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


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. Note the use of the PDL_INCLUDE, PDL_TYPEMAP, PDL_AUTO_INCLUDE and PDL_BOOT functions that were imported from PDL::Core::Dev. They are used in conjunction with an Inline Config call to ensure that the PDL typemap, the PDL include files and the PDL Core routines are found during compilation and later runtime execution.

   use PDL::LiteF;
   use PDL::Core::Dev;
   $a = myfloatseq(); # exercise our C piddle constructor
   print $a->info,"\n";
   # the reason for this config call is explained below
   use Inline C => Config =>
     INC           => &PDL_INCLUDE,  # make sure we find pdlcore.h etc
     TYPEMAPS      => &PDL_TYPEMAP,  # use the PDL typemap
     AUTO_INCLUDE  => &PDL_AUTO_INCLUDE,  # global declarations and includes
     BOOT          => &PDL_BOOT;     # boot code to load the Core struct
   use Inline C;
   Inline->init; # useful if you want to be able to 'do'-load this script
   static pdl* new_pdl(int datatype, PDL_Long 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_Long dims[] = {5,5,5};
     pdl *p = new_pdl(PDL_F,dims,3);
     PDL_Float *dataf = (PDL_Float *) p->data;
     int i;
     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 C => Config =>
     INC           => &PDL_INCLUDE,
     BOOT          => &PDL_BOOT;
   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_Long 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;
     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_Long 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 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:

   print PDL_BOOT;
   perl_require_pv ("PDL::Core"); /* make sure PDL::Core is loaded */
   CoreSV = perl_get_sv("PDL::SHARE",FALSE);  /* SV* value */
 #ifndef aTHX_
 #define aTHX_
   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_ "The code needs to be recompiled against the newly installed PDL");

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., continuning 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.



the Inline manpage


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


Copyright (c) 2001, Christian Soeller. All Rights Reserved. This module is free software. It may be used, redistributed and/or modified under the same terms as PDL itself (see http://pdl.perl.org).

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