR = [VAR1, VAR2, ...] in ARRAY) STMT"?

[Nan Xiao]

Hi Frank,

BTW, I think in this
page(https://sourceware.org/systemtap/langref/Statement_types.html#SECTION00076000000000000000),

use
    foreach (VALUE = [VAR1, VAR2, ...] in ARRAY) STMT (man stap)

is better and more understandable than

    foreach (VAR = [VAR1, VAR2, ...] in ARRAY) STMT

Thanks!

Best Regards
Nan Xiao


On Fri, Nov 27, 2015 at 10:23 AM, Nan Xiao <xiaonan830818@gmail.com> wrote:
> Hi Frank,
>
> Yeah, it works! Thanks very much for your help!
> Best Regards
> Nan Xiao
>
>
> On Fri, Nov 27, 2015 at 3:56 AM, Frank Ch. Eigler <fche@redhat.com> wrote:
>>
>> xiaonan830818 wrote:
>>
>>> [...]
>>> I can see there is a foreach usage:
>>>         foreach (VALUE = [VAR1, VAR2, ...] in ARRAY) STMT
>>
>> See [man stap].  This syntax is for automatically fetching
>> the ARRAY[VAR1,VAR2,...] value into a VALUE variable, so:
>>
>>> But execute the following script generate errors:
>>> [...]
>>> probe timer.s(3)
>>> {
>>>         foreach (var = [pid, name] in reads)
>>>                 printf("%d\n", reads[var]);
>>
>> ... would be correctly written as
>>
>>>                 printf("%d\n", var)
>>
>>
>> - FChE