perlmulticore.h man page

perlmulticore.h ā€” implements the Perl Multicore Specification


  #include "perlmulticore.h"

  // in your XS function:

  perlinterp_release ();
  do_the_C_thing ();
  perlinterp_acquire ();


This documentation is the abridged version of the full documention at <>. It's recommended to go there instead of reading this document.

This header file implements a very low overhead (both in code and runtime) mechanism for XS modules to allow re-use of the perl interpreter for other threads while doing some lengthy operation, such as cryptography, SQL queries, disk I/O and so on.

The newest version of the header file itself, can be downloaded from <>.

How Do I Use This in My Modules?

The usage is very simple - you include this header file in your XS module. Then, before you do your lengthy operation, you release the perl interpreter:

   perlinterp_release ();

And when you are done with your computation, you acquire it again:

   perlinterp_acquire ();

And that's it. This doesn't load any modules and consists of only a few machine instructions when no module to take advantage of it is loaded.

More documentation and examples can be found at the perl multicore site at <>.

The Hard and Fast Rules

As with everything, there are a number of rules to follow.

Never touch any perl data structures after calling "perlinterp_release".

Anything perl is completely off-limits after "perlinterp_release", until you call "perlinterp_acquire", after which you can access perl stuff again.

That includes anything in the perl interpreter that you didn't prove to be safe, and didn't prove to be safe in older and future versions of perl: global variables, local perl scalars, even if you are sure nobody accesses them and you only try to "read" their value.

Always call "perlinterp_release" and "perlinterp_acquire" in pairs.

For each "perlinterp_release" call there must be a "perlinterp_acquire" call. They don't have to be in the same function, and you can have multiple calls to them, as long as every "perlinterp_release" call is followed by exactly one "perlinterp_acquire" call at runtime.

Never nest calls to "perlinterp_release" and "perlinterp_acquire".

That simply means that after calling "perlinterp_release", you must call "perlinterp_acquire" before calling "perlinterp_release" again. Likewise, after "perlinterp_acquire", you can call "perlinterp_release" but not another "perlinterp_acquire".

Always call "perlinterp_release" first.

You must not call "perlinterp_acquire" without having called "perlinterp_release" before.

Never underestimate threads.

While it's easy to add parallel execution ability to your XS module, it doesn't mean it is safe. After you release the perl interpreter, it's perfectly possible that it will call your XS function in another thread, even while your original function still executes. In other words: your C code must be thread safe, and if you use any library, that library must be thread-safe, too.

Always assume that the code between "perlinterp_release" and "perlinterp_acquire" is executed in parallel on multiple CPUs at the same time.

Disabling Perl Multicore at Compile Time

You can disable the complete perl multicore API by defining the symbol "PERL_MULTICORE_DISABLE" to 1 (e.g. by specifying -DPERL_MULTICORE_DISABLE as compiler argument).

This could be added to perl's "CPPFLAGS" when configuring perl on platforms that do not support threading at all for example.


   Marc A. Lehmann <>


The perlmulticore.h header file is put into the public domain. Where this is legally not possible, or at your option, it can be licensed under creativecommons CC0 license: <>.


2018-02-08 perl v5.26.1 User Contributed Perl Documentation