How to understand "?" in probe alias?

How to understand "?" in probe alias?

[Nan Xiao]

Hi all,

Below is "Testing target variable available Example"
(https://sourceware.org/systemtap/SystemTap_Beginners_Guide/targetavailable.html):

probe vm.pagefault = kernel.function("__handle_mm_fault@mm/memory.c") ?,
                     kernel.function("handle_mm_fault@mm/memory.c") ?
{
        name = "pagefault"
        write_access = (@defined($flags) ? $flags & FAULT_FLAG_WRITE :
$write_access)
        address =  $address
}

How to understand "?" in probe alias? After checking Language
reference(https://sourceware.org/systemtap/langref/Components_SystemTap_script.html#SECTION00041000000000000000),
I can't find the explanations of it.

Thanks in advance!

Best Regards
Nan Xiao

Re: How to understand "?" in probe alias?

[Frank Ch. Eigler]

Nan Xiao <xiaonan830818@gmail.com> writes:

> [...]
> probe vm.pagefault = kernel.function("__handle_mm_fault@mm/memory.c") ?,
>                      kernel.function("handle_mm_fault@mm/memory.c") ?
> [...]  How to understand "?" in probe alias? After checking Language
> reference [...] I can't find the explanations of it.

You're right; that particular book doesn't seem to mention it.
But [man stap] refers to [man stapprobes], which includes:


    However, a probe point may be followed by a "?" character, to
    indicate that it is optional, and that no error should result if
    it fails to resolve. Optionalness passes down through all levels
    of alias/wildcard expansion. Alternately, a probe point may be
    followed by a "!" character, to indicate that it is both optional
    and sufficient. (Think vaguely of the Prolog cut operator.) If it
    does resolve, then no further probe points in the same
    comma-separated list will be resolved. Therefore, the "!"
    sufficiency mark only makes sense in a list of probe point
    alternatives.


- FChE