PDL::API - making piddles from Perl and C/XS code


A simple cookbook how to create piddles manually. It covers both the Perl and the C/XS level. Additionally, it describes the PDL core routines that can be accessed from other modules. These routines basically define the PDL API. If you need to access piddles from C/XS you probably need to know about these functions.


  use PDL;
  sub mkmypiddle {

Creating a piddle manually from Perl

Sometimes you want to create a piddle manually from binary data. You can do that at the Perl level. Examples in the distribution include some of the IO routines. The code snippet below illustrates the required steps.

   use Carp;
   sub mkmypiddle {
     my $class = shift;
     my $pdl  = $class->new;
     my @dims = (1,3,4);
     my $size = 1;
     for (@dims) { $size *= $_ }
     my $dref = $pdl->get_dataref();

     # read data directly from file
     open my $file, '<data.dat' or die "couldn't open data.dat";
     my $len = $size*PDL::Core::howbig($pdl->get_datatype);
     croak "couldn't read enough data" if
       read( $file, $$dref, $len) != $len;
     close $file;

     return $pdl;

Creating a piddle in C

The following example creates a piddle at the C level. We use the Inline module which is really the way to interface Perl and C these days, using the with capability in Inline 0.68+.

   use PDL::LiteF;

   $a = myfloatseq(); # exercise our C piddle constructor

   print $a->info,"\n";

   use Inline with => 'PDL';
   use Inline C;
   Inline->init; # useful if you want to be able to 'do'-load this script



   static pdl* new_pdl(int datatype, PDL_Indx dims[], int ndims)
     pdl *p = PDL->pdlnew();
     PDL->setdims (p, dims, ndims);  /* set dims */
     p->datatype = datatype;         /* and data type */
     PDL->allocdata (p);             /* allocate the data chunk */

     return p;

   pdl* myfloatseq()
     PDL_Indx dims[] = {5,5,5};
     pdl *p = new_pdl(PDL_F,dims,3);
     PDL_Float *dataf = (PDL_Float *) p->data;
     PDL_Indx i; /* dimensions might be 64bits */

     for (i=0;i<5*5*5;i++)
       dataf[i] = i; /* the data must be initialized ! */
     return p;

Wrapping your own data into a piddle

Sometimes you obtain a chunk of data from another source, for example an image processing library, etc. All you want to do in that case is wrap your data into a piddle struct at the C level. Examples using this approach can be found in the IO modules (where FastRaw and FlexRaw use it for mmapped access) and the Gimp Perl module (that uses it to wrap Gimp pixel regions into piddles). The following script demonstrates a simple example:

   use PDL::LiteF;
   use PDL::Core::Dev;
   use PDL::Graphics::PGPLOT;

   $b = mkpiddle();

   print $b->info,"\n";

   imag1 $b;

   use Inline with => 'PDL';
   use Inline C;



   /* wrap a user supplied chunk of data into a piddle
    * You must specify the dimensions (dims,ndims) and 
    * the datatype (constants for the datatypes are declared
    * in pdl.h; e.g. PDL_B for byte type, etc)
    * when the created piddle 'npdl' is destroyed on the
    * Perl side the function passed as the 'delete_magic'
    * parameter will be called with the pointer to the pdl structure
    * and the 'delparam' argument.
    * This gives you an opportunity to perform any clean up
    * that is necessary. For example, you might have to
    * explicitly call a function to free the resources
    * associated with your data pointer.
    * At the very least 'delete_magic' should zero the piddle's data pointer:
    *     void delete_mydata(pdl* pdl, int param)
    *     {
    *       pdl->data = 0;
    *     }
    *     pdl *p = pdl_wrap(mydata, PDL_B, dims, ndims, delete_mydata,0);
    * pdl_wrap returns the pointer to the pdl
    * that was created.
   typedef void (*DelMagic)(pdl *, int param);
   static void default_magic(pdl *p, int pa) { p->data = 0; }
   static pdl* pdl_wrap(void *data, int datatype, PDL_Indx dims[],
                        int ndims, DelMagic delete_magic, int delparam)
     pdl* npdl = PDL->pdlnew(); /* get the empty container */

     PDL->setdims(npdl,dims,ndims); /* set dims      */
     npdl->datatype = datatype;     /* and data type */
     npdl->data = data;             /* point it to your data */
     /* make sure the core doesn't meddle with your data */
     if (delete_magic != NULL)
       PDL->add_deletedata_magic(npdl, delete_magic, delparam);
       PDL->add_deletedata_magic(npdl, default_magic, 0);
     return npdl;

   #define SZ 256
   /* a really silly function that makes a ramp image
    * in reality this could be an opaque function
    * in some library that you are using
   static PDL_Byte* mkramp(void)
     PDL_Byte *data;
     int i; /* should use PDL_Indx to support 64bit pdl indexing */

     if ((data = malloc(SZ*SZ*sizeof(PDL_Byte))) == NULL)
       croak("mkramp: Couldn't allocate memory");
     for (i=0;i<SZ*SZ;i++)
       data[i] = i % SZ;

     return data;

