# Copyright (C) 2001-2005, Parrot Foundation. # $Id: struct.pod 38815 2009-05-16 03:50:07Z coke $ =head1 NAME Structures - Accessing C Structs from Parrot =head1 DESCRIPTION Parrot provides two PMC classes to deal with C structures. These are UnManagedStruct and ManagedStruct. The former has no allocated memory and is typically used to access structures returned by NCI calls, while the latter can be used to define a structure and pass it over to a C function - pointers to structures in both cases of course. =head1 Structure definition The Struct PMCs take an array of triples per structure element, either as initializer or with the B<assign> opcode to define the struct elements. =over 4 =item Datatype The datatype is defined by constants declared in F<datatypes.pasm>. =item Array Size The second initializer item, if set to a value greater then 1, defines the struct element to consist of an array of the given data type. =item Byte Offset The third initializer is the byte offset of the data item in the structure. This entry can be 0 if packing of the structure is aligned to the item's sizes or the alignment is the item's size. Otherwise these offsets must be set correctly as Parrot doesn't know how your C compiler packs arbitrary data. Parrot only knows the size of each item. =back =head2 Alignment Parrot tries to do The Right Thing that is currently align items at their size. struct { char c; int i; } The C<i> above is aligned at 4 (for i386 or such). =head2 Example The C structure struct { double d; float f; int i[4]; char *s; }; can be declared with this initializer: =begin PIR_FRAGMENT_INVALID new P2, 'ResizablePMCArray' .include "datatypes.pasm" push P2, .DATATYPE_DOUBLE push P2, 0 # no array i.e. 1 element push P2, 0 # calculate offset by just adding item size push P2, .DATATYPE_FLOAT push P2, 0 push P2, 0 push P2, .DATATYPE_INT push P2, 4 # 4 elem array push P2, 0 push P2, .DATATYPE_CSTR push P2, 0 push P2, 0 =end PIR_FRAGMENT_INVALID =head2 Named Structure Elements The initializer can be an OrderedHash PMC too. When all elements are defined in the correct order this can be used to define and access struct elements by name and by index: =begin PIR_FRAGMENT_INVALID new P2, 'OrderedHash' .include "datatypes.pasm" set P2["d"], .DATATYPE_DOUBLE push P2, 0 # no array i.e. 1 element push P2, 0 # calculate offset by just adding item size set P2["f"], .DATATYPE_FLOAT =end PIR_FRAGMENT_INVALID =head1 Size of a Structure For ManagedStruct (a new structure passed over to a C function) the storage for data items has to be allocated. This is done automatically, when the initializer is attached to the Struct PMC. The size can be obtained by: =begin PIR_FRAGMENT_INVALID new P5, 'ManagedStruct', P2 # P2 be some initializer set I0, P5 # allocated size =end PIR_FRAGMENT_INVALID =head1 Accessing Structure Items Setting or getting items is done by keyed access to the Struct PMC. The first key is the structure item, an optional second key can access the n-th array element. =head2 Example =begin PIR_FRAGMENT_INVALID set P5[0], N0 # set d set N0, P5[0] # get d set N0, P5["d"] # get d if initializer is an OrderedHash set P5[1], N1 # set f set N1, P5[1] # get f set N1, P5["f"] # get f if initializer is an OrderedHash set P5[2;0], I2 # set i[0] set I3, P5[2;3] # get i[3] set P5["i"; 2] # set i[2] if initializer is an OrderedHash set S0, P5[3] # get string at *s set S0, P5["s"] # same =end PIR_FRAGMENT_INVALID =head2 Strings When passing a STRING to a structure that needs a 0-terminated C-string (char *s), then you have to provide the terminating NUL char in the string. struct { ... char *s; }; =begin PIR_FRAGMENT_INVALID set P5["s"], "a string\x0" =end PIR_FRAGMENT_INVALID Please also note, that the C function currently gets a pointer to string memory, so any code that might trigger GC should be avoided (or GC turned off). Passing constant strings like above is safe though. =begin PIR_FRAGMENT_INVALID set P5["s"], S0 # S0 shouldn't move until function call =end PIR_FRAGMENT_INVALID =head2 Callback Functions in the C Library Given a C function that returns a structure containing a callback function like in this example: static struct { int (*f)(char *); } t = { call_back }; return &t; The PASM would look like: =begin PASM_INVALID push P2, .DATATYPE_FUNC_PTR # attach function signature property to this type set P1, P2[-1] new P3, 'String' set P3, "it" setprop P1, "_signature", P3 push P2, 0 push P2, 0 # P5 is the return UnManagedStruct PMC assign P5, P2 # now we get a callable NCI PMC set P0, P5[0] set S5, "hello call_back" # call the call_back function invoke =end PASM_INVALID =head2 Nested Structures or Pointers to Nested Structures Each contained structure needs its own UnManagedStruct initializer. The UnManagedStruct of the contained structures has to be attached to the structure type PMC as the property B<"_struct">. If a C function returns a pointer to this structure: static struct xt { char x; struct yt { char i; int j; } _y; char z; } _x; ... access to elements could look like: =begin PIR_FRAGMENT_INVALID # the nested structure new P3, 'OrderedHash' set P3["i"], .DATATYPE_CHAR push P3, 0 push P3, 0 set P3["j"], .DATATYPE_INT push P3, 0 push P3, 0 new P4, 'UnManagedStruct', P3 # outer structure new P2, 'OrderedHash' set P2["x"], .DATATYPE_CHAR push P2, 0 push P2, 0 set P2["_y"], .DATATYPE_STRUCT # attach the unmanaged struct as property to the type PMC set P1, P2[-1] # last element setprop P1, "_struct", P4 push P2, 0 push P2, 0 set P2["z"], .DATATYPE_CHAR push P2, 0 push P2, 0 # attach struct initializer to return value in P5 assign P5, P2 # now access values set I0, P5[0] # x set I0, P5[1;0] # _y.i set I0, P5[1;1] # _y.j set I0, P5[2] # z # or by name set I0, P5["x"] set I0, P5["_y"; "i"] set I0, P5["_y"; "j"] set I0, P5["z"] =end PIR_FRAGMENT_INVALID If the structure has a pointer to another structure the datatype is: =begin PIR_INVALID push P2, .DATATYPE_STRUCT_PTR =end PIR_INVALID =head1 Passing A Structure to a C function For a shared library B<libnci_test.so> (or whatever) and a C function typedef struct _dfi_t { double d; float f; int i[4]; } dfi_t; int nci_ip(dfi_t *p) {} a pointer to the structure is passed with the B<p> signature char: =begin PIR loadlib P1, "libnci_test" dlfunc P0, P1, "nci_ip", "ip" # P5 is ManagedStruct from above invoke # I5 is result =begin PIR =head1 BUGS Not all datatypes are implemented. Alignment is barely tested on different machines. Arrays of structures aren't handled yet. Passing nested structures to C isn't finished. =head1 FILES F<src/pmc/unmanagedstruct.pmc>, F<src/pmc/managedstruct.pmc> =head1 SEE ALSO F<docs/pdds/pdd03_calling_conventions.pod> F<t/pmc/nci.t>, F<src/nci_test.c> =head1 AUTHOR Leopold Toetsch <lt@toetsch.at>