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How use $^M?

by xiaoyafeng (Deacon)
on Apr 05, 2021 at 13:30 UTC ( [id://11130839]=perlquestion: print w/replies, xml ) Need Help??

xiaoyafeng has asked for the wisdom of the Perl Monks concerning the following question:

perl lack global variable/memory for use.But when I look up the doc I find $^M seem to meet this requirement. Below is the introduction of $^M

By default, running out of memory is an untrappable, fatal error. However, if suitably built, Perl can use the contents of $^M as an emergency memory pool after "die()"ing. Suppose that your Perl were compiled with "-DPERL_EMERGENCY_SBRK" and used Perl's malloc. Then $^M = 'a' x (1 << 16); would allocate a 64K buffer for use in an emergency.

But I can't find any excerpt code about this variable. Could someone show code about this? Thanks in advance!


Sadly, $^M is not for usage of global memory as I expect. It's only for a seldom scenario: when perl is out of memory even can't call die signal function to clean up, then if you define $^M already, perl would free $^M first and gain memory to clean up. I didn't try it though, but if you are interested in it, you can refer Is $^M a leftover April Fool? to get more discussion about $^M.

I am trying to improve my English skills, if you see a mistake please feel free to reply or /msg me a correction

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Re: How use $^M?
by Fletch (Bishop) on Apr 05, 2021 at 13:35 UTC

    There's not much more to show short of the example from the documentation. $^M isn't for general control over use of memory; it's a specific place you can pre-allocate a buffer to be used formatting error messages in the case that your process hits a memory limit (and hence couldn't request more memory from the OS at that point in time). Think of it like pulling a sheet of paper out of the ream and setting it in a safe place next to the copier so you have someplace to write a "Copier out of paper" sign when you run out.

    The cake is a lie.
    The cake is a lie.
    The cake is a lie.

      I have problems understanding the doc.

      If "running out of memory is untrappable" how can I be able to run code anyway?

      Or is it still trappable via $SIG{__DIE__} ?

      In that case why can't I just pre-allocate variables in the sig-handler's closure instead of requesting more memory?

      Why $^M at all?

      Honestly I don't know how to simulate "running out of memory" for experimenting.

      Cheers Rolf
      (addicted to the Perl Programming Language :)
      Wikisyntax for the Monastery

        There's a bit more of an explanation in perldiag for "Out of memory during request for %s" and I think this is presuming that you've compiled using perl's malloc.

            Out of memory during request for %s
                (X)(F) The malloc() function returned 0, indicating there was
                insufficient remaining memory (or virtual memory) to satisfy the
                The request was judged to be small, so the possibility to trap it
                depends on the way perl was compiled. By default it is not
                trappable. However, if compiled for this, Perl may use the contents
                of $^M as an emergency pool after die()ing with this message. In
                this case the error is trappable *once*, and the error message will
                include the line and file where the failed request happened.

        So iff you've got a perl with perl's malloc and you give a buffer to $^M and then run out of memory perl will die and write its error message to that buffer. In that case yes then you could trap that one exception, and you could see where the exception occurred (as opposed to just getting an "Out of memory" error from the OS' malloc maybe).

        As far as testing I *think* you could use something like ulimit -m to artificially lower how much memory's available to child processes. Allocate a small buffer as shown in the example, then try and create a large string (run ulimit -m 512 then try perl -E '$^M=(q{}x(1<<16));eval { $x = (q{FOO} x (512 * 1024 * 1024)); }; if($@){die $@;}')

        The cake is a lie.
        The cake is a lie.
        The cake is a lie.

        Honestly I don't know how to simulate "running out of memory" for experimenting

        That is one of those things that seems like it should be quite easy...that is, until you start to think about actually doing it!
        If I were going to try, I think I would start with a virtual machine with very little memory allocated and then get my program to deliberately create lots of arrays of arrays.

Re: How use $^M?
by Discipulus (Canon) on Apr 05, 2021 at 16:17 UTC
    Hello xiaoyafeng,

    iirc $^M is a leftover or similar.

    It worked only if perl is built to use its own malloc instead of the system one (see comments in malloc.c). UPDATE Perl's malloc is used only if -Dusemymalloc is used when building perl.

    See also: Is $^M a leftover April Fool?


    There are no rules, there are no thumbs..
    Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.
Re: How use $^M?
by eyepopslikeamosquito (Archbishop) on Apr 05, 2021 at 23:29 UTC
      Many Thanks! the link you paste is a very useful collection!

      I am trying to improve my English skills, if you see a mistake please feel free to reply or /msg me a correction

Re: How use $^M?
by 1nickt (Canon) on Apr 05, 2021 at 13:44 UTC


    "perl lack global variable/memory for use"

    Please see MCE::Shared which works with all perls back to 5.8 and on all OSes and under threaded and non-threaded perls.

    Hope this helps!

    The way forward always starts with a minimal test.

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