   /* this function takes care of the required clean-up */
   static void delete_myramp(pdl* p, int param)
     if (p->data)
     p->data = 0;

   pdl* mkpiddle()
     PDL_Indx dims[] = {SZ,SZ};
     pdl *p;

     p = pdl_wrap((void *) mkramp(), PDL_B, dims, 2, 
                  delete_myramp,0); /* the delparam is abitrarily set to 0 */
     return p;

The gory details

The Core struct -- getting at PDL core routines at runtime

PDL uses a technique similar to that employed by the Tk modules to let other modules use its core routines. A pointer to all shared core PDL routines is stored in the $PDL::SHARE variable. XS code should get hold of this pointer at boot time so that the rest of the C/XS code can then use that pointer for access at run time. This initial loading of the pointer is most easily achieved using the functions PDL_AUTO_INCLUDE and PDL_BOOT that are defined and exported by PDL::Core::Dev. Typical usage with the Inline module has already been demonstrated:

   use Inline with => 'PDL';

In earlier versions of Inline, this was achieved like this:

   use Inline C => Config =>
     INC           => &PDL_INCLUDE,
     AUTO_INCLUDE  => &PDL_AUTO_INCLUDE, # declarations
     BOOT          => &PDL_BOOT;         # code for the XS boot section

The code returned by PDL_AUTO_INCLUDE makes sure that pdlcore.h is included and declares the static variables to hold the pointer to the Core struct. It looks something like this:


 #include <pdlcore.h>
 static Core* PDL; /* Structure holds core C functions */
 static SV* CoreSV;       /* Gets pointer to Perl var holding core structure */

The code returned by PDL_BOOT retrieves the $PDL::SHARE variable and initializes the pointer to the Core struct. For those who know their way around the Perl API here is the code:

   perl_require_pv ("PDL/"); /* make sure PDL::Core is loaded */
#ifndef aTHX_
#define aTHX_
   if (SvTRUE (ERRSV)) Perl_croak(aTHX_ "%s",SvPV_nolen (ERRSV));
   CoreSV = perl_get_sv("PDL::SHARE",FALSE);  /* SV* value */
   if (CoreSV==NULL)
     Perl_croak(aTHX_ "We require the PDL::Core module, which was not found");
   PDL = INT2PTR(Core*,SvIV( CoreSV ));  /* Core* value */
   if (PDL->Version != PDL_CORE_VERSION)
     Perl_croak(aTHX_ "[PDL->Version: \%d PDL_CORE_VERSION: \%d XS_VERSION: \%s] The code needs to be recompiled against the newly installed PDL", PDL->Version, PDL_CORE_VERSION, XS_VERSION);

The Core struct contains version info to ensure that the structure defined in pdlcore.h really corresponds to the one obtained at runtime. The code above tests for this

   if (PDL->Version != PDL_CORE_VERSION)

For more information on the Core struct see PDL::Internals.

With these preparations your code can now access the core routines as already shown in some of the examples above, e.g.

  pdl *p = PDL->pdlnew();

By default the C variable named PDL is used to hold the pointer to the Core struct. If that is (for whichever reason) a problem you can explicitly specify a name for the variable with the PDL_AUTO_INCLUDE and the PDL_BOOT routines:

   use Inline C => Config =>
     INC           => &PDL_INCLUDE,
     BOOT          => &PDL_BOOT 'PDL_Corep';

Make sure you use the same identifier with PDL_AUTO_INCLUDE and PDL_BOOT and use that same identifier in your own code. E.g., continuing from the example above:

  pdl *p = PDL_Corep->pdlnew();

Some selected core routines explained

The full definition of the Core struct can be found in the file pdlcore.h. In the following the most frequently used member functions of this struct are briefly explained.




Handy macros from pdl.h

Some of the C API functions return PDL_Anyval C type which is a structure and therefore requires special handling.

You might want to use for example get_pdl_badvalue function:

 /* THIS DOES NOT WORK! (although it did in older PDL) */
 if( PDL->get_pdl_badvalue(a) == 0 )  { ... }

 double bad_a;
 ANYVAL_TO_CTYPE(bad_a, double, PDL->get_pdl_badvalue(a));
 if( bad_a == 0 ) { ... }

In pdl.h there are the following macros for handling PDL_Anyval from C code:

 ANYVAL_TO_SV(out_SV, in_anyval)
 ANYVAL_FROM_CTYPE(out_anyval, out_anyval_type, in_variable)
 ANYVAL_TO_CTYPE(out_variable, out_ctype, in_anyval)

As these macros where not available in older PDL versions you might want to add the following defines into your C/XS code to make compatible with older PDL versions.

 #define ANYVAL_TO_SV(outsv,inany)              outsv  = newSVnv((NV)(inany)
 #define ANYVAL_FROM_CTYPE(outany,avtype,inval) outany = (PDL_Double)(inval)
 #define ANYVAL_TO_CTYPE(outval,ctype,inany)    outval = (ctype)(inany)
 #define ANYVAL_EQ_ANYVAL(x,y)                  (x == y)


This manpage is still under development. Feedback and corrections are welcome.


Copyright 2013 Chris Marshall (

Copyright 2010 Christian Soeller ( You can distribute and/or modify this document under the same terms as the current Perl license.